CHROMIUM GEOCHEMISTRY OF SERPENTINITES AND SERPENTINE SOILS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF GEOLOGICAL & ENVIRONMENTAL SCIENCES AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Christopher John-Paul Oze August 2003
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CHROMIUM GEOCHEMISTRY
OF
SERPENTINITES AND SERPENTINE SOILS
A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF
GEOLOGICAL amp ENVIRONMENTAL SCIENCES
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Christopher John-Paul Oze
August 2003
copy Copyright by Christopher John-Paul Oze 2003
All Rights Reserved
ii
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Dennis K Bird Principal Adviser
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Scott Fendorf Principal Adviser
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Robert Coleman
Approved for the University Committee on Graduate Studies
iii
ABSTRACT
Chromium persists in the environment as Cr(III) a non-hazardous species and a
micronutrient in human nutrition or as Cr(VI) a strong oxidizing agent a toxin to living
cells and a Class A human carcinogen by inhalation Serpentinites or metamorphosed
ultramafic rocks are distributed worldwide and contain Cr concentrations typically
greater than 200 mg kg-1 Additionally serpentine soils a generic term used to describe
any soil derived from serpentinite regardless of its physical or chemical properties
contain elevated Cr concentrations often exceeding their corresponding protolith
Chromium in serpentinites and serpentine soils is dominantly in the form of Cr(III)
however geochemical interactions during serpentinite weathering and soil formation
provide oxidative pathways allowing the formation of Cr(VI) The relationship between
rock and soil and the processes governing the Cr chemistry are difficult to assess due to
the inherent complexity of evaluating and correlating the mineralogy and geochemistry of
both systems Ultimately a multidisciplinary approach combining field analytical and
laboratory studies is necessary to evaluate the Cr geochemistry related to these rocks and
soils and to determine whether or not serpentinites and serpentine soils are sources of
non-anthropogenic Cr contamination
Weathering of ultramafic rocks and serpentinites in the Franciscan Complex of
California produces serpentine soils containing high concentrations of Cr as well as other
potentially toxic elements including Ni Co and Mn Chromium concentrations in
serpentine soils from Jasper Ridge Biological Preserve in the Central Coast Range are as
high as 4760 mg kg-1 nearly three times greater than the serpentinite protolith (~1800
mg kg-1) Chromium-containing minerals within the bedrock include chlorite (~03 Cr
fluctuate between 1725 to 4760 mg kg-1 and do not correspond to variations in soil pH
organic matter or electrical conductivity The enrichment and variability of soil Cr is
directly related to the modal abundance and weathering of chromite Cr-magnetite and
the spinel-silicate mixture By comparison Cr silicates account for lt10 of the total soil
Cr Chemical analyses and X-ray microprobe maps demonstrate that Cr-spinels in these
iv
soils undergo incongruent dissolution progressively enriching the spinel toward a Cr-
enriched end-member (FeCr2O4) Chromium occurs in the trivalent state in both the rock
and soil samples The apparent resistance of Cr-spinels to weathering evident from
extraction experiments suggests that these minerals are not large inputs for Cr in soil
solutions and vegetation associated with serpentine soils Chromium-bearing igneous
and metamorphic silicates in the protolith and Cr-bearing clay minerals in the soil are
more likely sources of chemically mobile and bioavailable Cr
Despite the resistance of chromite to weathering Cr release and oxidation from
chromite is a potential environmental hazard in sediments and soils and a pathway for
soil development (weathering of primary minerals) related to ultramafic rocks and their
metamorphic derivatives (serpentinites) Birnessite is a common pedologic mineral in
these sediments and soils capable of oxidizing aqueous Cr(III) The interaction between
chromite ((Fe046Mg052Mn002)(Cr061Al029Fe010)2O4) and birnessite was investigated with
an interest in the potential generation of Cr(VI) The rate of Cr(VI) in solution increases
with increasing chromite suspension densities and decreasing pH but is independent of
birnessite suspension densities at values greater than ~20 m2 L-1 at 25degC The overall rate
expression of Cr released and oxidized in solution from chromite can be expressed as
d[Cr(VI))]dt = krsquo sdotChromite069
where krsquo is 00016sdotpH2 ndash 0025sdotpH + 0097 microM L m-2 h-1 Chromite is the chromite
suspension density in m2 L-1 birnessite suspension densities are greater than 20 m2 L-1
and pH values are between 3 to 8 Experiments involving serpentine soils containing
chromite and birnessite produce Cr(VI) rates consistent with those predicted by the
overall rate expression--rates ranging from 9times10-4 to 44times10-3 microM Cr(VI) h-1 These
experiments therefore demonstrate that serpentine soils are a potential source of non-
anthropogenic Cr(VI)
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
v
chemistry of Cr in serpentine soils including its protolith is reviewed with a focus on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils have widely varying pH contain a variety Fe(III) oxides (magnetite and
hematite) phyllosilicates (serpentine and chlorite) and clays (smectites and
vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1) and
Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Additionally Cr(III) is the only valence state
observed in the serpentine soil solids however Cr(VI) has been identified in New
Caledonia and California serpentine soil solutions at concentrations below 30 microM The
enrichment and range of Cr concentrations in serpentine soils are directly related to the
presence of Cr-spinels specifically chromite and Cr-magnetite These spinels are
resistant to weathering and are preserved in the soil environment however oxidation of
Cr(III) from Cr-spinels by high valent Mn oxides or other strong oxidants is a potential
source of Cr(VI) identified in serpentine soil solutions Due to the chemically and
physically resistant nature of the Cr-spinels Cr-bearing silicates including clay minerals
Cr-chlorite Cr-garnet Cr-mica and Cr-epidote are more viable sources of Cr identified
in vegetation soil extractions soil solutions and related waters
vi
PREFACE
The chemistry and mineralogy of Cr related to serpentinites and serpentine soils
are assessed in this dissertation using a multidisciplinary field analytical experimental
and theoretical approach The four chapters that constitute this dissertation describe my
investigations related to these rocks and soils Additionally the common theme shared
by these chapters is that they all examine the Cr mineralogy in serpentinites and
serpentine soils and the potential of Cr to be mobilized and oxidized in near-surface
environments Chromite a Cr-bearing spinel is the dominant source of Cr in
serpentinite and serpentine soils Although chromite is resistant to weathering the
presence of birnessite a high valent Mn oxide will increase the release of Cr and the
production of Cr(VI) from chromite over a range of pH Chromium-clinochlore or
kaumlmmererite contains Cr concentrations less than 3 Wt however the rate at which Cr-
clinochlore weathers compared to chromite makes this mineral a more probable source of
Cr released into soil solutions These types of concerns are addressed in this thesis Brief
descriptions of the four individual dissertation chapters including their publication status
and a list of the co-authors are presented in the following paragraphs
Chapter 1 presents the main objectives of this dissertation The chemistry
regulations and toxicity of Cr are discussed as well as the general characteristics of
serpentinites and serpentine soils Additionally a brief summary of each chapter is
provided
Chapter 2 focuses on serpentinites and serpentine soils at Jasper Ridge Biological
Preserve Stanford CA The results have been submitted for publication in an article
entitled ldquoChromium geochemistry of serpentinized ultramafic rocks and serpentine soils
in the Franciscan Complex of Californiardquo to the American Journal of Science (March
2003) Scott Fendorf Dennis Bird and Robert Coleman are the second third and fourth
co-authors respectively
Chapter 3 is an experimental study evaluating the release of Cr and the
production of Cr(VI) from chromite in the presence of birnessite over a range of pH The
results for this study will be submitted for publication in an article entitled ldquoOxidative
promoted dissolution of chromite by manganese dioxide and concurrent production of
vii
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
119
magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
124
125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C
DISCUSSION
Chromium Spinel in Soils and Rocks
CONCLUDING STATEMENT
ACKNOWLEDGEMENTS
CHAP4Tablepdf
Ulvoumlspinel
copy Copyright by Christopher John-Paul Oze 2003
All Rights Reserved
ii
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Dennis K Bird Principal Adviser
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Scott Fendorf Principal Adviser
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Robert Coleman
Approved for the University Committee on Graduate Studies
iii
ABSTRACT
Chromium persists in the environment as Cr(III) a non-hazardous species and a
micronutrient in human nutrition or as Cr(VI) a strong oxidizing agent a toxin to living
cells and a Class A human carcinogen by inhalation Serpentinites or metamorphosed
ultramafic rocks are distributed worldwide and contain Cr concentrations typically
greater than 200 mg kg-1 Additionally serpentine soils a generic term used to describe
any soil derived from serpentinite regardless of its physical or chemical properties
contain elevated Cr concentrations often exceeding their corresponding protolith
Chromium in serpentinites and serpentine soils is dominantly in the form of Cr(III)
however geochemical interactions during serpentinite weathering and soil formation
provide oxidative pathways allowing the formation of Cr(VI) The relationship between
rock and soil and the processes governing the Cr chemistry are difficult to assess due to
the inherent complexity of evaluating and correlating the mineralogy and geochemistry of
both systems Ultimately a multidisciplinary approach combining field analytical and
laboratory studies is necessary to evaluate the Cr geochemistry related to these rocks and
soils and to determine whether or not serpentinites and serpentine soils are sources of
non-anthropogenic Cr contamination
Weathering of ultramafic rocks and serpentinites in the Franciscan Complex of
California produces serpentine soils containing high concentrations of Cr as well as other
potentially toxic elements including Ni Co and Mn Chromium concentrations in
serpentine soils from Jasper Ridge Biological Preserve in the Central Coast Range are as
high as 4760 mg kg-1 nearly three times greater than the serpentinite protolith (~1800
mg kg-1) Chromium-containing minerals within the bedrock include chlorite (~03 Cr
fluctuate between 1725 to 4760 mg kg-1 and do not correspond to variations in soil pH
organic matter or electrical conductivity The enrichment and variability of soil Cr is
directly related to the modal abundance and weathering of chromite Cr-magnetite and
the spinel-silicate mixture By comparison Cr silicates account for lt10 of the total soil
Cr Chemical analyses and X-ray microprobe maps demonstrate that Cr-spinels in these
iv
soils undergo incongruent dissolution progressively enriching the spinel toward a Cr-
enriched end-member (FeCr2O4) Chromium occurs in the trivalent state in both the rock
and soil samples The apparent resistance of Cr-spinels to weathering evident from
extraction experiments suggests that these minerals are not large inputs for Cr in soil
solutions and vegetation associated with serpentine soils Chromium-bearing igneous
and metamorphic silicates in the protolith and Cr-bearing clay minerals in the soil are
more likely sources of chemically mobile and bioavailable Cr
Despite the resistance of chromite to weathering Cr release and oxidation from
chromite is a potential environmental hazard in sediments and soils and a pathway for
soil development (weathering of primary minerals) related to ultramafic rocks and their
metamorphic derivatives (serpentinites) Birnessite is a common pedologic mineral in
these sediments and soils capable of oxidizing aqueous Cr(III) The interaction between
chromite ((Fe046Mg052Mn002)(Cr061Al029Fe010)2O4) and birnessite was investigated with
an interest in the potential generation of Cr(VI) The rate of Cr(VI) in solution increases
with increasing chromite suspension densities and decreasing pH but is independent of
birnessite suspension densities at values greater than ~20 m2 L-1 at 25degC The overall rate
expression of Cr released and oxidized in solution from chromite can be expressed as
d[Cr(VI))]dt = krsquo sdotChromite069
where krsquo is 00016sdotpH2 ndash 0025sdotpH + 0097 microM L m-2 h-1 Chromite is the chromite
suspension density in m2 L-1 birnessite suspension densities are greater than 20 m2 L-1
and pH values are between 3 to 8 Experiments involving serpentine soils containing
chromite and birnessite produce Cr(VI) rates consistent with those predicted by the
overall rate expression--rates ranging from 9times10-4 to 44times10-3 microM Cr(VI) h-1 These
experiments therefore demonstrate that serpentine soils are a potential source of non-
anthropogenic Cr(VI)
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
v
chemistry of Cr in serpentine soils including its protolith is reviewed with a focus on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils have widely varying pH contain a variety Fe(III) oxides (magnetite and
hematite) phyllosilicates (serpentine and chlorite) and clays (smectites and
vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1) and
Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Additionally Cr(III) is the only valence state
observed in the serpentine soil solids however Cr(VI) has been identified in New
Caledonia and California serpentine soil solutions at concentrations below 30 microM The
enrichment and range of Cr concentrations in serpentine soils are directly related to the
presence of Cr-spinels specifically chromite and Cr-magnetite These spinels are
resistant to weathering and are preserved in the soil environment however oxidation of
Cr(III) from Cr-spinels by high valent Mn oxides or other strong oxidants is a potential
source of Cr(VI) identified in serpentine soil solutions Due to the chemically and
physically resistant nature of the Cr-spinels Cr-bearing silicates including clay minerals
Cr-chlorite Cr-garnet Cr-mica and Cr-epidote are more viable sources of Cr identified
in vegetation soil extractions soil solutions and related waters
vi
PREFACE
The chemistry and mineralogy of Cr related to serpentinites and serpentine soils
are assessed in this dissertation using a multidisciplinary field analytical experimental
and theoretical approach The four chapters that constitute this dissertation describe my
investigations related to these rocks and soils Additionally the common theme shared
by these chapters is that they all examine the Cr mineralogy in serpentinites and
serpentine soils and the potential of Cr to be mobilized and oxidized in near-surface
environments Chromite a Cr-bearing spinel is the dominant source of Cr in
serpentinite and serpentine soils Although chromite is resistant to weathering the
presence of birnessite a high valent Mn oxide will increase the release of Cr and the
production of Cr(VI) from chromite over a range of pH Chromium-clinochlore or
kaumlmmererite contains Cr concentrations less than 3 Wt however the rate at which Cr-
clinochlore weathers compared to chromite makes this mineral a more probable source of
Cr released into soil solutions These types of concerns are addressed in this thesis Brief
descriptions of the four individual dissertation chapters including their publication status
and a list of the co-authors are presented in the following paragraphs
Chapter 1 presents the main objectives of this dissertation The chemistry
regulations and toxicity of Cr are discussed as well as the general characteristics of
serpentinites and serpentine soils Additionally a brief summary of each chapter is
provided
Chapter 2 focuses on serpentinites and serpentine soils at Jasper Ridge Biological
Preserve Stanford CA The results have been submitted for publication in an article
entitled ldquoChromium geochemistry of serpentinized ultramafic rocks and serpentine soils
in the Franciscan Complex of Californiardquo to the American Journal of Science (March
2003) Scott Fendorf Dennis Bird and Robert Coleman are the second third and fourth
co-authors respectively
Chapter 3 is an experimental study evaluating the release of Cr and the
production of Cr(VI) from chromite in the presence of birnessite over a range of pH The
results for this study will be submitted for publication in an article entitled ldquoOxidative
promoted dissolution of chromite by manganese dioxide and concurrent production of
vii
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
119
magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
124
125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C
DISCUSSION
Chromium Spinel in Soils and Rocks
CONCLUDING STATEMENT
ACKNOWLEDGEMENTS
CHAP4Tablepdf
Ulvoumlspinel
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Dennis K Bird Principal Adviser
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Scott Fendorf Principal Adviser
I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy
____________________________ Robert Coleman
Approved for the University Committee on Graduate Studies
iii
ABSTRACT
Chromium persists in the environment as Cr(III) a non-hazardous species and a
micronutrient in human nutrition or as Cr(VI) a strong oxidizing agent a toxin to living
cells and a Class A human carcinogen by inhalation Serpentinites or metamorphosed
ultramafic rocks are distributed worldwide and contain Cr concentrations typically
greater than 200 mg kg-1 Additionally serpentine soils a generic term used to describe
any soil derived from serpentinite regardless of its physical or chemical properties
contain elevated Cr concentrations often exceeding their corresponding protolith
Chromium in serpentinites and serpentine soils is dominantly in the form of Cr(III)
however geochemical interactions during serpentinite weathering and soil formation
provide oxidative pathways allowing the formation of Cr(VI) The relationship between
rock and soil and the processes governing the Cr chemistry are difficult to assess due to
the inherent complexity of evaluating and correlating the mineralogy and geochemistry of
both systems Ultimately a multidisciplinary approach combining field analytical and
laboratory studies is necessary to evaluate the Cr geochemistry related to these rocks and
soils and to determine whether or not serpentinites and serpentine soils are sources of
non-anthropogenic Cr contamination
Weathering of ultramafic rocks and serpentinites in the Franciscan Complex of
California produces serpentine soils containing high concentrations of Cr as well as other
potentially toxic elements including Ni Co and Mn Chromium concentrations in
serpentine soils from Jasper Ridge Biological Preserve in the Central Coast Range are as
high as 4760 mg kg-1 nearly three times greater than the serpentinite protolith (~1800
mg kg-1) Chromium-containing minerals within the bedrock include chlorite (~03 Cr
fluctuate between 1725 to 4760 mg kg-1 and do not correspond to variations in soil pH
organic matter or electrical conductivity The enrichment and variability of soil Cr is
directly related to the modal abundance and weathering of chromite Cr-magnetite and
the spinel-silicate mixture By comparison Cr silicates account for lt10 of the total soil
Cr Chemical analyses and X-ray microprobe maps demonstrate that Cr-spinels in these
iv
soils undergo incongruent dissolution progressively enriching the spinel toward a Cr-
enriched end-member (FeCr2O4) Chromium occurs in the trivalent state in both the rock
and soil samples The apparent resistance of Cr-spinels to weathering evident from
extraction experiments suggests that these minerals are not large inputs for Cr in soil
solutions and vegetation associated with serpentine soils Chromium-bearing igneous
and metamorphic silicates in the protolith and Cr-bearing clay minerals in the soil are
more likely sources of chemically mobile and bioavailable Cr
Despite the resistance of chromite to weathering Cr release and oxidation from
chromite is a potential environmental hazard in sediments and soils and a pathway for
soil development (weathering of primary minerals) related to ultramafic rocks and their
metamorphic derivatives (serpentinites) Birnessite is a common pedologic mineral in
these sediments and soils capable of oxidizing aqueous Cr(III) The interaction between
chromite ((Fe046Mg052Mn002)(Cr061Al029Fe010)2O4) and birnessite was investigated with
an interest in the potential generation of Cr(VI) The rate of Cr(VI) in solution increases
with increasing chromite suspension densities and decreasing pH but is independent of
birnessite suspension densities at values greater than ~20 m2 L-1 at 25degC The overall rate
expression of Cr released and oxidized in solution from chromite can be expressed as
d[Cr(VI))]dt = krsquo sdotChromite069
where krsquo is 00016sdotpH2 ndash 0025sdotpH + 0097 microM L m-2 h-1 Chromite is the chromite
suspension density in m2 L-1 birnessite suspension densities are greater than 20 m2 L-1
and pH values are between 3 to 8 Experiments involving serpentine soils containing
chromite and birnessite produce Cr(VI) rates consistent with those predicted by the
overall rate expression--rates ranging from 9times10-4 to 44times10-3 microM Cr(VI) h-1 These
experiments therefore demonstrate that serpentine soils are a potential source of non-
anthropogenic Cr(VI)
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
v
chemistry of Cr in serpentine soils including its protolith is reviewed with a focus on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils have widely varying pH contain a variety Fe(III) oxides (magnetite and
hematite) phyllosilicates (serpentine and chlorite) and clays (smectites and
vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1) and
Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Additionally Cr(III) is the only valence state
observed in the serpentine soil solids however Cr(VI) has been identified in New
Caledonia and California serpentine soil solutions at concentrations below 30 microM The
enrichment and range of Cr concentrations in serpentine soils are directly related to the
presence of Cr-spinels specifically chromite and Cr-magnetite These spinels are
resistant to weathering and are preserved in the soil environment however oxidation of
Cr(III) from Cr-spinels by high valent Mn oxides or other strong oxidants is a potential
source of Cr(VI) identified in serpentine soil solutions Due to the chemically and
physically resistant nature of the Cr-spinels Cr-bearing silicates including clay minerals
Cr-chlorite Cr-garnet Cr-mica and Cr-epidote are more viable sources of Cr identified
in vegetation soil extractions soil solutions and related waters
vi
PREFACE
The chemistry and mineralogy of Cr related to serpentinites and serpentine soils
are assessed in this dissertation using a multidisciplinary field analytical experimental
and theoretical approach The four chapters that constitute this dissertation describe my
investigations related to these rocks and soils Additionally the common theme shared
by these chapters is that they all examine the Cr mineralogy in serpentinites and
serpentine soils and the potential of Cr to be mobilized and oxidized in near-surface
environments Chromite a Cr-bearing spinel is the dominant source of Cr in
serpentinite and serpentine soils Although chromite is resistant to weathering the
presence of birnessite a high valent Mn oxide will increase the release of Cr and the
production of Cr(VI) from chromite over a range of pH Chromium-clinochlore or
kaumlmmererite contains Cr concentrations less than 3 Wt however the rate at which Cr-
clinochlore weathers compared to chromite makes this mineral a more probable source of
Cr released into soil solutions These types of concerns are addressed in this thesis Brief
descriptions of the four individual dissertation chapters including their publication status
and a list of the co-authors are presented in the following paragraphs
Chapter 1 presents the main objectives of this dissertation The chemistry
regulations and toxicity of Cr are discussed as well as the general characteristics of
serpentinites and serpentine soils Additionally a brief summary of each chapter is
provided
Chapter 2 focuses on serpentinites and serpentine soils at Jasper Ridge Biological
Preserve Stanford CA The results have been submitted for publication in an article
entitled ldquoChromium geochemistry of serpentinized ultramafic rocks and serpentine soils
in the Franciscan Complex of Californiardquo to the American Journal of Science (March
2003) Scott Fendorf Dennis Bird and Robert Coleman are the second third and fourth
co-authors respectively
Chapter 3 is an experimental study evaluating the release of Cr and the
production of Cr(VI) from chromite in the presence of birnessite over a range of pH The
results for this study will be submitted for publication in an article entitled ldquoOxidative
promoted dissolution of chromite by manganese dioxide and concurrent production of
vii
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
119
magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
124
125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C
DISCUSSION
Chromium Spinel in Soils and Rocks
CONCLUDING STATEMENT
ACKNOWLEDGEMENTS
CHAP4Tablepdf
Ulvoumlspinel
ABSTRACT
Chromium persists in the environment as Cr(III) a non-hazardous species and a
micronutrient in human nutrition or as Cr(VI) a strong oxidizing agent a toxin to living
cells and a Class A human carcinogen by inhalation Serpentinites or metamorphosed
ultramafic rocks are distributed worldwide and contain Cr concentrations typically
greater than 200 mg kg-1 Additionally serpentine soils a generic term used to describe
any soil derived from serpentinite regardless of its physical or chemical properties
contain elevated Cr concentrations often exceeding their corresponding protolith
Chromium in serpentinites and serpentine soils is dominantly in the form of Cr(III)
however geochemical interactions during serpentinite weathering and soil formation
provide oxidative pathways allowing the formation of Cr(VI) The relationship between
rock and soil and the processes governing the Cr chemistry are difficult to assess due to
the inherent complexity of evaluating and correlating the mineralogy and geochemistry of
both systems Ultimately a multidisciplinary approach combining field analytical and
laboratory studies is necessary to evaluate the Cr geochemistry related to these rocks and
soils and to determine whether or not serpentinites and serpentine soils are sources of
non-anthropogenic Cr contamination
Weathering of ultramafic rocks and serpentinites in the Franciscan Complex of
California produces serpentine soils containing high concentrations of Cr as well as other
potentially toxic elements including Ni Co and Mn Chromium concentrations in
serpentine soils from Jasper Ridge Biological Preserve in the Central Coast Range are as
high as 4760 mg kg-1 nearly three times greater than the serpentinite protolith (~1800
mg kg-1) Chromium-containing minerals within the bedrock include chlorite (~03 Cr
fluctuate between 1725 to 4760 mg kg-1 and do not correspond to variations in soil pH
organic matter or electrical conductivity The enrichment and variability of soil Cr is
directly related to the modal abundance and weathering of chromite Cr-magnetite and
the spinel-silicate mixture By comparison Cr silicates account for lt10 of the total soil
Cr Chemical analyses and X-ray microprobe maps demonstrate that Cr-spinels in these
iv
soils undergo incongruent dissolution progressively enriching the spinel toward a Cr-
enriched end-member (FeCr2O4) Chromium occurs in the trivalent state in both the rock
and soil samples The apparent resistance of Cr-spinels to weathering evident from
extraction experiments suggests that these minerals are not large inputs for Cr in soil
solutions and vegetation associated with serpentine soils Chromium-bearing igneous
and metamorphic silicates in the protolith and Cr-bearing clay minerals in the soil are
more likely sources of chemically mobile and bioavailable Cr
Despite the resistance of chromite to weathering Cr release and oxidation from
chromite is a potential environmental hazard in sediments and soils and a pathway for
soil development (weathering of primary minerals) related to ultramafic rocks and their
metamorphic derivatives (serpentinites) Birnessite is a common pedologic mineral in
these sediments and soils capable of oxidizing aqueous Cr(III) The interaction between
chromite ((Fe046Mg052Mn002)(Cr061Al029Fe010)2O4) and birnessite was investigated with
an interest in the potential generation of Cr(VI) The rate of Cr(VI) in solution increases
with increasing chromite suspension densities and decreasing pH but is independent of
birnessite suspension densities at values greater than ~20 m2 L-1 at 25degC The overall rate
expression of Cr released and oxidized in solution from chromite can be expressed as
d[Cr(VI))]dt = krsquo sdotChromite069
where krsquo is 00016sdotpH2 ndash 0025sdotpH + 0097 microM L m-2 h-1 Chromite is the chromite
suspension density in m2 L-1 birnessite suspension densities are greater than 20 m2 L-1
and pH values are between 3 to 8 Experiments involving serpentine soils containing
chromite and birnessite produce Cr(VI) rates consistent with those predicted by the
overall rate expression--rates ranging from 9times10-4 to 44times10-3 microM Cr(VI) h-1 These
experiments therefore demonstrate that serpentine soils are a potential source of non-
anthropogenic Cr(VI)
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
v
chemistry of Cr in serpentine soils including its protolith is reviewed with a focus on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils have widely varying pH contain a variety Fe(III) oxides (magnetite and
hematite) phyllosilicates (serpentine and chlorite) and clays (smectites and
vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1) and
Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Additionally Cr(III) is the only valence state
observed in the serpentine soil solids however Cr(VI) has been identified in New
Caledonia and California serpentine soil solutions at concentrations below 30 microM The
enrichment and range of Cr concentrations in serpentine soils are directly related to the
presence of Cr-spinels specifically chromite and Cr-magnetite These spinels are
resistant to weathering and are preserved in the soil environment however oxidation of
Cr(III) from Cr-spinels by high valent Mn oxides or other strong oxidants is a potential
source of Cr(VI) identified in serpentine soil solutions Due to the chemically and
physically resistant nature of the Cr-spinels Cr-bearing silicates including clay minerals
Cr-chlorite Cr-garnet Cr-mica and Cr-epidote are more viable sources of Cr identified
in vegetation soil extractions soil solutions and related waters
vi
PREFACE
The chemistry and mineralogy of Cr related to serpentinites and serpentine soils
are assessed in this dissertation using a multidisciplinary field analytical experimental
and theoretical approach The four chapters that constitute this dissertation describe my
investigations related to these rocks and soils Additionally the common theme shared
by these chapters is that they all examine the Cr mineralogy in serpentinites and
serpentine soils and the potential of Cr to be mobilized and oxidized in near-surface
environments Chromite a Cr-bearing spinel is the dominant source of Cr in
serpentinite and serpentine soils Although chromite is resistant to weathering the
presence of birnessite a high valent Mn oxide will increase the release of Cr and the
production of Cr(VI) from chromite over a range of pH Chromium-clinochlore or
kaumlmmererite contains Cr concentrations less than 3 Wt however the rate at which Cr-
clinochlore weathers compared to chromite makes this mineral a more probable source of
Cr released into soil solutions These types of concerns are addressed in this thesis Brief
descriptions of the four individual dissertation chapters including their publication status
and a list of the co-authors are presented in the following paragraphs
Chapter 1 presents the main objectives of this dissertation The chemistry
regulations and toxicity of Cr are discussed as well as the general characteristics of
serpentinites and serpentine soils Additionally a brief summary of each chapter is
provided
Chapter 2 focuses on serpentinites and serpentine soils at Jasper Ridge Biological
Preserve Stanford CA The results have been submitted for publication in an article
entitled ldquoChromium geochemistry of serpentinized ultramafic rocks and serpentine soils
in the Franciscan Complex of Californiardquo to the American Journal of Science (March
2003) Scott Fendorf Dennis Bird and Robert Coleman are the second third and fourth
co-authors respectively
Chapter 3 is an experimental study evaluating the release of Cr and the
production of Cr(VI) from chromite in the presence of birnessite over a range of pH The
results for this study will be submitted for publication in an article entitled ldquoOxidative
promoted dissolution of chromite by manganese dioxide and concurrent production of
vii
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
119
magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
124
125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C
DISCUSSION
Chromium Spinel in Soils and Rocks
CONCLUDING STATEMENT
ACKNOWLEDGEMENTS
CHAP4Tablepdf
Ulvoumlspinel
soils undergo incongruent dissolution progressively enriching the spinel toward a Cr-
enriched end-member (FeCr2O4) Chromium occurs in the trivalent state in both the rock
and soil samples The apparent resistance of Cr-spinels to weathering evident from
extraction experiments suggests that these minerals are not large inputs for Cr in soil
solutions and vegetation associated with serpentine soils Chromium-bearing igneous
and metamorphic silicates in the protolith and Cr-bearing clay minerals in the soil are
more likely sources of chemically mobile and bioavailable Cr
Despite the resistance of chromite to weathering Cr release and oxidation from
chromite is a potential environmental hazard in sediments and soils and a pathway for
soil development (weathering of primary minerals) related to ultramafic rocks and their
metamorphic derivatives (serpentinites) Birnessite is a common pedologic mineral in
these sediments and soils capable of oxidizing aqueous Cr(III) The interaction between
chromite ((Fe046Mg052Mn002)(Cr061Al029Fe010)2O4) and birnessite was investigated with
an interest in the potential generation of Cr(VI) The rate of Cr(VI) in solution increases
with increasing chromite suspension densities and decreasing pH but is independent of
birnessite suspension densities at values greater than ~20 m2 L-1 at 25degC The overall rate
expression of Cr released and oxidized in solution from chromite can be expressed as
d[Cr(VI))]dt = krsquo sdotChromite069
where krsquo is 00016sdotpH2 ndash 0025sdotpH + 0097 microM L m-2 h-1 Chromite is the chromite
suspension density in m2 L-1 birnessite suspension densities are greater than 20 m2 L-1
and pH values are between 3 to 8 Experiments involving serpentine soils containing
chromite and birnessite produce Cr(VI) rates consistent with those predicted by the
overall rate expression--rates ranging from 9times10-4 to 44times10-3 microM Cr(VI) h-1 These
experiments therefore demonstrate that serpentine soils are a potential source of non-
anthropogenic Cr(VI)
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
v
chemistry of Cr in serpentine soils including its protolith is reviewed with a focus on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils have widely varying pH contain a variety Fe(III) oxides (magnetite and
hematite) phyllosilicates (serpentine and chlorite) and clays (smectites and
vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1) and
Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Additionally Cr(III) is the only valence state
observed in the serpentine soil solids however Cr(VI) has been identified in New
Caledonia and California serpentine soil solutions at concentrations below 30 microM The
enrichment and range of Cr concentrations in serpentine soils are directly related to the
presence of Cr-spinels specifically chromite and Cr-magnetite These spinels are
resistant to weathering and are preserved in the soil environment however oxidation of
Cr(III) from Cr-spinels by high valent Mn oxides or other strong oxidants is a potential
source of Cr(VI) identified in serpentine soil solutions Due to the chemically and
physically resistant nature of the Cr-spinels Cr-bearing silicates including clay minerals
Cr-chlorite Cr-garnet Cr-mica and Cr-epidote are more viable sources of Cr identified
in vegetation soil extractions soil solutions and related waters
vi
PREFACE
The chemistry and mineralogy of Cr related to serpentinites and serpentine soils
are assessed in this dissertation using a multidisciplinary field analytical experimental
and theoretical approach The four chapters that constitute this dissertation describe my
investigations related to these rocks and soils Additionally the common theme shared
by these chapters is that they all examine the Cr mineralogy in serpentinites and
serpentine soils and the potential of Cr to be mobilized and oxidized in near-surface
environments Chromite a Cr-bearing spinel is the dominant source of Cr in
serpentinite and serpentine soils Although chromite is resistant to weathering the
presence of birnessite a high valent Mn oxide will increase the release of Cr and the
production of Cr(VI) from chromite over a range of pH Chromium-clinochlore or
kaumlmmererite contains Cr concentrations less than 3 Wt however the rate at which Cr-
clinochlore weathers compared to chromite makes this mineral a more probable source of
Cr released into soil solutions These types of concerns are addressed in this thesis Brief
descriptions of the four individual dissertation chapters including their publication status
and a list of the co-authors are presented in the following paragraphs
Chapter 1 presents the main objectives of this dissertation The chemistry
regulations and toxicity of Cr are discussed as well as the general characteristics of
serpentinites and serpentine soils Additionally a brief summary of each chapter is
provided
Chapter 2 focuses on serpentinites and serpentine soils at Jasper Ridge Biological
Preserve Stanford CA The results have been submitted for publication in an article
entitled ldquoChromium geochemistry of serpentinized ultramafic rocks and serpentine soils
in the Franciscan Complex of Californiardquo to the American Journal of Science (March
2003) Scott Fendorf Dennis Bird and Robert Coleman are the second third and fourth
co-authors respectively
Chapter 3 is an experimental study evaluating the release of Cr and the
production of Cr(VI) from chromite in the presence of birnessite over a range of pH The
results for this study will be submitted for publication in an article entitled ldquoOxidative
promoted dissolution of chromite by manganese dioxide and concurrent production of
vii
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
119
magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
124
125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C
DISCUSSION
Chromium Spinel in Soils and Rocks
CONCLUDING STATEMENT
ACKNOWLEDGEMENTS
CHAP4Tablepdf
Ulvoumlspinel
chemistry of Cr in serpentine soils including its protolith is reviewed with a focus on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils have widely varying pH contain a variety Fe(III) oxides (magnetite and
hematite) phyllosilicates (serpentine and chlorite) and clays (smectites and
vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1) and
Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Additionally Cr(III) is the only valence state
observed in the serpentine soil solids however Cr(VI) has been identified in New
Caledonia and California serpentine soil solutions at concentrations below 30 microM The
enrichment and range of Cr concentrations in serpentine soils are directly related to the
presence of Cr-spinels specifically chromite and Cr-magnetite These spinels are
resistant to weathering and are preserved in the soil environment however oxidation of
Cr(III) from Cr-spinels by high valent Mn oxides or other strong oxidants is a potential
source of Cr(VI) identified in serpentine soil solutions Due to the chemically and
physically resistant nature of the Cr-spinels Cr-bearing silicates including clay minerals
Cr-chlorite Cr-garnet Cr-mica and Cr-epidote are more viable sources of Cr identified
in vegetation soil extractions soil solutions and related waters
vi
PREFACE
The chemistry and mineralogy of Cr related to serpentinites and serpentine soils
are assessed in this dissertation using a multidisciplinary field analytical experimental
and theoretical approach The four chapters that constitute this dissertation describe my
investigations related to these rocks and soils Additionally the common theme shared
by these chapters is that they all examine the Cr mineralogy in serpentinites and
serpentine soils and the potential of Cr to be mobilized and oxidized in near-surface
environments Chromite a Cr-bearing spinel is the dominant source of Cr in
serpentinite and serpentine soils Although chromite is resistant to weathering the
presence of birnessite a high valent Mn oxide will increase the release of Cr and the
production of Cr(VI) from chromite over a range of pH Chromium-clinochlore or
kaumlmmererite contains Cr concentrations less than 3 Wt however the rate at which Cr-
clinochlore weathers compared to chromite makes this mineral a more probable source of
Cr released into soil solutions These types of concerns are addressed in this thesis Brief
descriptions of the four individual dissertation chapters including their publication status
and a list of the co-authors are presented in the following paragraphs
Chapter 1 presents the main objectives of this dissertation The chemistry
regulations and toxicity of Cr are discussed as well as the general characteristics of
serpentinites and serpentine soils Additionally a brief summary of each chapter is
provided
Chapter 2 focuses on serpentinites and serpentine soils at Jasper Ridge Biological
Preserve Stanford CA The results have been submitted for publication in an article
entitled ldquoChromium geochemistry of serpentinized ultramafic rocks and serpentine soils
in the Franciscan Complex of Californiardquo to the American Journal of Science (March
2003) Scott Fendorf Dennis Bird and Robert Coleman are the second third and fourth
co-authors respectively
Chapter 3 is an experimental study evaluating the release of Cr and the
production of Cr(VI) from chromite in the presence of birnessite over a range of pH The
results for this study will be submitted for publication in an article entitled ldquoOxidative
promoted dissolution of chromite by manganese dioxide and concurrent production of
vii
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
119
magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
124
125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C
DISCUSSION
Chromium Spinel in Soils and Rocks
CONCLUDING STATEMENT
ACKNOWLEDGEMENTS
CHAP4Tablepdf
Ulvoumlspinel
PREFACE
The chemistry and mineralogy of Cr related to serpentinites and serpentine soils
are assessed in this dissertation using a multidisciplinary field analytical experimental
and theoretical approach The four chapters that constitute this dissertation describe my
investigations related to these rocks and soils Additionally the common theme shared
by these chapters is that they all examine the Cr mineralogy in serpentinites and
serpentine soils and the potential of Cr to be mobilized and oxidized in near-surface
environments Chromite a Cr-bearing spinel is the dominant source of Cr in
serpentinite and serpentine soils Although chromite is resistant to weathering the
presence of birnessite a high valent Mn oxide will increase the release of Cr and the
production of Cr(VI) from chromite over a range of pH Chromium-clinochlore or
kaumlmmererite contains Cr concentrations less than 3 Wt however the rate at which Cr-
clinochlore weathers compared to chromite makes this mineral a more probable source of
Cr released into soil solutions These types of concerns are addressed in this thesis Brief
descriptions of the four individual dissertation chapters including their publication status
and a list of the co-authors are presented in the following paragraphs
Chapter 1 presents the main objectives of this dissertation The chemistry
regulations and toxicity of Cr are discussed as well as the general characteristics of
serpentinites and serpentine soils Additionally a brief summary of each chapter is
provided
Chapter 2 focuses on serpentinites and serpentine soils at Jasper Ridge Biological
Preserve Stanford CA The results have been submitted for publication in an article
entitled ldquoChromium geochemistry of serpentinized ultramafic rocks and serpentine soils
in the Franciscan Complex of Californiardquo to the American Journal of Science (March
2003) Scott Fendorf Dennis Bird and Robert Coleman are the second third and fourth
co-authors respectively
Chapter 3 is an experimental study evaluating the release of Cr and the
production of Cr(VI) from chromite in the presence of birnessite over a range of pH The
results for this study will be submitted for publication in an article entitled ldquoOxidative
promoted dissolution of chromite by manganese dioxide and concurrent production of
vii
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
119
magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
124
125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C
DISCUSSION
Chromium Spinel in Soils and Rocks
CONCLUDING STATEMENT
ACKNOWLEDGEMENTS
CHAP4Tablepdf
Ulvoumlspinel
chromaterdquo to Environmental Science and Technology The co-authors for this paper are
Scott Fendorf and Dennis Bird
Chapter 4 is a review paper that critically evaluates the chemistry of serpentine
soils and serpentinites primarily focusing on Cr geochemistry Additionally analytical
studies of serpentine soils collected from California Oregon and New Caledonia are
presented This study will be presented at the Robert Coleman Symposium and has been
submitted as an article entitled ldquoChromium geochemistry of serpentine soilsrdquo for an
International Geology Review Special Publication Scott Fendorf Dennis Bird and
Robert Coleman are the second third and fourth co-authors respectively
Appendix A is a preliminary investigation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite) a mineral common in serpentinites and present in serpentine
soils This appendix is entitled ldquoEstimation of the thermodynamic properties of Cr-
clinochlore (kaumlmmererite)rdquo The results of this chapter will be included in a study that
estimates the thermodynamic properties of Cr-bearing silicates including fuchsite (Cr-
muscovite) tawmawite (Cr-epidote) and uvarovite (Cr-garnet)
viii
ACKNOWLEDGEMENTS
Serpentinites serpentine soils and chromium are the Geologic Trinity of this
dissertationhellipeach topic is intimately related to one another The scope and breadth of
this research required a significant amount of time and funding None of this research
would have been possible without the generous support of several foundations
organizations and Stanford University I am grateful for the financial aid I have
received from the Stanford Fellowship the Stanford University McGee and Shell funds
the National Science Foundation the Geologic Society of America Graduate Research
Fund the Stanford Office of Teaching and Learning grant and the Jasper Ridge
Biological Preserve Melon Grant
I would not be where am I today if it were not for my parents Robert and Sue
Oze and my brother Rob Oze Thank you for the sandbox and all the hose-driven
volcanoeshellipthose Wookies never stood a chance While some families vacation in theme
parks my family frequented gem and mineral conventions collected rocks and shells
from mist-shrouded shores and sought lucky stones including an infamous quartz orb
Sometimes I think I was destined to be a geologist I would also like to thank everyone
in my family and all our family friends who have provided that extra little push and
support along the way These people include Tudor Bennatts Louis Oze Margaret and
John Chambers John Bennatts Nancy Skiles the Izumis the memory of Witchie Poo
hijacking Harleys from Bremerton bikers and the Man in the Sound waiting for that one
last catch Close friends I would like to acknowledge who are not related to the Stanford
crowd include Dave and Leigh Anderson and Rebecca Parker These satellite friends
helped me keep in touch with the world outside of academia and proved to me that I
wasnrsquot meant to have a real job A special thank you goes to Katherine Akako
Izumihellipyou have made this whole trip worthwhile
My PhD advisors Scott Fendorf and Dennis Bird provided an invaluable amount
of help patience and energy over the past five years Scott always made the time to
discuss and refine many of my projects making them relevant to many current
geochemical issues Additionally he helped me hone my soil science skills in the field
and in the lab Out in the field Scott can dig a pretty mean hole Dennis devoted a
significant amount of time and sweat in order to make this dissertation happen His
ix
insightful knowledge of geochemistry and of the geology of the Sierra Nevadas provided
the cornerstone for this dissertation
Robert Coleman played a key role in the production of this dissertation He
always knew what to say at the right time and helped me understand the complexity of
ophiolites and serpentinites through experienced eyes Additionally Keith Loague and
Gary Ernst deserve recognition for their outstanding support as committee members for
this dissertation over the last four years I would also like to thank Jonathan Stebbins and
Gail Mahood for providing me the opportunity to attend Stanford
Bob and Vicki Jones always offered an open ear whether it was science or solving
the worldrsquos problems Rick Hazlett (Pomona College) and John Winter (Whitman
College) both had a significant impact on shaping my future science goals and objectives
during my undergrad years and both encouraged me to pursue a career in geology They
were the catalyst for this whole experiment
I owe a lot to my research group members (both the Bird Cage and Soil
Chemistry Group) and those people who didnrsquot mind me tagging along Before I
introduce these people one person has been my friend and co-conspirator for many years
at Stanford Tim Copehellipyou are a pain in my ass Besides never picking up any of your
crap and breaking all my stuff this whole grad school experience would not have been
the same without you Other friends and acquaintances that will not be forgotten in
chronological order of me walking into my office for the first time on September 11
1998 include Anne Dyreborg Esra Inan Tom Bawden Patrick Redmond Phil
Neuhoff Thrainn Fredrickson Lara Heister Kaye Savage Andrea Fildani Mike Schultz
Trayle Kushan Kevin Theissen Angela Hessler Chris Van de Ven Kono Lemke Colin
Doyle Colleen Hansel Ben Bostick Matthew LaForce Nancy Grumet Scott Kroeker
Anders Meibom Brian Ebel Matt Ginder-Vogel Matt Pollizitto Spud Amy Weislogel
Travis Horton and Jake Waldbauer I would like to end this Acknowledgement section
and begin my dissertation with a haikuhellip
Better than outcrops
Are the people Irsquove met here
Thanks for all the beer
x
TABLE OF CONTENTS
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip iv Prefacehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip vii Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip ix Table of Contentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xi List of Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xiv List of Illustrationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip xvi Chapter 1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 1
Toxicity and Regulations of Chromiumhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 2 Mineralogy of Chromium in Serpentiniteshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 3 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5 Chapter Summarieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 5
Chapter 2 Chromium Geochemistry of Serpentinized Ultramafic Rocks and Serpentine Soils in the Franciscan Complex of Californiahelliphelliphelliphelliphelliphellip 8
Geologic Background of the Serpentinite at Jasper Ridgehelliphelliphelliphellip 10 Serpentine Soils Characteristics and Reviewhelliphelliphelliphelliphelliphelliphelliphellip 11 Sources of Chromium ProtolithBedrockhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 12 Chromium-Bearing Phases in Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 13
Materials and Methodshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Location and Sample Collectionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Mineral Identification in Rocks and Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 14 Soil Chemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 15 X-ray Diffraction and Clay-Size Mineral Identificationhelliphelliphelliphelliphellip 16 X-ray Absorption Spectroscopyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17 Sequential Extraction Procedurehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 17
Resultshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Serpentinite Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 18 Distribution of Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphellip21 Serpentine Soil Chemistry and Mineralogy at Site JR3helliphelliphelliphelliphellip 22 Sequential Extractionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 26
Results helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite Extraction Experimentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 69 Chromite and Birnessite Batch Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 70 Secondary and Solid Phase Examination of Chromite and Birnessitehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 72 Serpentine Soil and Birnessite Experimentshelliphelliphelliphelliphelliphelliphelliphelliphellip 72
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 73 Chromium(VI) Pathways from Chromite and Birnessite into Solutionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 76 Serpentine Soils and Cr(VI)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 77
Chromium Geochemistryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 102 Mineralogy of Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 103 Sources of Chromium for Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphellip 105 Chromium in Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 106 Soils Solutions and Related Waterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 108 Serpentine Soil Vegetationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 110 Soil Organic Matterhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 111
California Oregon and New Caledonia Serpentine Soilshelliphelliphelliphelliphelliphelliphellip 112 Sample Locations and General Soil Characteristicshelliphelliphelliphelliphelliphellip 112 BedrockProtolith Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 113 Soil Chemistry and Physical Propertieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 114 Soil Mineralogyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116
Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 116 Chromium and Serpentine Soilshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 117
xii
Chromium Spinel in Soils and Rockshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 119 Concluding Statementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 120 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 121 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 122 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 129 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 131
Appendiceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 141 Appendix Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 142 Appendix Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 143 Appendix A Estimation of the Thermodynamic Properties of Kaumlmmererite (Cr-Clinochlore)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip145
Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 145 Nomenclature and Crystal Chemistry of Cr-chloritehelliphelliphelliphelliphelliphelliphelliphelliphellip 146 Estimation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 147
Estimation of theStandard Molar Volume Using Unit Cell Measurementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of the Standard Molar Volume Heat Capacity and Entropyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 148 Estimation of Standard Molar Enthalpy and Gibbs Free Energyhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 150 Critique and Discussion of the Estimation Algorithms and SUPCRT92helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 151
Evaluation of the Thermodynamic Properties of Kaumlmmereritehelliphelliphelliphelliphellip 152 Tableshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 155 Figure Captionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 161 Figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 162
List of References
References to Chapter 1helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 166 References to Chapter 2helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 172 References to Chapter 3helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 179 References to Chapter 4helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 183 References to Appendix Ahelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 195
xiii
LIST OF TABLES Chapter 2 Table 21 Whole rock elemental concentrations of serpentinites chert and sandstone Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in
serpentinite rock at Jasper Ridge Table 22b Selected Cr-bearing minerals and compositions in oxide Wt in
serpentine soil at Jasper Ridge Table 23 Mineral compositions in oxide Wt of minerals identified in Figures 25
and 26 Table 24 Bulk soil elemental concentrations for soils and bedrock collected along
the transect in Figure 21b Table 25 Chemical and physical properties of the soil at Site JR3 Table 26 Selected trace metal concentrations for each size fraction versus depth at
Site JR3 Table 27 X-ray diffraction analyses of the clay-sized fraction from site JR3 Table 28 X-ray diffraction d-spacings for common silicates Table 29 Results of the Diablo clay serpentine soil and chromite sequential
extraction experiments Table 210 Chromite masses required for determined Cr concentrations (mg kg-1) in 1
kg of sample Chapter 3 Table 31 Chromite and birnessite masses and concentrations used for each batch
experiment Table 32 Serpentine soil and birnessite experimental parameters Table 33 Summary of Cr(VI) rates obtained from batch experiments Table 34 Comparative analyses of batch experiment solutions and chemical
extractions of chromite and birnessite solids Chapter 4 Table 41 Measured Cr concentrations in serpentine soils worldwide Table 42 Summary of Cr Fe and Ni extractions from serpentine soils Table 43 Serpentine soil plant Cr concentrations Table 44 Chemical and textural properties of collected soils Table 45 Total major and trace element concentrations of the collected soils Table 46 Compositions for soil Cr-containing minerals identified in Figure 49 Table 47 Chromium-bearing mineral masses and volumes required to influence Cr
concentrations in soils Appendix A Table A1 Cr and Al end-members in the Cr-clinochlore solid solution
xiv
Table A2 Standard state thermodynamic properties of Cr and Al-bearing minerals Table A3 Unit cell measurements and molar volumes of clinochlore and Cr-
clinochlores Table A4 Entropy volume and heat capacity for kaumlmmererite using the Ransom and
Helgeson Method Table A5 Fictive polyhedral considered in the estimations Table A6 Estimated thermodynamic properties for kaumlmmererite
xv
LIST OF FIGURES Chapter 2 Figure 21a Soils map of Jasper Ridge Biological Preserve CA Figure 21b Serpentinite and serpentine soil sampling sites at Jasper Ridge Biological
Preserve CA Figure 22 Chromium concentrations versus depth for four serpentine soils in
Maryland California and New Foundland Figure 23 Petrographic images of serpentinite at Jasper Ridge Figure 24 Elemental concentrations relative to Cr for chlorite pyroxene magnetite
chromite and a chromite-silicate mixture in the serpentinite at Jasper Ridge
Figure 25 Backscattering electron images of minerals in the serpentinite at Jasper Ridge
Figure 26 Backscattered electron images of Cr-chlorite and Cr-magnetite assemblage in the serpentinite bedrock
Figure 27 Highest and lowest Cr concentrations at each site in the serpentine soil transect
Figure 28 A graphical representation of Cr Ni Mn and Co concentrations collected for total gt2 mm sand silt and clay size fractions
Figure 29 Correlation diagrams of Cr-bearing minerals in the serpentine soils at Jasper Ridge
Figure 210 X-ray diffraction clay analyses of sample JR3515 Figure 211 Cr K XANES spectra for soils analyzed from Jasper Ridge serpentine soil
at specific depths Figure 212 X-ray microprobe elemental maps of Cr and Fe for the serpentine soil and
serpentinite at site JR3 Figure 213 A graphical representation of the exchangeable Fe oxide and silicate
extraction experiments Figure 214 Scanning electron microprobe images of the chromite standard after each
sequential extraction experiment Figure 215 Tie-lines connecting serpentinite chromite and end-member chromite with
soil chromite values Chapter 3 Figure 31 Scanning electron microprobe images of chromite and birnessite standards Figure 32 Chromite extraction experiments evaluating Cr Fe and Al Figure 33 pH and Eh versus time (hours) for each chromite-birnessite chromite and
birnessite batch experiments Figure 34 pH and Eh with Cr stabilities Figure 35 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant chromite and variable birnessite at pH 5 Figure 36 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for constant birnessite and variable chromite at pH 5
xvi
Figure 37 Concentrations of Cr Cr(VI) Fe Fe(II) Mn and Mg as a function of time
for variable pH and constant chromite and birnessite Figure 38 Rate of Cr(VI) production (d[Cr(VI)]dt) as a function of the surface area
concentration of chromite birnessite and pH Figure 39 SEM examination of solids following the termination of the batch
experiments Figure 310 Serpentine soil and birnessite experiments Figure 311 Iron analyses and the solubility of amorphous Fe(OH)3 and goethite Figure 312 Congruent dissolution of chromite based on Fe and Cr Figure 313 Three pathways of Cr release and oxidation Chapter 4 Figure 41 Serpentine soil localities worldwide Figure 42 Field site pictures of serpentine soils Figure 43 Stability field diagram of Cr over a range of Eh and pH conditions Figure 44 Chromium and Al Wt plot and octahedral Cr and octahedral Al plots
for chromite and Cr-silicates including Cr mica (fuchsitemariposite) Cr-garnet (uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite)
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (m) for selected serpentine soils
Figure 46 pH of serpentine soils and related groundwater worldwide Figure 47 Soil pictures with Munsel color values and names from collected soils Figure 48 Cr XANES spectra for collected serpentine soils Figure 49 Backscattering images of mineral phases within each serpentine soil Figure 410 Chromite magnetite and hematite compositions from analyzed soils as
well as analyses for chromite reported from around the world Appendix A Figure A1 Compositional variation of Cr2O3 and Al2O3 Wt in Cr-chlorite Figure A2 Compositional variation of Cr and VIAl in Cr-chlorite based on 14 oxygens Figure A3 Molar volume estimation of kaumlmmererite based on XRD unit-cell
parameters Figure A4 Natural and calculated compositions of co-existing Cr-clinochlore and Cr-
spinel solid solution in the system Mg0-FeO-Cr2O3-Al2O3-SiO2-H2O at 2 kbar pressure
xvii
CHAPTER 1
INTRODUCTION
Serpentinites or metamorphosed ultramafic rocks are distributed worldwide and
are commonly associated with ophiolite complexes (Coleman 1977 Coleman and Jove
1992) These rocks weather to form serpentine soils a generic term used to describe any
soil derived from serpentinite regardless of its physical or chemical properties
Serpentinites and serpentine soils contain Cr-bearing minerals the most prevalent being
chromite and Cr released from these minerals into surface and subsurface environments
is potentially toxic and carcinogenic The main objective of this dissertation is to
evaluate the mineralogy and chemistry of Cr related to serpentinites and serpentine soils
over geologic time using an integrated analytical experimental and theoretical approach
Additionally the potential of serpentinites and serpentine soils as sources of non-
anthropogenic Cr contamination is investigated A discussion of Cr and its significance
to serpentinites and serpentine soils begins with its discovery by a Frenchman
investigating an ore named ldquored leadrdquo from the Urals
Nicolas-Louis Vauquelin discovered Cr in 1797 in the mineral crocoite PbCrO4
(Burns and Burns 1975) The metalelement was named chromium after the Greek word
chroma meaning color due to its strong influence on mineral pigmentation (Burns and
Burns 1975) For example rubies and emeralds derive their color from trace amounts of
Cr in their structures More importantly JT Lowitz in 1798 discovered Cr in chromite
or Cr-spinel the primary source of Cr for industrial purposes (Burns and Burns 1975)
Chromium is a group VIB transition metal with two common oxidation states (Cr(III) and
Cr(VI)) an average crustal abundance of 100 mg kg-1 and an abundance of 513x105 in
the solar system (relative to [H]=1x1012) (Mason and Moore 1982)
The majority of Cr on the earth exists as Cr(III) where it is an immobile cation
that complexes strongly with organic matter and adsorbs on oxides and silicates even at
low pHs (pHgt4) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) Additionally
the chemical mobility of Cr(III) is limited due to the formation of highly stable
(oxy)hydroxides (Rai and Zachara 1988 Rai et al 1989 Ball and Nordstrom 1998)
1
Chromium(III) has the highest octrahedral site preference energy (OSPE) of any trace
element and it isomorphically substitutes into octahedral sites in oxides
(oxy)hydroxides sulfides and silicates Although Cr(III) is known to substitute for both
octahedral Al(III) and Fe(III) the strongest correlation for substitution appears to be for
Al(III) in oxides and silicates as demonstrated in Chapters 2 and 4 and Appendix A
Chromium(VI) is a strongly oxidizing and weakly adsorbing oxyanion in the form of
chromate (HxCrO42-x) (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995 Shiraki
1997 Ball and Nordstrom 1998) Consequently Cr(VI) is chemically mobile and
bioavailable over a wide range of pH (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Chromium(VI) is tetrahedrally coordinated and behaves similarly to
sulfate in minerals such as jarosite (Baron and Palmer 1996)
The pathways and chemistry of Cr oxidation and reduction in soils and related
waters are covered in depth by Fendorf (1995) and Ball and Nordstrom (1998) The
reduction of Cr(VI) to Cr(III) may occur by reactions with Fe(II)aq Fe(II)-containing
minerals such as magnetite organic matter H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996) Oxidation of Cr(III)
to Cr(VI) related to serpentinite-related water and serpentine soil solutions is still a matter
of conjecture Chromium(III) oxidation in serpentine soils has been hypothesized to be
the result of Mn oxides such as birnessite as suggested by both Gough et al (1989) and
Becquer et al (2003) This idea is explored in depth in Chapter 3 where birnessite is
shown to influence the release and oxidation of Cr from chromite over a range of pH (3
to 8) at 25oC
Toxicity and Regulations of Chromium
The examination of Cr based on its oxidation state is necessary due to the
juxtaposing behavior and toxicity of Cr(III) and Cr(VI) Chromium(III) is an essential
micronutrient in human nutrition used by the body to process sugar protein and fat
(Vincent 2000) Inhalation of Cr(VI) is known to cause irritation in the nose including
symptoms such as nosebleeds (epistaxis) and ulcers andor perforations in the nasal
septum (Katz and Salem 1993 Kuo et al 1997) Ingesting considerable quantities of
Cr(VI) can result in stomach ulcers convulsions kidney and liver damage and death
2
(Katz and Salem 1993) Skin contact with certain Cr(VI) compounds causes allergic
contact dermatitis (ulcers) and irritant dermatitis (Katz and Salem 1993) Chromate
induced ulcers or chromeholes are typically red swollen crusted painless lesions
(Katz and Salem 1993) The nosology of Cr is beyond the scope of this dissertation
however Lees (1991) is an excellent source for further information Overall the
Environmental Protection Agency (EPA) the Occupational Safety and Health
Administration (OSHA) the World Health Organization (WHO) and the Department of
Health and Human Services (DHHS) have determined that that Cr(VI) compounds can
increase the risk of lung cancer primarily by inhalation as well as other forms of cancer
caused by ingestion
Federal and state regulation standards appropriate for both Cr(III) and Cr(VI) are
summarized and discussed by Proctor et al (1997) Human health action levels for Cr
vary for each state and the ranges for total Cr in soils include the following 200-66000
Total digestion ICP-AES analyses daggerSerpentinites are dominantly composed of the minerals serpentine chlorite talc and spinels DaggerSee Figure 21b for sample locations
33
Table 22a Selected Cr-bearing minerals and compositions in oxide Wt in serpentinite rock at Jasper Ridge
a Chromite chemical composition is (Fe046Mg052Mn002)(Cr061Al029Fe010)2O4 b Synthetic birnessite (δ-MnO2) c10 mM acetate buffered solution pH = 5 non-buffered pH=3 5 mM MES buffer pH 67 non-buffered pH 8 dMeasured BET analyses
81
Table 32 Serpentine soil and birnessite experiments with soil and birnessite masses used and the estimated chromite suspension density
daggerMolarity values are normalized to the final solution analysis of each experiment based on volume of solution and mass of the sample in each experiment aSolution refers to the final solution analysis before the experiment was terminated bmicroMoles extracted from the batch experiment solids using a 1 mM phosphate buffer pH 7 3 h cmicroMoles extracted from the batch experiment solids using a 10 mM CaCl2 solution pH 7 3 h dmicroMoles extracted from the batch experiment solids using a 05 N HCl solution 3 h emicroMoles extracted from the batch experiment bottles using a 05 N HCl solution 15 h
83
Figure Captions
Figure 31 Scanning electron microscopic (SEM) images of chromite and birnessite at
varying magnifications prior to the batch experiments
Figure 32 A summary of Cr Fe and Mn released from chromite during extraction
experiments Solutions used for these experiments include 1) 10 H2O2 solution 2) 01
M KMnO4 solution 3) 10 M HF 4) 1 M NH2OHmiddotHCl with 25 (vv) HOAc and 5) 1mM
BaCl2 solution () Fe concentration for the HF extraction is 106 mg L-1
Figure 33 pH and Eh measurements as a function of time (h) for the batch
experiments Chromite (Ch) and birnessite (Bt) suspension densities used for the batch
experiments (Table 31) are listed adjacent their corresponding symbol in the key
Figure 34 Eh and pH batch experiment analyses (Figure 33) plotted on a stability field
diagram for Cr (log a Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom
1a) Cr dissolution from chromite FeCr2O3(s) + 4H+ rarr 2Cr(OH)2+
aq + Fe2+(Oxyhydroxide) + H2O
1b) Adsorption of CrOH2+ on the surface of birnessite Cr(OH)2+
aq + MnO2(s) rarr Cr(III)-MnO2(s) + OH- 1c) Oxidation of CrOH2+ on the surface of birnessite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
1d) Desorption of Cr(VI) from birnessite Cr(VI)-Mn(II) + 4H2O rarr Mn2+
(aq) + 2H2O 2b) Mn4+ adsorption on the surface of chromite Mn4+
(aq) + FeCr2O4(s) rarr Mn(IV)-FeCr2O4 2c) Oxidation of Cr(III) on the surface of chromite
1st electron transfer Cr(III)-Mn(IV) rarr Cr(IV)-Mn(III) 2nd electron transfer Cr(IV)-Mn(IV) rarr Cr(V)-Mn(III) Cr(IV)-Mn(III) rarr Cr(V)-Mn(II) 3rd electron transfer Cr(V)-Mn(III) rarr Cr(VI)-Mn(II)
2d) Desorption of CrO24- from chromite
Cr(VI)-Mn(II) + 4H2O rarr Mn2+(aq) + HCrO4
-(aq) + 7H+
Pathway 3 Solid Solution Reaction 3) Solid solution interaction between chromite and birnessite
Figure 313
99
CHAPTER 4
CHROMIUM GEOCHEMISTRY OF SERPENTINE SOILS
ABSTRACT
Serpentine soils derived from the weathering of ultramafic rocks mainly
ophiolitic serpentinites are characterized by Cr concentrations in excess of 200 mg kg-1
higher concentrations Fe Ni Co and Cd low concentrations of plant nutrients and
higher concentrations of ammonium acetate extractable Mg compared to Ca The
chemistry of Cr in serpentine soils including its protolith is reviewed focusing on
serpentine soils collected from New Caledonia Oregon and California Overall
serpentine soils are slightly acidic (average pH of ~6) contain a variety Fe(III) oxides
(magnetite and hematite) phyllosilicates (serpentine and chlorite) and clays (smectites
and vermiculites) and contain concentrations of Cr (gt200 mg kg-1) Ni (gt1000 mg kg-1)
and Mn (gt200 mg kg-1) exceeding values of non-serpentine soils Although Cr
concentrations in serpentine soils have been reported as high as 6 Wt in New
Caledonia Cr in New Caledonia Oregon and California serpentine soils evaluated in this
study range from 827 to 9528 mg kg-1 Chromium(III) is the only valence state observed
in the serpentine soil solids however Cr(VI) has been identified in New Caledonia and
California serpentine soil solutions at concentrations below 30 microM The enrichment and
range of Cr concentrations in serpentine soils are directly related to the presence of Cr-
spinels specifically chromite and Cr-magnetite These spinels are resistant to weathering
and are preserved in the soil environment however oxidation of Cr(III) from Cr-spinels
by high valent Mn oxides or other strong oxidants is a potential source of Cr(VI)
identified in serpentine soil solutions Due to the chemically and physically resistant
nature of the Cr-spinels Cr-bearing silicates including clay minerals Cr-chlorite Cr-
garnet Cr-mica and Cr-epidote are more viable sources of Cr identified in vegetation
soil extractions soil solutions and related waters
100
INTRODUCTION
Serpentine soils a generic term used to describe any soil derived from an
ultramafic rock (peridotites and pyroxenites) or serpentinite (metamorphosed ultramafic
rock) are often characterized by 1) low concentrations of plant nutrients such as N P
and K 2) high concentrations of biologically toxic elements including Ni Co and Cd 3)
low ammonium acetate extractable CaMg ratios and 4) unique flora and physical
properties (Brooks 1987) Measured Cr concentrations in serpentine soils range from
~200 mg kg-1 to 6 Wt values enriched compared to non-serpentine soils lt200 mg kg-
1 (Kabata-Pendias and Pendias 1984 Schwertmann and Latham 1986 Brooks 1987)
Chromite and Cr-magnetite are common primary sources of Cr in these soils (Rabenhorst
et al 1982 Schwertmann and Latham 1986 Alexander et al 1989 Gough et al 1989
Gasser and Dahlgren 1994 Kaupenjohann and Wilcke 1995 Becquer et al 2003)
however the plethora of factors relating the chemistry of Cr in serpentine soils and the
weathering of chromite has not been fully addressed using an integrative approach
Overall serpentinites and serpentine soils cover ~1 of the earthrsquos exposed
surface and these rocks and soils are commonly associated with ophiolite complexes
(Coleman 1977) Serpentinites and serpentine soils related to ophiolite complexes are
the primary interest of this paper A map illustrating the worldwide distribution of major
ophiolitic serpentinite bodies and their related soils is shown in Figure 41 A majority of
the sites are near convergent boundaries (Figure 41) Although these rocks and soils
cover a minimal area of the earthrsquos surface they are present in populated areas within the
Circum-Pacific Margin and the Mediterranean Due to the wide-distribution and
occurrence of serpentinites and serpentine soils weathering of these rocks and soils will
differ from location to location due to varying climatic conditions as well as the nature of
the parent material and other factors including topography biota and time (Proctor and
Nagy 1992) As a result the chemistry of these soils as well as the nature of Cr in the
serpentinites and serpentine soils will vary from site to site potentially allowing Cr to be a
source of non-anthropogenic contamination
The objective of this paper is to review the chemical characteristics of serpentine
soils globally evaluate mineralogical and compositional variations between several soils
derived from ultramafic sources and assess the geochemistry of Cr in diverse soil-
101
forming environments Additionally chemical and spectroscopic analyses were
performed on soils derived from ultramafic rocks collected from 1) New Caledonia
South Pacific 2) Eight Dollar Mountain Selma OR 3) Nickel Mountain Riddle OR 4)
Harvard Mine Jamestown CA 5) Jasper Ridge Biological Preserve Stanford CA and 6)
Pillikin Mine El Dorado CA shown geographically in Figure 41 A basalt-derived soil
from Hawaii is included in this study in order to compare a soil formed from a mafic
protolith to those from ultramafic protoliths The various field sites are shown in Figure
42 These soils allow a comparative examination of how soil-forming factors such as
climate topography biota parent material and time affect the chemistry of Cr
REVIEW
Chromium Geochemistry
Chromium is a group VIB transition metal with two common oxidation states
Cr(III) and Cr(VI) differentiated by noticeably different chemical characteristics and
toxicities It should be noted that Cr(II) has been observed in lunar basalt olivines and in
high temperature terrestrial olivines (Sutton et al 1993 Li et al 1995) however this
oxidation state is not observed in soil environments A majority of Cr commonly persists
in ultramafic rocks serpentine soils and related waters as Cr(III) a non-hazardous
species and a micronutrient in human nutrition (Daugherty 1992 Cohen et al 1993
Fendorf 1995 James et al 1997) Chromium(III) has the highest crystal field
stabilization energy for all transition metals strongly favors octahedral coordination and
has 3 d-electrons with a high spin state Chromium(III) is strongly hydrolyzed in
aqueous solutions and the predominant species based on pH redox potential and O2(aq)
are shown in Figure 43 Chromium(III) species are more prevalent under reducing
conditions and its mobility is increased only under strongly acidic conditions At pH
values greater than 9 the hydrolyzed Cr(III) species is negatively charged demonstrating
the amphoteric properties of Cr(III) (Figure 43) Due to the factors described above
Cr(III) forms low solubility compounds and adsorbs strongly on mineral surfaces and
organic matter at pHgt4 (Rai and Zachara 1988 Rai et al 1989 Fendorf 1995) For
example the adsorption of Cr(III) on HFO (hydrous ferric oxide) as a function of pH is
shown in Figure 43 illustrating an adsorption edge at a pH of ~4 (Leckie et al 1984
102
Stumm 1992) Chromium(III) oxidation to Cr(VI) has been observed in reactions
involving hydrogen peroxide (H2O2) and high valent Mn oxides (Eary and Rai 1987
Johnson and Xyla 1991 Fendorf et al 1992 Fendorf et al 1993 Rock et al 2001)
Chromium(VI) is a strong oxidizing agent a highly corrosive substance a toxin to
living cells and a Class A human carcinogen by inhalation (Daugherty 1992 Cohen et
al 1993 Fendorf 1995) The hydrolyzed Cr(VI) species are also shown in Figure 43
with respect to pH redox potential and O2(aq) Chromium(VI) is more favorable under
oxidizing conditions and its mobility is enhanced under basic conditions In solution
Cr(VI) is a weakly adsorbing (oxy)anion (HxCrO42-x) on common weathering products
meaning that it forms highly labile complexes that can be readily displaced by ions
including phosphate and sulfate (Rai and Zachara 1988 Fendorf 1995 Ball and
Nordstrom 1998) Additionally the adsorption of CrO42- on HFO shown in Figure 43
demonstrates the anionic character in which the mol bound of CrO42- increases with
decreasing pH and an adsorption edge is at pH ~72 (Leckie et al 1980 Stumm 1992)
The reduction of Cr(VI) to Cr(III) is known to occur by reactions with Fe(II)aq Fe(II)-
containing minerals such as magnetite H2S and organic matter including microbial
processes (Ilton and Veblen 1994 Fendorf 1995 Peterson 1996)
Mineralogy of Serpentine Soils
Serpentine soils have been identified on every continent excluding Antarctica
(Figure 41) The occurrence and distribution of ultramafic rocks specifically related to
ophiolites are closely associated with convergent plate boundaries (Figure 41 Coleman
1977 Coleman and Jove 1992) Tectonic blocks of ultramafic rock are hypothesized to
be detached from the down-going slab of oceanic crust and incorporated into the
subduction meacutelange Subsequent serpentinization of peridotite forms low density
serpentine group minerals (245-250 g cm-3) resulting in the upward diapiric migration of
the serpentinite masses along zones of structural weakness (fault zones) and through the
denser enclosing subduction meacutelange (~265 g cm-3) For example North America has
numerous exposed ophiolites containing serpentinite located in western California and
Oregon where subduction zones were once active in the Early Cretaceous (Harden
1998)
103
Serpentinization of ultramafic rocks is a significant factor controlling mineral
assemblages and textures in serpentinite Hydration and oxidation of olivine and
pyroxene during serpentinization forms serpentine (lizardite chrysotile and antigorite)
illustrate Cr(VI) levels are below the detection limits of this method (~10 Cr mg kg-1) due
to the lack of a pre-edge feature The box in Figure 48 illustrates where a Cr(VI) pre-
115
edge peak should be present Chromium(VI) concentrations in these soils are possibly
present in the parts per billion range
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the
Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49
and corresponding Cr-bearing mineral compositions are listed in Table 46 The New
Caledonia soil exhibits a wide range of particle sizes and a Fe-(oxy)hydroxide was
identified containing 165 Cr2O3 Wt Chromium-spinel has been identified in the New
Caledonia laterite by Becquer et al (2003) Eight Dollar Mountain soils exhibit a coarse
texture and both Cr-magnetite (2387 Cr2O3 Wt ) and chromite (3343 Cr2O3 Wt )
are present Nickel Mountain soils are also coarse and chromite (1315 Cr Wt ) was
identified Soils at Harvard Mine contain numerous chromite grains and the mineral
trevorite (a Ni-bearing spinel) was identified with 3991 Cr2O3 Wt and 2054 NiO Wt
Chromite and magnetite are abundant in soils at Jasper Ridge as well as a fine-
grained mixture of chromite magnetite and an unidentifiable silicate with up to 1878
Cr2O3 Wt (Figure 49) Veins of Cr-chlorite crosscut the spinel-silicate mixture in
Figure 49 and this Cr-chlorite is the only Cr-silicate identified using this method
Chromite (5355 Cr2O3 Wt ) is the dominant Cr-bearing mineral detected in soils at the
Pillikin Mine and these grains contain the highest Cr concentrations compared to the
other minerals in the seven soils The Hawaiian Oxisol possesses ulvoumlspinels (007
Cr2O3 Wt and 2465 TiO2 Wt ) which are evenly distributed throughout the soil
Overall microprobe observations reveal that the Cr-bearing spinels are present in every
soil Additionally Cr-bearing spinels at Jasper Ridge have been shown to undergo
incongruent dissolution progressively enriching the spinel towards a FeCr2O4 end-
member (Oze et al 2003)
DISCUSSION
The physical and chemical characteristics of serpentine soils are directly related to
the alteration and tectonic history of the serpentinite as well as the soil-forming factors
including the parent material climate biota topography and time Identifying Cr-
116
bearing minerals their chemical compositions and abundance in the soils allows a more
definitive explanation for Cr enrichment in these soils as compared to total Cr
concentrations listed in Tables 41 and 45 and sequential extraction experiments reported
in Table 42 Detrital chromite grains from the serpentinite protolith are resistant to
weathering and accumulate in the soil over time Although serpentine soils contain Cr
concentrations typically gt200 mg kg-1 the presence of Cr-spinels and their resistance to
weathering demonstrate that a majority of the Cr is not chemically available to interact
within the soil The role of organic matter and vegetation are important factors related to
the chemistry of serpentine soils however their role for influencing a majority of the Cr
geochemistry is limited due to the nature of the Cr-spinels
Chromium and Serpentine Soils
Total Cr concentrations exceed 200 mg kg-1 for most of the serpentine soils
reported in Table 41 and in the six serpentine soils of this study (Table 45) These Cr
concentrations can be accounted for by assessing the Cr-bearing minerals associated with
these soils and their relative abundance Small grains of Cr-spinels or chromite identified
in the soils (Figure 49 and Table 46) have the potential to appreciably impact Cr
concentrations Table 47 demonstrates the mineral masses and volumes of Cr-bearing
minerals required to cause predetermined Cr concentrations in soil samples A minimal
amount of chromite (lt02 of a soil sample mass or 005 of the volume of serpentine)
can result in Cr concentrations of 1000 mg kg-1 A few grains of chromite could easily
be responsible for the Cr concentrations in Tables 41 and 45 In fact residual grains of
Cr-enriched spinels are observed in every soil analyzed (Figure 49 and Table 46)
Ultimately grains of spinel in the soil do appreciably impact Cr concentrations however
they do not provide a readily available source of Cr based on extraction experiments
performed by Oze et al (2003) Serpentine minerals could also provide minor Cr
concentrations to the soils however soil extractions have consistently shown that a
majority of the Cr is in highly resistant phases capable of resisting dissolution even in
concentrated hydrochloric acid nitric acid and hydroxylamine with acetic acid (Table
42) Chromium end-member minerals including eskolaite Cr-muscovite (fuchsite) Cr-
garnet (uvarovite) Cr-epidote (tawmawite) and Cr-chlinochlore (kaumlmmererite) are also
117
included in Table 47 to demonstrate that relatively low abundances of these minerals are
all that is necessary to increase Cr concentrations in rocks and soils Chromium-
clinochlore is often observed developing adjacent Cr-spinels in serpentinites and is
perhaps a more accessible mineral source of Cr in the serpentinites compared to Cr-
spinels (Phillips et al 1980 Schreyer et al 1981 Kerrich et al 1987 Treloar 1987a
Pan and Fleet 1991 Schandl and Wicks 1993 Christofides et al 1994 Challis et al
1995 Sanchez-Vizcaino et al 1995 Mathiesen 1999 Oze et al 2003)
As shown in Table 44 and Figures 43 and 46 the chemical properties (pH EC
N etc) of serpentine soils are not correlated to Cr concentrations measured in the soil
solids (Tables 41 Figure 45) due to Cr-spinels accounting for a majority of the Cr
(Figure 49 and 410 Tables 46 and 47) However extraction experiments and soil
solution analyses demonstrate that a limited fraction of the total Cr (lt1) is extractable
If only 1 of the Cr in a serpentine soil is extractable a serpentine soil containing 10000
Cr mg kg-1 could possibly contribute up to100 Cr mg kg-1 still a significant Cr
concentration The chemical properties of the soil such as pH will influence this limited
fraction of the Cr in the soils Acidic pHs may aid in mobilizing Cr(III) however this
will only occur at low pHs (lt4) which exceed the range of most serpentine soils (Figures
43 and 46) pH values greater than 8 like those in some serpentine soils and
serpentinite groundwater may help sustain Cr(VI) concentrations in solution (Figure
43) In addition to pH organic matter in the soils may be a significant factor related to
the mobility of Cr due to the production of organic acids promoting the dissolution of the
Cr-bearing phases but particulate organic matter is known to bind Cr(III) in stable stable
organometallic complexes (Kaupenjohann and Wilcke 1995 Brady and Weil 1999)
Evaluating the valence state of Cr is crucial for understanding the chemistry as
well as the potential toxicity related to serpentine soils The soil solids do not contain
detectable concentrations of Cr(VI) (Figure 48) however Cr(VI) concentrations are
present in soil solutions and groundwater at values lt30 microg L-1 (Gough et al 1989
Becquer et al 2003) A majority of serpentinite Cr-bearing minerals only contain
Cr(III) therefore there must be a reaction or process by which Cr(III) is oxidized in a
water saturated soil High valence Mn oxides such as birnessite are present within
serpentine soils and provide a feasible explanation for Cr(III) oxidation Additionally
118
Mn oxides or other strong oxidants in the soil might accelerate the weathering of Cr-
bearing minerals including chromite and generate the Cr(VI) concentrations observed in
soil solutions and groundwater After Cr(III) is oxidized Cr(VI) is a chemically mobile
species and would be transported away from the serpentine soils before it can be
observed in the solid phase Additionally Cr(VI) may be subsequently reduced by
reacting with organic matter or Fe(II)-bearing minerals such as magnetite which are
present in a majority of serpentine soils Perhaps the best way to evaluate the potential of
serpentine soils being a non-anthropogenic source of Cr contamination is to monitor
Cr(III) and Cr(VI) in the soil solutions over time as demonstrated by Gasser and
Dahlgren (1994)
It is curious that plants do uptake Cr in serpentine soils especially when Cr is
tightly bound in the soil matrix (Table 42) In order for plants to uptake Cr it must be in
a form to be absorbed by plant roots Soil pH in the rhizosphere can be altered by plant
roots by as much as 2 units within two days (Fischer et al 1989) This acidification is
known to increase the solubility of the minerals in the soils and could serve as a
mechanism by which plants can incorporate this relatively insoluable element Another
possibility is that Cr(III) in the soil is oxidized by either organic or inorganic means and
then incorporated into the plant in its more chemically mobile and bioavailable form
Cr(VI) How and why plants incorporate as much Cr as they do needs to be examined
more thoroughly
Chromium Spinel in Soils and Rocks
Compositions of chromite identified in serpentinites worldwide (Figure 44) and
Cr-bearing spinels identified in the six serpentine soils are compared in Figure 10 in order
to determine whether the soil spinels are geochemically similar to those in serpentinites
This comparison is ascertained by examining how Al Fe and Mg Wt values varied
with respect to Cr Wt Trends for magnetitehematite chromite and the
microcrystalline spinel-silicate mixture observed in the soils are noted in each diagram in
Figure 410 Additionally the highest Cr concentrations for soil chromite
magnetitehematite and the microcrystalline spinel-silicate mixture are ~40 Cr Wt
~25 Cr Wt and ~15 Cr Wt respectively Based on Al Wt soil
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magnetitehematite deviates significantly from the chromite trend with Al Wt values
lt5 Magnetitehematites are enriched with Fe relative to serpentinite and soil chromite
and Mg concentrations in the soil chromite are slightly higher than serpentinite chromite
Soil magnetitehematite and chromite are similar by demonstrating a linear trend when
Al Fe and Mg are summed The microcrystalline spinel-silicate mixture (CSM) does
not follow this trend because of Si present in this mineral mixture Overall soil
chromites are similar to serpentinite chromite and soil magnetiteshematites can be
differentiated from the chromites when Cr Wt is plotted against Al Fe and Mg Wt
CONCLUDING STATEMENT
Serpentine soils are abundant among convergent margins and the chemistry of
these soils is ultimately a function of the ultramaficserpentinite rock mineralogy and
texture as well as all the common soil-forming factors Although ultramafic rocks and
serpentinites produce groundwater that is highly basic most serpentine soil pHs are
slightly acidic Although Proctor and Nagy (1992) argued that it is futile to categorize
serpentine soils based on chemistry and biology (otherwise referred to as the lsquoserpentine
factorrsquo) as proposed by Brooks (1987) Cr concentrations greater than 200 mg kg-1 in
serpentine soils are consistently present Chromium in the soils originates from minerals
in the bedrock where the valence is primarily Cr(III) the exception being Cr(VI) in the
rarely present mineral crocoite (PbCrO4) Increased Cr-concentrations in serpentine soils
are largely the result of chromite or other resistant Cr-bearing minerals causing a
localized enrichment in the soils Although Cr in serpentine soils is not readily released
from the soil matrix serpentine soil vegetation can incorporate Cr concentrations as high
as 700 mg kg-1 (Table 34) Finally the limited extractability of Cr and site dependent
factors such as the presence of organic matter clays Fe(II)-bearing minerals pH and the
poor drainage indicates that the possible formation and sustained presence of Cr(VI) is
not a portentous matter related to serpentine soils
120
ACKNOWLEDGEMENTS
We appreciate technical and field support from Robert Jones Anders Meibom
Rob Dunbar and Guangchao Li Funding from the JRBP Mellon Grant Stanford Shell
Foundation Stanford McGee Fund GSA Graduate Student Research Grants and the
Office of Teaching and Learning at Stanford University is gratefully acknowledged
121
Table 41 Measured Cr concentrations in serpentine soils worldwide
Location Cr concentration (mg kg-1) Reference
Canyon Creek area OR 920 (White 1971) 66-13200 (Brooks et al 1992)
Ireland 29-5000 (Proctor and Woodell 1971 Jeffrey 1992) Italy 1300-3900 (Gambi 1992) Japan 5200 (Shewry and Peterson 1976) Maryland 1500-6000 (Rabenhorst et al 1982) Mt Tamalpais CA 1700 (White 1971) New Caledonia 6300-80000 (Jaffre 1980) New Caledonia 2-6 (Schwertmann and Latham 1986) New Idria CA 1640 (White 1971) New Zealand 850-3250 (Lee 1992) Newfoundland Canada 309-1447 (Roberts 1992) Nickel Mountain OR 1730 (White 1971) Santa Barbara County CA 2080 (Woodell et al 1975) Southern Africa 045-14 (Morrey et al 1992) Tehama County 023-12 (Gough et al 1989) Transvaal Lowveld South Africa 120-800 (Cole 1992) Unst Shetland 3000-4000 (Proctor 1969) Wenatchee Mnts WA 895 (White 1971) Zimbabwe 110-46000 (Soane and Saunder 1959 Proctor et al 1980)
Goias State Brazil
122
123
Table 42 Summary of Cr Fe and Ni extractions from serpentine soils
Range of Extractable Cr Fe and Ni Location Cr Fe Ni
Lophostachys villosa 203 Goias State Brazil (Brooks et al 1992) Ruellia geminiflora
56 Goias State Brazil (Brooks et al 1992) Minuartia ophiolitica 83 Italy (Gambi 1992) Sanguisorba minor 194 Italy (Gambi 1992) Plantago lanceolata 135 Italy (Gambi 1992) Cerastium latifolium 110 Italy (Gambi 1992) Funaria flavicans 34-899 Maryland (Shaw 1991) Avena sativa 168-583 New Zealand (Lee 1992) Aerobryopsis longissima 200-700 New Caledonia (Lee et al 1977) Arenaria humifusa 530 Newfoundland Canada (Roberts 1992) Senecio pauperculus 406 Newfoundland Canada (Roberts 1992) Rhalomitrium lanuginosum 246 Newfoundland Canada (Roberts 1992) Santolia semidentata 95 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Asplenium cuneifolium 228 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992) Armeria eriophylla 38 Northeast Portugal (Menezes de Sequeira and Pinto da Silva 1992)
Arctostaphylos viscida 029-705 Tehama County CA (Gough et al 1989) Calluna vulgaris 417 Unst (Shewry and Peterson 1976) Armeria maritima 149 Unst (Shewry and Peterson 1976) Agrostis stolonifera 120 Unst (Proctor and Woodell 1971) Dicoma niccolifera 77 Zimbabwe (Proctor 1992) Xerophyta equisetoides 68 Zimbabwe (Proctor 1992)
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125
Table 44 Chemical and textural properties of the collected soils
Ammonium Acetate Extractable ------------------------Hydrometer--------------------- Ca K Mg Na
dagger Electron microprobe analyses This is the form of iron unless otherwise stated a FeO is in the form Fe2O3 b FeO is in the form of Fe3O4 1CSM Microcrystalline chromitesilicate mixture
128
Table 47 Chromium-bearing mineraldagger masses and volumes required to influence Cr concentrations in soils
Cr Concentration Mineral mass (grams) required for 1000 g bulk soil analysis Mineral Mass Percent () (mg kg-1) Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite Chromite Eskolaite Fuchsite Uvarovite Tawmawite Kaumlmmererite
Note Molar volumes used in the calculations above are from their Al-enriched counterparts in Robie and Hemingway (1995) The molar volume for kaumlmmererite was estimated using unit cell volumes from XRD unit cell measurements from Robie and Hemingway (1995) Christofides et al (1994) and Phillips et al (1980)
129
Figure Captions
Figure 41 Selected serpentine soil localities as well as soils collected for analyses are
shown on a world map Active plate boundaries are shown Serpentine soil localities not
adjacent the plate boundaries on this map are related to ancient plate boundaries that are
no longer active
Figure 42 Field pictures of soils collected from a) Eight Dollar Mountain b) Nickel
Mountain c) Harvard Mine d) Pillikin Mine and f) Jasper Ridge Biological Preserve
Figure 43 Stability field diagram of Cr under a range of Eh and pH conditions (log a
Cr+3 = -16 and log a H2O = 0) modeled after Ball and Nordstrom (1998) Adsorption
isotherms for Cr3+ and CrO42- on hydrous ferric oxide (HFO) are also shown (Leckie et
al 1980 Leckie et al 1984)
Figure 44 Microprobe analyses demonstrating Cr versus Al Wt for Cr-spinel
(chromite) and Cr-silicates including Cr-mica (fuchsitemariposite) Cr-garnet
(uvarovite) Cr-clinochlore (kaumlmmererite) and Cr-epidote (tawmawite) (Whitmore et al
1946 Leo et al 1965 Chen and Lee 1974 Neiva 1978 Cooper 1980 Phillips et al
1980 Grapes 1981 Schreyer et al 1981 Max et al 1983 Nutman et al 1983 Raase
et al 1983 Chao et al 1986 Von Knorring et al 1986 Kerrich et al 1987 Treloar
1987b a Morand 1990 Pan and Fleet 1991 Tracy 1991 Schandl and Wicks 1993
Christofides et al 1994 Challis et al 1995 Sanchez-Vizcaino et al 1995 Mathiesen
1999 Proenza et al 1999) Chromium-spinel and Cr-silicate compositions based on
octahedral Cr and octahedral Al are also shown with tie-lines (dotted) denoting ideal
isomorphic substitution between Cr and Al end-members
Figure 45 Chromium concentrations (mg kg-1) as a function of depth (meters) for
selected serpentine soils indicated in the key above Each dotted line connecting the
points represents a separate soil profile at each site
129
Figure 46 Ranges of pH for serpentine soils and related groundwater worldwide Soil
pH values were obtained from A(Kruckeberg 1992) B (Graham et al 1990) C
(Alexander et al 1989) D (Gough et al 1989) E (Rabenhorst et al 1982) F
(Roberts 1992) G (Bulmer et al 1992) H (Proctor 1992a) I (Jeffrey 1992) J
(Gambi 1992) K (Verger 1992) L (Proctor and Cole 1992) M (Cole 1992b) N
(Morrey et al 1992) O (Proctor 1992b) P (Mizuno and Nosaka 1992) Q (Cole
1992a) and R (Lee 1992) Groundwater pH values from serpentinite are from Barnes
and ONeil (1969) and Barnes et al (1972) Note the range of pH environments possible
for ultramafic rocks and their related soils
Figure 47 Soil pictures with Munsel color values and names from soils collected from
a) New Caledonia b) Hawaii c) Eight Dollar Mountain d) Nickel Mountain e) Harvard
Mine f) Jasper Ridge Biological Preserve and g) Pillikin Mine
Figure 48 Cr XANES spectra for soils analyzed from Jasper Ridge New Caledonia
Pillikin Mine Eight Dollar Mountain and Harvard Mine The pre-edge area indicative of
Cr(VI) peaks is noted in the diagram No definitive Cr(VI) peaks are present in any of
these spectra
Figure 49 Backscattering images of mineral phases within each soil are presented
Electron microprobe analyses are noted by a circle and letter for the Cr-bearing minerals
and the results are listed in Table 46
Figure 410 Chromite magnetite and hematite compositions from the collected soils
(gray circles) as well as analyses for serpentinite chromite (white squares) from Figure
44 are shown in these diagrams with Cr Wt versus Al Fe Mg and Al+Fe+Mg Wt
Chapter 3 Oxidative Promoted Dissolution of Chromite by
Manganese Dioxide and Concurrent Production of Ch
Chapter 4 Chromium Geochemistry of Serpentine S
List of References
Chapter 2
Chapter 3
Chapter 4
Appendix A
Chapter 2
Chapter 3
Chapter 4
Appendix A
CHAP1pdf
Toxicity and Regulations of Chromium
Mineralogy of Chromium in Serpentinites
Chapter Summaries
CHAP2Tablepdf
JR305CLAY Depth 0-5cm
JR3515CLAY Depth 5-15cm
JR33045CLAY Depth 30-45cm
HCAP2Figspdf
Figure 27
CHAP3Tablepdf
Figure 36
Figure 37
CHAP4pdf
ABSTRACT
REVIEW
Chromium Geochemistry
Soil Organic Matter
CALIFORNIA OREGON AND NEW CALEDONIA SERPENTINE SOILS
BedrockProtolith Mineralogy
Soil Chemistry and Physical Properties
Soil Mineralogy
Backscattered electron images and microprobe analyses were used to identify the Cr-bearing minerals in the seven soils backscattered images are shown for in Figure 49 and corresponding Cr-bearing mineral compositions are listed in Table 46 The New C