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1-5667-0608-4/01/$0.00+$1.50© 2004 by CRC Press LLC
2
Chemistry, Geochemistry, and Geology
of Chromium and Chromium Compounds
William E. Motzer and Todd Engineers
CONTENTS
2.1 Chromium Chemistry .................................................................................242.1.1 Background ......................................................................................242.1.2 Elemental/Metallic Chromium Characteristics .........................252.1.3 Ionic Radii ........................................................................................292.1.4 Oxidation States...............................................................................302.1.5 Stable and Radioactive Isotopes ...................................................312.1.6 Characteristics of Chromium Compounds .................................34
2.2 Natural Chromium Concentrations..........................................................342.2.1 Mantle ...............................................................................................462.2.2 Chromium Minerals........................................................................462.2.3 Chromium Ore Deposits ................................................................46
2.2.3.1 Stratiform Mafic-Ultramafic Chromite Deposits .........622.2.3.2 Podiform- or Alpine-Type Chromite Deposits ............63
2.2.4 Crude Oil, Tars and Pitch, Asphalts, and Coal ..........................632.2.5 Rock ...................................................................................................642.2.6 Soil .....................................................................................................662.2.7 Precipitation (Rain Water) and Surface Water ...........................672.2.8 Groundwater....................................................................................672.2.9 Sea Water ..........................................................................................672.2.10 Air ......................................................................................................672.2.11 Biogeochemical Cycling .................................................................68
2.3 Chromium Geochemistry ...........................................................................702.3.1 Cr(III) Geochemistry.......................................................................702.3.2 Cr(VI) Geochemistry.......................................................................712.3.3 Chromium Reaction Rates (Kinetics)...........................................73
2.4 Chromium Distribution in Primary Environments ...............................742.4.1 Possible Sources of Natural Cr(VI) in Rocks..............................742.4.2 Known Sources of Natural Cr(VI) in Rocks ...............................77
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24
Chromium(VI) Handbook
2.5 Chromium Distribution In Secondary Environments ...........................782.5.1 Known Natural Cr(VI) Occurrences in Surface
Water and Groundwater ................................................................782.6 Forensic Geochemistry................................................................................80
2.6.1 Soil .....................................................................................................802.6.2 Groundwater....................................................................................802.6.3 Air ......................................................................................................81
2.7 Acknowledgments.......................................................................................81Bibliography ........................................................................................... 82
2.1 Chromium Chemistry
2.1.1 Background
In 1797, the French chemist Nicholas-Louis Vauquelin hypothesized thatchromium (Cr) was a separate and distinct element. He had isolated theoxide of this element from a Siberian mineral known as crocoite (PbCrO
4
).In 1798, Vauquelin successfully isolated metallic chromium by heating(reducing) chromic oxide (Cr
2
O
3
) with charcoal. He then named the newelement after the Greek word
χρωµα
(chro
^
ma), pronounced khrma, for colorbecause it produced chemical compounds with distinct and unique colors.Vauquelin also analyzed a Peruvian emerald, determining that its green colorwas due to the presence of chromium. About two years after chromium’sdiscovery, Tassaert, a German chemist, determined that chromium waspresent in an ore that we now know as chromite (Greenwood and Earnshaw,1998; ChemGlobe, 2000; Papp, 2000; Winter, 2002).
Since its discovery, chromium has become a very important industrialmetal because of its many applications in ferrous (cast iron and stainlesssteel) and in nonferrous (aluminum, copper, and nickel) alloy metal fabrica-tion, and in the chemical industry (metal finishing, plating, corrosion control,pigments and tanning compounds, and wood preservatives) (Papp, 2000).Chromium and chromium compounds are used in a wide variety of indus-trial and manufacturing applications including steel alloy fabrication, wherethey enhance corrosion and heat resistance in other metals, and in platedproduct fabrication where they are used for metal decoration or increasedwear resistance. They are also used in nonferrous alloy metal fabrication toimpart special qualities to the alloys; in production and processing of insol-uble salts, as chemical intermediates; in the textile industry for dyeing, silktreating, printing, and moth proofing wool; in the leather industry for tan-ning; in the manufacture of green varnishes, inks, paints, and glazes; ascatalysts for halogenation, alkylation, and catalytic cracking of hydrocar-bons; as fuel and propellant additives; and in ceramics (Spectrum Labora-tories, 1998).
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Chemistry, Geochemistry, and Geology of Chromium
25
While chromium in its Cr(III) form is not considered a toxic element andis a required diet nutrient with recommended daily adult dosages rangingfrom 0.5 to 2 mg per day (required for glucose metabolism), in its Cr(VI)form, it does have toxic effects (see Guertin, Section 6, this volume). Acuteexposure to Cr(VI)-laden dust results in skin rashes, ulcers, sores, andeczema in occupational workers. In humans, Cr(VI) exposure causedmarked irritation of the respiratory tract and ulceration and perforation ofthe nasal septum in workers in the chromate producing and -using indus-tries. Ingestion of 1.0 to 5.0 g of Cr(VI) as chromate results in severe acutegastrointestinal disorders, hemorrhagic diathesis, and convulsions. Deathmay occur following cardiovascular shock. Doses in animals of Cr(VI)greater than 10 mg/kg mainly affect the gastrointestinal tract, kidneys, andhematopic system (IPCS, 1988). Cr(VI) causes cancerous tumors in mice byinhalation and is considered a possible human carcinogen by this routebecause workers engaged in the production of chromate salts and chromatepigments experience an increased risk of developing bronchial carcinomas.However, ingestion of Cr(VI) has not been observed to cause cancerbecause it is believed that Cr(VI) is reduced to Cr(III) in the gastrointestinaltract (IPCS, 1988; WHO, 1988 and 1996; Smith and Huyck, 1999; CDHS,2003).
The understanding of chromium chemistry and geochemistry is thereforeimportant in developing remediation systems that can deal with industrial-caused pollution (see Chapter 8). This chapter is a review of the character-istics of chromium in the natural environment; its concentration within theearth’s crust, atmosphere, and biosphere; and its geochemistry.
2.1.2 Elemental/Metallic Chromium Characteristics
Chromium (atomic number 24) is a transition metal occurring in Group VIBof the periodic table. General elemental chromium characteristics are sum-marized in Tables 2.1a to 2.1d. Chromium has a ground state electron con-figuration of 1s
2
2s
2
2p
6
3s
2
3p
6
4s
1
3d
5
(Table 2.2). In the periodic table, transitionmetals (Groups IB to XIIIB) occur between the main group elements (GroupsIA to IIA and Groups IIIA to VIIA and the inert gases—Group VIIIA) (Drew,1972; Timberlake, 2003). The atoms of transition elements have electronsfilling d subshells consisting of 5d orbitals.
The transition metals are noteworthy because they:
1. Form alloys with one another and the main group metals. 2. Commonly are white lustrous metals with high melting and boiling
points. The transition metals vary in abundance in the continentalcrust from iron, which is common at 5.63% to scandium which israre at 22 (parts per million) ppm (Ronov and Yaroshevsky, 1972).
3. Have high melting points and densities because the electrons inthe d orbitals, bind atoms together in the crystal lattice.
L1608_C02.fm Page 25 Thursday, July 15, 2004 6:57 PM
26
Chromium(VI) Handbook
TAB
LE 2
.1A
Ele
men
tal C
hrom
ium
Pro
pert
ies
Per
iod
ic T
able
Ato
mic
no.
2
4A
tom
ic m
ass
51
.996
1G
roup
no.
6
B
Mas
s N
um
ber
52
Gro
up n
ame
Tr
ansi
tion
met
als
Sym
bol
Cr
Peri
od n
o.
4B
lock
d
Ato
mic
No.
(Z)
24
Ch
emic
al R
egis
try
CA
S no
.74
40-4
7-3
(mos
t co
mm
on is
otop
e: 8
3.78
9%)
Pro
per
ties
Ph
ysic
alE
lect
rica
lT
her
mal
Ato
mic
rad
ius
(nm
)0.
185
e
–
in s
hell
1,2,
3,4
2,8,
13,1
Boi
ling
poin
t29
44 K
; 263
1 K
Ato
mic
vol
ume
(cm
3
/m
ol)
0.13
9e
–
con
figu
rati
onA
r4s
1
3d
5
Mel
ting
poi
nt
2180
°
C; 1
907
°
CB
ond
leng
th: C
r-C
r (n
m)
0.25
0
1
/
2
Fill
s of
sub
shel
l3d
5
Hea
t of
fus
ion
(kJ/
mol
)0.
325
Cov
alen
t ra
diu
s (n
m)
0.11
8E
lect
ron
bind
ing
ener
gies
Tabl
e 2.
1bH
eat
of v
apor
izat
ion
(kJ/
g)6.
622
Cry
stal
str
uctu
reC
ubic
bod
yce
nter
edE
ffec
tive
nuc
lear
cha
rges
Tabl
e 2.
1cSp
ecifi
c he
at c
apac
ity
(J/
g
⋅
K)
0.45
1
Den
sity
(g/
cm
3
at
293
K)
7.19
Ele
ctri
cal c
ond
ucti
vity
(cm
Ω
)0.
0774
×
10
6
The
rmal
con
duc
tivi
ty (
W/c
m
⋅
K)
0.93
7
Elas
tic
Prop
erti
es:
Elec
tron
egat
ivit
y:
Youn
g’s
mod
ulus
(G
Pa)
279
Paul
ing
1.66
Rig
idit
y m
odul
us (
GPa
)11
5A
bsol
ute
(eV
)3.
72B
ulk
mod
ulus
(G
Pa)
160
Ele
ctri
cal r
esis
tivi
ty (
10
−
8
Ω
)12
.7
L1608_C02.fm Page 26 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium
27
Har
dnes
s:Io
niza
tion
Pot
enti
al (
eV):
Min
eral
: Moh
s (n
o un
its)
8.
5Fi
rst
6.76
66B
rine
ll (M
N/
m
2
)11
20Se
cond
16.5
0V
icke
rs (
MN
/m
2
)10
60T
hird
30.9
6Io
nic
rad
ius
(nm
)0.
62O
xid
atio
n st
ates
(Ta
ble
3.4)
+
2 to
+
6Su
cces
sive
ioni
zati
on E
nerg
ies
Tabl
e 2.
1d
Sour
ces:
Che
mG
lobe
(20
00);
Che
mPr
os (
2000
); W
inte
r: W
ebE
lem
ents
(20
01).
L1608_C02.fm Page 27 Thursday, July 15, 2004 6:57 PM
28
Chromium(VI) Handbook
4. Form compounds that are commonly brightly colored [e.g., Cr(III)chloride is violet]. This occurs because lower energy electrons movefrom a lower energy electrons move formula lower energy d orbitalto higher energy d orbitals resulting in energy being taken in. Whenthese electrons return to their original position, they release specificenergies producing light of specific colors.
5. Like the main group metals, they form salts. However, where themain group salts will have cations that balance anions [e.g., halite
TABLE 2.1B
Chromium Electron Binding Energies
Label Orbital
eV
K 1s 5989L
I
2s 696L
II
2p
1
/
2
583.8L
III
2p
1
/
2
574.1M
I
3s 74.1M
II
3p
1
/
2
42.2M
III
3p
3
/
2
42.2
TABLE 2.1C
Chromium Effective Nuclear Charges
Orbital Z
eff
Orbital Z
eff
Orbital Z
eff
Orbital
1s 23.41 — — — —2s 16.98 2p 20.08 — — —3s 12.37 3p 11.47 3d 9.76 —4s 5.13 4p — 4d — 4f5s — 5p — 5d — —6s — 6p — — — —7s — — — — — —
TABLE 2.1D
Chromium Ionization Energies
Ionization State kJ/molIonization
State kJ/mol
Cr
0
–Cr
+
652.7 Cr
5
+
–Cr
6
+
8,738Cr
+
–Cr
2
+
1,592 Cr
6
+
–Cr
7
+
15,550Cr
2
+
–Cr
3
+
2,987 Cr
7
+
–Cr
8
+
17,830Cr
3
+
–Cr
4
+
4,740 Cr
8
+
–Cr
9
+
20,220Cr
4
+
–Cr
5
+
6,640 Cr
9
+
–Cr
10
+
23,580
Note:
Values in bold involve the removal of innershell electrons; for references see Table 2.1a.
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Chemistry, Geochemistry, and Geology of Chromium
29
or sodium chloride (NaCl) when dissolved in water forms an ionicsolution of Na
+
+
Cl
−
], transition metals are more likely to formcomplex ions with varying predominantly negative valences [e.g.,Cd(OH)
42
−
and CrO
42
−
] (Royal Society of Chemistry, 2000).
At ambient temperatures (20 to 25
°
C), metallic chromium is very hard,brittle,* blue-white to steel-gray, corrosion resistant, and capable of taking ahigh polish (ChemGlobe, 2000). Chromium can also be considered as botha heavy metal and trace element; heavy metals are those with densitiesgreater than 5 g/cm
3
at ambient temperature (de Haan and Bolt, 1979). Inmany cases, in the natural environment (i.e., in soils, rocks, etc.), chromiumcan also be considered as a trace element in that a trace element is definedas any chemical element that has a solid phase mass concentration less than100 ppm (Sposito, 1989).
2.1.3 Ionic Radii
The radius of the neutral chromium atom is 0.130 nm (Chang, 1994); chro-mium ionic radii vary from 0.04 nm in Cr(VI) to 0.094 nm in Cr(II). Radiivariations depend on coordination type, species, and spin (Winter, 2001;Table 2.3). Ionic radii are important in determining ionic substitution forvarious chromium species.
TABLE 2.2
Electronic Configuration of Elements in Period 4
AtomicNo.
Element Name
Subshell
1s 2s 2p 3s 3p 3d 4s 4p 4d 4f
21 Sc 2 2 6 2 6 1 2 — — —22 Ti 2 2 6 2 6 2 2 — — —23 V 2 2 6 2 6 3 2 — — —24 Cr 2 2 6 2 6 5 1 — — —25 Mn 2 2 6 2 6 5 2 — — —26 Fe 2 2 6 2 6 6 2 — — —27 Co 2 2 6 2 6 7 2 — — —28 Ni 2 2 6 2 6 8 2 — — —29 Cu 2 2 6 2 6 10 1 — — —30 Zn 2 2 6 2 6 10 2 — — —
Note:
By energy 4s fills before the 3d as in Cr = 1s
2
2s
2
2p
6
3s
2
3p
6
4s
1
3d
5
; for references see Table 2.1a.
* Chromium metal’s reported brittleness in most of the literature may be caused by oxidizedimpurities. Pure chromium metal is extremely susceptible to extracting and combining withatmospheric oxygen. Therefore, it is almost impossible to have pure chromium in an oxygen-richatmosphere (see Kohl, 1967).
L1608_C02.fm Page 29 Thursday, July 15, 2004 6:57 PM
30
Chromium(VI) Handbook
2.1.4 Oxidation States
Oxidation states in the transition metals are important in that transition metalions that have charges greater than
+
3 cannot exist in aqueous solution.Chromium oxidation states range from
−
4 to
+
6 (Table 2.4). The differentoxidation states are important in determining what chromium compoundsform in the environment (Smith, 1972). Oxidation states
−
2,
−
1, 0, and
+
1primarily occur in synthetic organic-chromium compounds such as the chro-mium carbonyls, chromium bipyridine, carbonyl nitrosyls, and organome-tallic complexes (Kotz et al., 2000; Luis, 2001).
Only three oxidation states are readily found in nature; these are:
1. Cr(0) which occurs in metallic or native chromium2. Cr(III) which occurs in chromic compounds3. Cr(VI) which occurs in chromate and dichromate compounds
Cr(0) is rarely found in the natural environment, although many referencesindicate that it does not occur. However, native chromium occurs as metallicinclusions in cryptocrystalline diamonds (carbonado) from kimberlite pipesin the Siberian Yakutia diamond deposits of Russia (Gorshkov et al., 1996).Native chromium also has been found in vein deposits from Sichuan, China(Guisewite, 2001), in meteorites such as the Agpalilik meteorite fragmentfrom Cape York, Greenland, and as metal alloys in placer deposits (seeTable 2.8 in Section 2.2.2).
Cr(III) occurs as insoluble chromium oxide (Cr
2
O
3
) and chromium hydrox-ide [Cr(OH)
3
]; it also occurs as soluble chromium hydroxide cations: CrOH
2
+
and Cr(OH)
2
+
. Cr(VI) generally occurs as soluble dichromate (Cr
2
O
72
−
) andchromate (CrO
42
−
) anions.
TABLE 2.3
Chromium Ionic Radii
OxidationState
CoordinationTypea Species Spin
Radius (nm)
Cr(IV) 4 Tetrahedral 0.055Cr(V) 4 Tetrahedral 0.0485Cr(VI) 4 Tetrahedral 0.040Cr(II) 6 Octahedral Low 0.087Cr(II) 6 Octahedral High 0.094Cr(III) 6 Octahedral Low 0.0755Cr(IV) 6 Octahedral Low 0.069Cr(V) 6 Octahedral Low 0.063Cr(VI) 8 Octahedral Low 0.058Cr(V) 0.071
a Coordination type refers to covalent bonding.
Source: Winter: WebElements (2001).
L1608_C02.fm Page 30 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 31
2.1.5 Stable and Radioactive Isotopes
Currently, there are 26 known chromium isotopes (Table 2.5), of which fourare stable (nonradioactive), naturally occurring isotopes (ChemGlobe, 2000;Winter, 2001; LBNL, 2002). These include chromium-50 (50Cr), 52Cr, 53Cr, and54Cr; their naturally occurring abundances are 4.345%, 83.789%, 9.509%, and2.465%, respectively (Winter, 2001).
Stable chromium isotopes are known to fractionate (Table 2.5), that is, whenone isotope is preferentially enriched over another relative to a known stan-dard, which represents its natural abundance (see footnote in Section 3.6).There are several environmental and geologic controlling processes in sta-ble isotopic fractionation; these may include (1) isotopic exchange reactions,(2) evaporation and condensation, (3) melting and crystallization of rocks,(4) adsorption and desorption, (5) mass dependent diffusion, (6) tempera-ture, (7) ultrafiltration in water-rock reactions, and (8) the preference for somebiological organisms in concentrating lighter over heavier isotopes (Hurst,1991). By experimental methods, Ottonello (2002) has identified chromiumfractionation under various conditions.
TABLE 2.4
Chromium Oxidation States
Oxidation State
Example Compound
Name Formula
−2 Sodium chromium (−II) carbonyl Na2[Cr(CO)5]−1 Sodium chromium (−I) carbonyl Na2[Cr2(CO)10]
0Chromium (0) (metal)Chromium (0) carbonyl
Cr0
Cr(CO)6
+1 Chromium bipyrydil (=L) [Cr(L)3]
+2
Chromium oxideChromium difluorideChromium dichlorideChromium sulfide
CrOCrF2
CrCl2
CrS
+3
Chromium oxideChromium trifluorideChromium trichlorideChromium hydroxide
Cr2O3
CrF3
CrCl3
Cr(OH)3
+4Chromium dioxide Chromium tetrafluoride
CrO2
CrF4
+5Barium chromate (V)Chromium pentafluoride
Ba3(CrO4)2
CrF5
+6
Barium chromate (VI)Chromate anionSodium dichromateDichromate anion
BaCrO4
Cr2O7−
CrO42−
Cr2O72−
Note: Oxidation states in bold are those commonly found in min-erals and compounds in the natural environment.
Source: Modified from USEPA (1984); Marques et al. (1999).
Au: Pleasecheck thechange.
L1608_C02.fm Page 31 Thursday, July 15, 2004 6:57 PM
32 Chromium(VI) Handbook
TAB
LE 2
.5
Chr
omiu
m N
uclid
e (I
soto
pe)
Prop
erti
es
Isot
ope
Ato
mic
Mas
s (m
a/u
)
Nat
ura
l A
bu
nd
ance
(a
tom
%)
Nu
clea
r S
pin
(I)
Nu
clea
r M
agn
etic
M
omen
t(µ
n/µ
N)
Hal
f L
ife
(t1/
2)M
ode
of
Dec
ayD
ecay
s to
Dec
ay E
ner
gy
(MeV
)
42C
r—
Art
ifici
al—
——
——
—43
Cr
43.9
8556
Art
ifici
al(3
/2+
)—
0.
21 s
ecε
43V
15.8
9043
Cr:
met
a st
ate
Art
ifici
al(3
/2+
)—
0.
21 s
ecε
+ α
39Sc
15.7
0043
Cr:
met
a st
ate:
0.0
00 M
eVA
rtifi
cial
——
0.
21 s
ecε
+ p
41Sc
11.9
30
44C
r43
.985
56A
rtifi
cial
0+—
0.
53 s
ecε
+ p
43Ti
8.50
044
V10
.310
45C
r44
.979
11A
rtifi
cial
7/2–
—
0.05
sec
ε +
β+
44Ti
10.8
50ε
45V
12.4
6046
Cr
45.9
6836
Art
ifici
al0+
—
0.26
sec
ε +
β+
46V
7.60
347
Cr
46.9
6290
5A
rtifi
cial
3/2–
—
0.51
sec
ε +
β+
47V
7.45
148
Cr
47.9
5403
3A
rtifi
cial
0+—
21.5
6 h
ε48
V1.
659
49C
r48
.951
338
Art
ifici
al5/
2–0.
476
42.3
min
ε49
V2.
631
50C
r49
.946
0464
4.34
50+
—R
el. S
tabl
e:1.
8 ×
1017
yr
ε50
Ti—
51C
r50
.944
768
7/2–
−0.9
3427
.702
5 d
ε51
V0.
753
52C
r51
.940
5098
83.7
890+
—St
able
NA
NA
NA
53C
r52
.940
6513
9.50
93/
2––0
.474
54St
able
NA
NA
NA
54C
r53
.938
8825
2.46
50+
—St
able
NA
NA
NA
55C
r54
.940
842
Art
ifici
al3/
2–—
3.
497
min
β−55
Mn
2.60
356
Cr
55.9
4064
3A
rtifi
cial
0+—
5.
94 m
inβ−
56M
n1.
617
57C
r56
.943
44A
rtifi
cial
3/2–
, 5/
2–, 7
/2–
—21
.1 s
ecβ−
57M
n5.
090
58C
r57
.944
12A
rtifi
cial
0+0.
0834
7.
0 se
cβ−
58M
n3.
970
59C
r58
.949
Art
ifici
al—
—
0.74
sec
β− 59
Mn
7.70
0
L1608_C02.fm Page 32 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 33
60C
r59
.95
Art
ifici
al0+
—
0.57
sec
β− 60
Mn
5.90
061
Cr
60.9
54A
rtifi
cial
(5/
2–)
—
0.27
0 se
cβ
−61
Mn
8.80
062
Cr
—A
rtifi
cial
0+—
0.
190
sec
β−—
—63
Cr
—A
rtifi
cial
(1/
2–)
—
0.19
0 se
cβ−
——
64C
r—
Art
ifici
al0+
—
0.11
0 se
cβ−
——
65C
r64
.97
Art
ifici
al(1
/2−
)—
—
β−
(?)
——
66C
r—
Art
ifici
al0+
——
β−—
—67
Cr
—A
rtifi
cial
(1/
2–)
——
β−—
—
Not
e:Is
otop
es i
n bo
ld a
re s
tabl
e no
n-ra
dio
acti
ve i
soto
pes
. ε =
ele
ctro
n ca
ptu
re; α
= a
lpha
em
issi
on β
− =
beta
em
issi
on; β
+ =
pos
itro
n em
issi
on;
p =
pro
ton
emis
sion
. µn/
µ N =
mag
neti
c m
omen
t in
nu
clea
r m
agne
tron
s. N
A =
not
ap
pli
cabl
e. s
ec =
sec
ond
s, m
in =
min
ute
s, h
= h
ours
, d =
day
s,
y =
year
s. D
ash
(—)
ind
icat
es n
o av
aila
ble
dat
a. V
alu
es i
n p
aren
thes
is a
re t
enta
tive
.
Sour
ces:
Win
ter:
Web
Ele
men
ts (
2001
); M
arqu
es e
t al
. (19
99);
Bar
bala
ce e
t al
. (20
01);
Law
renc
e B
erke
ley
Nat
iona
l Lab
orat
orie
s (2
002)
.
L1608_C02.fm Page 33 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 34
Radioactive isotopes of chromium have been artificially produced. Mosthave very short half-lives (t1/2). For example, Table 2.5 shows that the t1/2 forisotopes from 42Cr to 47Cr and from 57Cr to 64Cr are much less than 1 sec.
Chromium isotope studies have been important in determining the ageof solids (planetesimals) first formed in the solar nebula (Carlson andLugmair, 2000) and in investigations of the solar wind (Kitts et al., 2002).Stable isotope fractionation may be important for forensic geochemicalinvestigations (see Section 2.6).
2.1.6 Characteristics of Chromium Compounds
Chromium can be combined with various nonmetals (oxygen, fluorine, chlo-rine, etc.) and polyatomic anions (nitrate, sulfate, etc.), forming relativelystable, soluble and insoluble compounds (Table 2.6). More common are Cr(III)compounds such as chromium tribromide (insoluble), chromium nitrate (sol-uble), chromic hydroxide (insoluble), and chromic oxide (insoluble). In thechemical production industry, most chromium chemicals are produced fromsodium dichromate, which is the principal feedstock. Chemicals made fromsodium dichromate include chromic acid, chromic oxide, and potassiumdichromate (Papp, 2000).
Most chromium compounds are brightly colored and these colors arereflected in synonyms for their respective compounds. For example, basicchromium sulfate is known as chrome tan, chromic oxide is known as chromegreen, barium chromate is known as baryta yellow or lemon chrome, basiclead chromate is known as chrome orange and chrome red, calcium chromateis known as calcium chrome yellow, and lead chromate is also known aschrome green. All chromium compounds are considerably denser than waterwith specific gravities ranging from 1.77 (for hydrated chromium sulfate) to6.10 [for chromium(II) selenide] (Dean, 1992; ChemIDplus, 2001; Chem-finder, 2001). Therefore, saturated and very concentrated chromium com-pound solutions would tend to sink through the groundwater column.
2.2 Natural Chromium Concentrations
As with other elements in the periodic table, chromium concentrations innatural substances are quite variable. Chromium preferentially concentratesin various rocks throughout the earth’s crust with concentrations dependenton the rock’s origin and source (Table 2.7). Chromium concentrations arealso quite variable in secondary geochemical environments, particularly insoils, sediments, and stream and lake water. Concentrations may signifi-cantly vary because of anthropogenic influences and inputs, largely fromsmelting of chromium ore and the burning of fossil fuels such as coal andpetroleum products.
L1608_C02.fm Page 34 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 35
TAB
LE 2
.6
Phys
icoc
hem
ical
Pro
pert
ies
of S
ome
Chr
omiu
m C
ompo
und
s
Com
pou
nd
Nam
esan
dS
ynon
yms
Form
ula
CA
S
Nu
mb
er
Mol
ar
Mas
s (g
/mol
)D
ensi
ty(w
ater
= 1
)M
elti
ng
Poi
nt
(°°°°C
)B
oili
ng
Poi
nt
(°°°°C
)
Aq
ueo
us
Sol
ub
ilit
y at
20°°°° C
(m
g/l)
Ph
ysic
alD
escr
ipti
on
Chr
omiu
m:
(II)
acet
ate
Chr
omiu
m a
ceta
teC
r(C
2H3O
2)2
1759
3-70
-317
0.10
1.79
Solu
ble
(II)
ace
tate
hy
dra
teC
hrom
ium
ace
tate
; ch
rom
ium
ace
tate
m
onoh
ydra
te
C4H
8CrO
5;C
r(C
2H3O
2)2
· H2O
628-
52-4
188.
101
1.79
Red
dis
h- b
row
npo
wd
er; r
edm
onoc
linic
cry
stal
s(I
II)a
ceta
teC
hrom
ium
ace
tate
; ch
rom
ic a
ceta
te;
chro
miu
m t
riac
etat
e
Cr(
C2H
3O2)
3 ·
12H
2O10
66-3
0-4
229.
1295
Slig
htly
So
lubl
eG
rayi
sh--
gree
n po
wd
eror
vio
let
plat
es
(III
)ace
tate
he
xahy
dra
teC
hrom
ium
(II
I) a
ceta
tehe
xahy
dra
teC
r(C
2H3O
2)3
1066
-30-
428
5.22
6So
lubl
eB
lue
need
les
acet
ylac
eton
ate
Chr
omiu
m
acet
ylac
eton
ate
[CH
3CO
CH
C
(CH
3)O
] 3C
r21
634
0In
solu
ble
Purp
le p
owd
er o
rre
dd
ish-
viol
et c
ryst
als
amm
oniu
m
sulf
ate
Chr
omiu
m a
mm
oniu
m
sulf
ate
·12
hyd
rate
CrN
H4(
SO4)
·12H
2O1.
72
94So
lubl
eG
reen
pow
der
or
dee
p vi
olet
cry
stal
san
tim
onid
eC
hrom
ium
ant
imon
ide
CrS
b21
679-
31-2
349.
324
7.11
1110
–122
0H
exag
onal
cry
stal
sar
seni
de
Chr
omiu
m a
rsen
ide
Cr 2
As
1225
4-85
-217
8.91
47.
04Te
trah
edra
l cry
stal
sbr
omid
eC
hrom
ium
bro
mid
eC
rBr 3
1003
1-25
-129
1.70
84.
6811
30So
lubl
e in
hot
H
2OD
ark
gree
n he
xago
nal
crys
tals
(III
) br
omid
e he
xahy
dra
teC
hrom
ium
bro
mid
e he
xahy
dra
teC
r(H
2O) 6
Br 3
1003
1-25
-139
9.79
9So
lubl
eV
iole
t hy
dro
scop
iccr
ysta
lsbo
rid
eC
hrom
ium
bor
ide
CrB
1200
6-79
-062
.807
6.1
2100
Ref
ract
ory,
or
thor
hom
bic
crys
tals
(IV
)bor
ide
Chr
omiu
m b
orid
eC
rB2
1200
7-16
-873
.618
5.22
2200
Ref
ract
ory
solid
; he
xago
nal c
ryst
als
bori
de
Chr
omiu
m b
orid
eC
r 5B
312
007-
38-4
292.
414
6.10
1900
(II)
brom
ide
Chr
omiu
m b
rom
ide;
ch
rom
ium
dib
rom
ide
CrB
r 210
049-
25-9
211.
804
4.23
684
2So
lubl
eW
hite
mon
oclin
ic
crys
tals
; for
ms
blue
aq
ueou
s so
luti
on
(Con
tinu
ed)
L1608_C02.fm Page 35 Thursday, July 15, 2004 6:57 PM
36 Chromium(VI) Handbook
TAB
LE 2
.6
(Con
tinu
ed)
Com
pou
nd
Nam
es a
nd
S
ynon
yms
Form
ula
CA
S
Nu
mb
er
Mol
ar
Mas
s(g
/mol
)D
ensi
ty
(wat
er =
1)
Mel
tin
g P
oin
t (°
C)
Boi
lin
g P
oin
t (°
C)
Aq
ueo
us
Sol
ub
ilit
y at
20
°C
(mg/
l)P
hys
ical
D
escr
ipti
on
(III
)bro
mid
eC
hrom
ium
tri
brom
ide
CrB
r 310
031-
25-1
291.
708
4.68
081
2; 1
130
(?)
Inso
lubl
e;
Solu
ble
in h
ot
H2O
Oliv
e gr
een
or d
ark
gree
n so
lid
(IV
)bro
mid
eC
hrom
ium
te
trab
rom
ide
CrB
r 423
098-
84-2
371.
612
——
——
Gas
(III
)bro
mid
e he
xahy
dra
teC
hrom
ium
bro
mid
e he
xahy
dra
teC
rBr 3
·6H
2O10
060-
12-5
266.
445
Solu
ble
Vio
let
crys
tals
; hy
dro
scop
icca
rbid
eC
hrom
ium
car
bid
eC
r 3C
212
012−
35−0
180.
010
6.65
1890
–189
538
00In
solu
ble
Gra
y, o
rtho
rhom
bic
crys
tals
carb
ide
chro
miu
m c
arbi
de
Cr 2
3 C
612
105-
81-6
carb
onyl
Chr
omiu
m c
arbo
nyl;
chro
miu
m
hexa
carb
onyl
Cr(
CO
) 613
007-
92-6
220.
058
1.77
dec
: 11
0–13
012
0In
solu
ble
<1,
000
Whi
te c
ryst
allin
e so
lid
carb
onat
eC
hrom
ium
car
bona
te;
chro
mus
car
bona
teC
rCO
32.
75Sl
ight
ly
solu
ble
Gra
yish
-blu
e am
orph
ous
pow
der
(II)
chlo
rid
eC
hrom
ium
dic
hlor
ide;
ch
rom
us c
hlor
ide
CrC
l 210
049-
05-5
122.
902
2.87
881
5–82
411
20V
ery
solu
ble
Lus
trou
s w
hite
nee
dle
s or
fus
ed fi
brou
s m
ass;
hy
dro
scop
ic(I
II)c
hlor
ide
Chr
omiu
m t
rich
lori
de
(a)
CrC
l 3 (b
) C
rCl 3
·6H
2O10
025-
73-7
158.
355
(a)
2.76
(b)
2.87
011
5013
00In
solu
ble
in
cold
H2O
; sl
ight
ly
solu
ble
in h
ot
H2O
Red
dis
h-vi
olet
cr
ysta
lline
sol
id;
hyd
rosc
opic
(IV
)chl
orid
eC
hrom
ium
te
trac
hlor
ide
CrC
l 415
597-
88-3
193.
807
0.00
85 (
gas)
dec
: >60
0St
able
at
high
te
mpe
ratu
res
(II)
chlo
rid
e te
trah
ydra
teC
hrom
ium
chl
orid
e te
trah
ydra
teC
r(H
2O) 4
Cl 2⋅
4H
2O13
931-
94-7
267.
023
dec
: 51
Solu
ble
(II)
coba
ltC
obal
t ch
rom
ite
CoC
r 2O
413
455-
25-9
226.
923
5.14
Inso
lubl
eB
luis
h-gr
een
cubi
c cr
ysta
ls
L1608_C02.fm Page 36 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 37
(II)
chro
mit
eC
oppe
r ch
rom
ite
CuC
r 2O
412
018-
10-9
231.
536
5.4
Inso
lubl
eG
rayi
sh-b
lack
te
trah
edra
l cry
stal
s(I
I)fl
uori
de
Chr
omiu
m fl
uori
de;
ch
rom
ium
difl
uori
de;
ch
rom
us fl
uori
de
CrF
210
049-
10-2
89.9
928
3.79
089
413
00Sl
ight
ly
solu
ble
Blu
e-gr
een
mon
oclin
ic
crys
tals
; anh
ydro
us
(III
)fluo
rid
eC
hrom
ium
tri
fluo
rid
eC
rF3
7788
-97-
810
8.99
13.
814
04In
solu
ble
Gre
en c
ryst
als
(III
)fluo
rid
e tr
ihyd
rate
Chr
omiu
m fl
uori
de
trih
ydra
teC
rF3·3
H2O
1667
1-27
-516
3.03
72.
2So
lubl
eG
reen
hex
agon
al
crys
tals
(IV
)fluo
rid
eC
hrom
ium
tet
rafl
uori
de
CrF
410
049-
11-3
127.
990
2.9
277
400
Gre
en t
o vi
olet
cr
ysta
lline
sol
id(V
)fluo
rid
eC
hrom
ium
pe
ntafl
uori
de
CrF
514
884-
42-5
146.
988
3411
7R
ed o
rtho
rhom
bic
to
crim
son
crys
talli
ne
solid
(VI)
fluo
rid
eC
hrom
ium
hex
afluo
rid
eC
rF6
1384
3-28
-216
5.98
7d
ec: −
100
−100
Solu
ble
Yello
w c
ryst
allin
e so
lid;
stab
le a
t lo
w
tem
pera
ture
s(I
II)f
orm
ate
6-w
ater
Hyd
rate
d c
hrom
ium
fo
rmat
eC
r(C
HO
2)3
·6H
2O29
5.15
Solu
ble
(III
)hyd
roxi
de
Chr
omic
hyd
roxi
de;
ch
rom
ium
tr
ihyd
roxi
de
Cr(
OH
) 313
08-1
4-1
103.
0179
dec
: >10
0In
solu
ble
(III
)hyd
roxi
de
trih
ydra
teC
hrom
ium
hyd
roxi
de
trih
ydra
teC
r(O
H) 3
·3H
2O13
08-1
4-1
157.
063
Inso
lubl
eB
luis
h-gr
een
pow
der
(II)
iod
ide
Chr
omiu
m io
did
eC
rI2
1347
8-28
-930
5.80
55.
100-
5.32
868
1100
Solu
ble
Red
dis
h-br
own
crys
talli
ne s
olid
(III
)iod
ide
Chr
omiu
m t
riio
did
eC
rI3
1356
9-75
-043
2.71
5.30
0d
ec: 5
0050
0Sl
ight
ly
solu
ble
Dar
k gr
een
crys
talli
ne
solid
(IV
)iod
ide
Chr
omiu
m t
etra
iod
ide
CrI
423
518-
77-6
559.
614
(II)
iron
Iron
chr
omit
eFe
Cr 2
O4
1308
-31-
222
3.83
55.
0B
lack
cub
ic c
ryst
als
(III
)nit
rate
Chr
omic
nit
rate
Cr(
NO
3)3
1354
8-38
-423
8.01
0760
Ver
y so
lubl
eG
reen
, hyd
rosc
opic
po
wd
er(I
II)n
itra
te
9-w
ater
Hyd
rate
d c
hrom
ium
ni
trat
e; c
hrom
ium
ni
trat
e no
nahy
dra
te
Cr(
NO
3)3·9
H2O
7789
-02-
840
0.14
81.
8066
.3 (
dec
: 10
0)2,
080,
000
Gre
enis
h bl
ack
to
purp
le r
hom
bic
(mon
oclin
ic)
crys
tals
(Con
tinu
ed) Au:
Deletion OK?
L1608_C02.fm Page 37 Thursday, July 15, 2004 6:57 PM
38 Chromium(VI) Handbook
TAB
LE 2
.6
(Con
tinu
ed)
Com
pou
nd
Nam
es a
nd
S
ynon
yms
Form
ula
CA
S
Nu
mb
er
Mol
ar
Mas
s(g
/mol
)D
ensi
ty
(wat
er =
1)
Mel
tin
g P
oin
t (°
C)
Boi
lin
g P
oin
t (°
C)
Aq
ueo
us
Sol
ub
ilit
y at
20°C
(m
g/l)
Ph
ysic
al
Des
crip
tion
(III
)nit
rid
eC
hrom
ium
nit
rid
e;
chro
miu
m
mon
onit
rid
e
CrN
2409
4-93
-766
.003
5.9
dec
: 108
0G
ray
crys
talli
ne s
olid
nitr
ide
Chr
omiu
m n
itri
de
Cr 2
N12
053-
27-9
117.
999
6.8
1650
Hex
agon
al c
ryst
als
oxal
ate
Chr
omus
oxa
late
m
onoh
ydra
te;
chro
miu
m(I
I) o
xala
te
mon
ohyd
rate
CrC
2O4⋅H
2O81
4-90
-415
8.03
12.
468
Solu
ble
Yello
wis
h-gr
een
crys
talli
ne p
owd
er
(III
)oxi
de
Chr
omia
; chr
omic
ox
ide;
dic
hrom
ium
tr
ioxi
de;
chr
omiu
m
sesq
uiox
ide;
gre
en
cinn
abar
Cr 2
O3
1308
-38-
915
1.99
025.
2123
30; 2
435;
24
50
~30
00; 4
000
Inso
lubl
eD
ark
gree
n, a
mor
phou
s po
wd
er f
orm
ing
hexa
gona
l cry
stal
s up
on h
eati
ng;
hyd
rosc
opic
(II)
(III
)oxi
de
Chr
omiu
m o
xid
eC
r 3O
412
018-
34-7
219.
968
6.1
Cub
ic c
ryst
als
(IV
)oxi
de
Chr
omiu
m o
xid
e;
chro
miu
m d
ioxi
de
CrO
212
018-
01-8
83.9
948
4.89
dec
: 400
(app
roxi
ma
te; l
oses
O2)
Inso
lubl
eB
row
nish
bla
ck a
cicu
lar
crys
talli
ne (t
etra
gona
l)
solid
(VI)
oxid
eC
hrom
ic t
riox
ide;
ch
rom
ium
anh
ydri
de
CrO
313
33-8
2-0
99.9
942
2.70
019
0; 1
95;
197
dec
: ~
250
617,
000
Dar
k re
d o
rtho
rhom
bic
crys
talli
ne (
flak
es o
r po
wd
er)
solid
; hy
dro
scop
icpe
rchl
orat
eC
hrom
ic p
erch
lora
te;
chro
miu
m p
erch
lora
teC
r(C
lO4)
313
537-
21-8
(III
)pho
spha
teC
hrom
ic p
hosp
hate
CrP
O4
7789
-04-
014
6.96
74.
6>
1800
Inso
lubl
eB
lue
orth
orho
mbi
c cr
ysta
ls(I
II)p
hosp
hate
he
mih
epta
-hy
dra
te
chro
miu
m p
hosp
hate
he
mih
epta
hyd
rate
CrP
O4
⋅ 3.5
H2O
8435
9-31
-921
0.02
12.
15In
solu
ble
Blu
ish-
gree
n po
wd
er
(III
)pho
spha
te
hyd
rate
Chr
omiu
m p
hosp
hate
hy
dra
teC
rPO
4⋅4
H2O
7789
-04-
02.
12@
14°C
Inso
lubl
eG
reen
cry
stal
s
L1608_C02.fm Page 38 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 39
(III
)pho
spha
te
hexa
hyd
rate
Chr
omiu
m p
hosp
hate
he
xahy
dra
teC
rPO
4 ⋅6
H2O
8435
9-31
-925
5.05
92.
121
dec
: >50
0In
solu
ble
Vio
let
crys
tals
phos
phid
eC
hrom
ium
pho
sphi
de
CrP
2634
2-61
-082
.970
5.25
Ort
horh
ombi
c cr
ysta
lspo
tass
ium
su
lfat
ePo
tass
ium
chr
omic
su
lfat
eC
rK(S
O4)
210
141-
00-1
1.81
389
Solu
ble
Dar
k vi
olet
-red
cry
stal
s
(III
)pot
assi
umsu
lfat
ed
odec
ahyd
rate
Chr
ome
alum
; chr
ome
alum
(do
deca
hydr
ate)
;po
tass
ium
bis
ulfa
te
12-w
ater
CrK
(SO
4)2
⋅12H
2O77
88-9
9-0
499.
405
1.82
622
0,00
0Pu
rple
to
viol
et-b
lack
cu
bic
crys
tals
(II)
sele
nid
ech
rom
ium
sel
enid
eC
rSe
1205
3-13
-313
0.95
66.
100
~15
00H
exag
onal
cry
stal
ssi
licid
ech
rom
ium
sili
cid
eC
r 3Si
1201
8-36
-918
4.07
46.
417
70C
ubic
cry
stal
ssi
licid
ech
rom
ium
sili
cid
eC
rSi 2
1201
8-09
-610
8.16
74.
9114
90G
ray
hexa
gona
l cry
stal
s(I
II)s
ulfa
tech
rom
ium
sul
fate
Cr 2
(SO
4)3
1010
1-53
-839
2.18
33.
1In
solu
ble
Red
dis
h-br
own
hexa
gona
l cry
stal
ssu
lfat
e pe
ntah
ydra
tech
rom
ium
(II)
sul
fate
pe
ntah
ydra
teC
rSO
4 ⋅5
H2O
1382
5-66
-023
8.13
6So
lubl
eB
lue
crys
tals
sulf
ate
7-w
ater
Hyd
rate
d c
hrom
ium
su
lfat
eC
rSO
4 ⋅7
H2O
274.
1722
9,00
0
sulf
ate
12-w
ater
Dic
hrom
ium
sul
fate
, 12
-hyd
rate
; d
ichr
omiu
m t
risu
lfat
e
CrS
O4
⋅12H
2O10
101-
53-8
608.
3472
Inso
lubl
ePe
ach-
colo
red
sol
id
(III
)sul
fide
Chr
omiu
m s
ulfid
e;
dic
hrom
ium
tri
sulfi
de
Cr 2
S 312
018-
22-3
200.
190
3.8
Bro
wn
to b
lack
cr
ysta
lline
sol
idst
eara
teC
hrom
ium
ste
arat
eC
r(C
18H
35O
2)3
95–1
00D
ark
gree
n po
wd
er(I
II)t
ellu
rid
eC
hrom
ium
tel
luri
de
Cr 2
Te3
1205
3-39
-348
6.79
7.0
~13
00H
exag
onal
cry
stal
szi
ncZ
inc
chro
mit
eZ
nCr 2
O4
1201
8-19
-823
3.38
5.29
Gre
en c
ubic
cry
stal
s
Chr
omat
e:
amm
oniu
mA
mm
oniu
m c
hrom
ium
ox
ide;
dia
mm
oniu
m
chro
mat
e
(NH
4)2C
r 2O
477
88-9
8-9
152.
0702
185
Solu
ble
Yello
w c
ryst
als
bari
um(V
)B
ariu
m c
hrom
ate
Ba 3
(CrO
4)2
1234
5-14
-164
3.96
85.
25So
lubl
eG
reen
ish-
blac
k he
xago
nal c
ryst
als
(Con
tinu
ed)
L1608_C02.fm Page 39 Thursday, July 15, 2004 6:57 PM
40 Chromium(VI) Handbook
TAB
LE 2
.6
(Con
tinu
ed)
Com
pou
nd
Nam
es a
nd
S
ynon
yms
Form
ula
CA
S
Nu
mb
er
Mol
ar
Mas
s (g
/mol
)D
ensi
ty
(wat
er =
1)
Mel
tin
g P
oin
t (°
C)
Boi
lin
g P
oin
t (°
C)
Aq
ueo
us
Sol
ub
ilit
y at
20
°C
(mg/
l)P
hys
ical
D
escr
ipti
on
bari
um(V
I)B
ariu
m c
hrom
ate
BaC
rO4
1042
94-4
0-3
253.
3236
4.50
Inso
lubl
eYe
llow
, ort
horh
ombi
c cr
ysta
lsca
dm
ium
Cad
miu
m c
hrom
ate
Cd
CrO
414
312-
00-0
622
8.40
54.
5In
solu
ble
Yello
w, o
rtho
rhom
bic
crys
tals
calc
ium
Cal
cium
chr
omiu
m
oxid
e ca
lciu
m
chro
mat
e
CaC
rO4
1376
5-19
-015
6.07
362.
89Sl
ight
ly
solu
ble:
<10
0 @
22
°C
Bri
ght
yello
w p
owd
er
calc
ium
d
ihyd
rate
Cal
cium
chr
omat
e d
ihyd
rate
CaC
rO4 ⋅
2H2O
1376
5-19
-019
2.10
22.
50Sl
ight
ly
solu
ble
Yello
w, o
rtho
rhom
bic
crys
tals
chro
mic
Chr
omic
aci
d;
chro
miu
m c
hrom
ate
Cr 2
(CrO
4)3
2461
3-89
-6
coba
ltC
obal
tous
chr
omat
e:ba
sic
coba
lt c
hrom
ate
CoC
rO4
2461
3-38
-517
4.92
7~
4.0
Solu
ble
Bla
ck t
o br
own
crys
tals
or
yel
low
pow
der
copp
erC
oppe
r(II
) ch
rom
ate
CuC
rO4
1354
8-42
-017
9.54
0So
lubl
eR
edd
ish-
brow
n cr
ysta
lsco
pper
Cup
ric
chro
mat
e ba
sic
CuC
rO4⋅2
CuO
⋅ 2H
2OL
ight
cho
cola
te b
row
n po
wd
erir
onIr
on(I
II)
chro
mat
eFe
2(C
rO4)
310
294-
52-7
459.
671
Inso
lubl
eye
llow
pow
der
lead
Lea
d c
hrom
ate;
chr
ome
yello
wPb
CrO
477
58-9
7-6
323.
1936
6.12
384
4In
solu
ble:
<10
0Ye
llow
ish-
oran
ge
mon
oclin
ic c
ryst
als
or
oran
ge-b
row
n po
wde
r(I
I)ch
rom
ate
(VI)
oxid
eL
ead
chr
omat
e ox
ide
PbC
rO4 ⋅
PbO
1845
4-12
-154
6.4
Inso
lubl
eR
ed p
owd
er
lithi
umL
ithi
um c
hrom
ate
Li 2C
rO4
1430
7-35
-8So
lubl
eYe
llow
cry
stal
line,
d
eliq
uesc
ent
pow
der
lithi
um
dih
ydra
teL
ithi
um c
hrom
ate
dih
ydra
teL
i 2CrO
4 ⋅2H
2O77
89-0
1-7
165.
906
2.15
dec
: 75
Ver
y so
lubl
eYe
llow
ort
horh
ombi
c cr
ysta
lsm
ercu
ryM
ercu
ry c
hrom
ate
Hg 2
CrO
4d
ec o
n he
atin
gIn
solu
ble
Bri
ck r
ed p
owd
er
L1608_C02.fm Page 40 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 41
(II)
mer
cury
Mer
cury
chr
omat
eH
gCrO
413
444-
75-2
316.
586.
06So
lubl
eR
ed m
onoc
linic
cry
stal
ssi
lver
Silv
er c
hrom
ate
Ag 2
CrO
477
84-0
1-2
331.
730
5.62
5In
solu
ble
Bro
wni
sh-r
ed
mon
oclin
ic c
ryst
als
sod
ium
Chr
omat
e of
sod
a;
sod
ium
chr
omat
eN
a 2C
rO4
7775
-11-
316
1.97
314
2.72
792
Slig
htly
so
lubl
eYe
llow
ort
horh
ombi
c cr
ysta
lsst
ront
ium
Stro
ntiu
m c
hrom
ate
SrC
rO4
7789
-06-
220
3.61
363.
9d
ecSo
lubl
eYe
llow
mon
oclin
ic
crys
tals
pota
ssiu
mPo
tass
ium
chr
omat
eK
2CrO
477
89-0
0-6
2.73
997
1–97
5V
ery
solu
ble
Yello
w o
rtho
rhom
bic
crys
tals
zinc
Bas
ic z
inc
chro
mat
eZ
nCrO
413
530-
65-9
Solid
zinc
hyd
roxi
de
Zin
c hy
dro
xid
e ch
rom
ate
1593
0-94
-6
zinc
pot
assi
umPo
tass
ium
zin
c ch
rom
ate
hyd
roxi
de
1110
3-86
-9<
1,00
0G
reen
ish-
yello
w s
olid
Chr
omyl
:
chlo
rid
eC
hrom
ium
oxy
chlo
rid
eC
rO2C
l 214
977-
61-8
154.
901.
9145
–96.
511
6–11
7re
acts
wit
h H
2Od
ark
red
, tox
ic, f
umin
g liq
uid
fluo
rid
eC
hrom
ium
oxy
fluo
rid
eC
rF2O
3su
b at
29.
6bl
ack
crys
tals
; po
lym
eriz
es o
n ex
posu
re t
o lig
ht
Dic
hrom
ates
/Bic
hrom
ates
:
amm
oniu
mA
mm
oniu
m
dic
hrom
ate;
dic
hrom
ic
acid
(NH
4)2C
r 2O
777
88-0
9-5
252.
0644
2.15
170
310,
000
Bri
ght
oran
ge c
ryst
als
bari
um
dih
ydra
teB
ariu
m d
ichr
omat
e d
ihyd
rate
BaC
r 2O
7⋅2
H2O
1003
1-16
-038
9.34
6d
ecR
eact
s w
ith
wat
erB
row
nish
-red
nee
dle
s
calc
ium
Cal
cium
bic
hrom
ate
CaC
r 2O
714
307-
33-6
256.
0678
2.13
6So
lubl
eB
row
nish
-red
cry
stal
s
calc
ium
tr
ihyd
rate
Cal
cium
dic
hrom
ate
trih
ydra
teC
aCr 2
O7⋅3
H2O
1430
7-33
-631
0.11
22.
37V
ery
solu
ble
Red
dis
h or
ange
cry
stal
s
(III
)iro
nIr
on d
ichr
omat
eFe
2(C
r 2O
7)3
1029
4-53
-875
9.65
4So
lubl
eR
edd
ish-
brow
n so
lid
(Con
tinu
ed)
L1608_C02.fm Page 41 Thursday, July 15, 2004 6:57 PM
42 Chromium(VI) Handbook
TAB
LE 2
.6
(Con
tinu
ed)
Com
pou
nd
Nam
es a
nd
S
ynon
yms
Form
ula
CA
S
Nu
mb
er
Mol
ar
Mas
s (g
/mol
)D
ensi
ty
(wat
er =
1)
Mel
tin
g P
oin
t (°
C)
Boi
lin
g P
oin
t (°
C)
Aq
ueo
us
Sol
ub
ilit
y at
20
°C(m
g/L
)P
hys
ical
D
escr
ipti
on
(II)
dic
hrom
ate
dih
ydra
teC
oppe
r d
ichr
omat
e d
ihyd
rate
CuC
r 2O
7σ⋅
2H
2O13
675-
47-3
315.
565
2.28
6V
ery
solu
ble
Red
dis
h-br
own
tric
linic
cr
ysta
lslit
hium
Lit
hium
dic
hrom
ate
Li 2C
r 2O
713
843-
81-7
2.34
@ 3
0°C
130
Solu
ble
Yello
wis
h-re
d
crys
talli
ne p
owd
erlit
hium
d
ihyd
rate
Lit
hium
dic
hrom
ate
dih
ydra
teL
i 2Cr 2
O7 ⋅
2H2O
1002
2-48
-726
5.90
12.
34d
ec: 1
30V
ery
solu
ble
Yello
wis
h-re
d
hyd
rosc
opic
cry
stal
s H
eavy
red
cry
stal
line
pow
der
mer
cury
Mer
cury
dic
hrom
ate;
m
ercu
ric
dic
hrom
ate;
m
ercu
ric
bich
rom
ate
HgC
r 2O
777
89-1
9-8
416.
58In
solu
ble
pota
ssiu
mPo
tass
ium
dic
hrom
ate;
bi
chro
mat
e of
pot
ash
K2C
r 2O
777
78-5
0-9
294.
1678
2.67
639
850
049
,000
Bri
ght
oran
gish
-red
tr
iclin
ic c
ryst
als
silv
erSi
lver
dic
hrom
ate
Ag 2
Cr 2
O7
7784
-02-
343
1.72
44.
770
Solu
ble
Red
dis
h cr
ysta
lsso
diu
mD
isod
ium
dic
hrom
ate;
+
sod
ium
dic
hrom
ate
Na 2
Cr 2
O7
1058
8-01
-926
1.96
734
3.57
356.
740
0V
ery
solu
ble
Red
hyd
rosc
opic
cr
ysta
lsso
diu
m
tetr
ahyd
rate
Sod
ium
chr
omat
e te
trah
ydra
teN
a 2C
r 2O
7 ⋅ 4H
2O10
034-
82-9
234.
035
dec
Ver
y so
lubl
eYe
llow
hyd
rosc
opic
cr
ysta
lszi
ncZ
inc
bich
rom
ate
ZnC
r 2O
714
018-
95-2
281.
3778
Not
e:g/
mol
= g
ram
s pe
r m
ole;
mg/
l = m
illig
ram
s pe
r lit
er; °
C =
deg
rees
Cel
sius
; dec
= d
ecom
pose
s; s
ub =
sub
limes
.
Sour
ce:
Dea
n (1
992)
; Che
mID
plus
(20
01);
Che
mfi
nder
(20
01).
L1608_C02.fm Page 42 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 43
TABLE 2.7
Total Chromium Concentration in Natural Substances
ParameterReported
Units Average RangeEnrichmentFactor (EF) References
Universe ppm 15 — — (21)Solar system Relative to
H = 1 × 1012
5.13 × 105 — — (17)
Sun ppm 20 — — (21)Carbonaceous meteorites mg/kg — — — (21)Mantle mass percent 0.41–0.43 — (3)Crust mg/kg 100 — — (10) (22)Igneous rocks: mg/kg 117 — 1.17 (4) (14)Ultramafic mg/kg 2000 — 20.0 (4) (14)Mafic (basalts) mg/kg 220 40–600 2.20 (4) (14)Felsic (granites) mg/kg 20 2–100 0.20 (4) (14)
Sedimentary rocks:Limestone mg/kg 10 <1–120 0.10 (4) (14)Sandstone mg/kg 35 34–90 0.35 (4) (14)Shale mg/kg 90, 120 30–590 0.9–1.20 (14)Black shale mg/kg 90, 100 26–1000 0.9–1.0 (4) (13) (14)Sediments: mg/kg 72 — — (5)Deep sea clay mg/kg 90 — 0.90 (5)Shallow water mg/kg 60 — 0.60 (5)River suspended mg/kg 100 — 1.00 (5)Stream bed (U.S.A.) mg/kg 64a <1.0–700 0.64 (17)
Soil:World mg/kg 200 — 2.00 (1)U.S.A. mg/kg 54 1–2000 0.54 (6) (14)California mg/kg 15.4 4–32 0.15 (7)Coal ppm 15 250 (max) 0.15 (11)Petroleum ppm 0.09–3.15 10.7 (max) (13)Asphalt ppm 6.0 (max) — (2)Plant (ash) ppm 9 (1)
Water:Ground µg/L — 0.04–20 — (14)Ocean µg/L 0.05–0.3 0.156–0.26 — (8) (15) (19)
(22)North Sea µg/L — 0.7 — (16)Atlantic (surface) ppb 0.18 — — (15) (17)Atlantic (deep) ppb 0.23 — — (15) (17)Pacific (surface) ppb 0.15 — — (15) (17)Pacific (deep) ppb 0.25 — — (15) (17)River (total)b µg/L 0.5b — — (5)River (dissolved) µg/L — 0.02–0.3 — (16)Great Lakes (U.S.A/Canada)
µg/L 1.0 0.2–19 (8)
Central Canada(surfacewater)
µg/L — 0.2–44 — (8) (16)
(Continued)
L1608_C02.fm Page 43 Thursday, July 15, 2004 6:57 PM
44 Chromium(VI) Handbook
TABLE 2.7
(Continued)
ParameterReported
Units Average RangeEnrichmentFactor (EF) References
Atlantic Region-Canada µg/L — 0.2–24 (8)Antarctic Lakesc µg/L — <0.6–30 — (16)Precipitation µg/L — 0.2–1.0 — (16)
Air:Arctic pg/m3 — 50–70 — (16)U.S.A. ng/m3 <300 10–50 — (8) (16)California (2000) ng/m3 4.9 1–40 — (17)
ng/m3 0.13 0.1–1.2 — (20)Canada (5 remote areas) ng/m3 0.25–0.32 (8)Netherlands ng/m3 — 2–5 — (16)Indoor (tobacco smoke) ng/m3 ~1000 — — (16)
Human tissue:mg/kg 30 — —
Blood mg/dL — 0.006–0.11 — (17)Bone ppm — 0.1–0.33 — (17)Liver ppm — 0.02–3.3 — (17)Muscle ppm — 0.024–0.84 — (17)
Food:µg/kg <10–1300 — (16)
Milk and dairy products mg/kg 0.06 — — (8)Meat mg/kg 0.07 — — (8)Cereal mg/kg 0.17 — — (8)Potatoes mg/kg 0.05 — — (8)Fruits mg/kg 0.06 — — (8)Sugars mg/kg 0.34 — — (8)Intake from food and water
µg/day 52–943 — — (16)
Total intake from air,water, and food in U.K.
µg/day 76–106 — (16)
Total intake from air, water, and ford in
Netherlands
µg/day 100 50–200 — (16)
a Median concentration.b Background concentration.c Concentration increases with depth.
Note: EF = average materials concentration/average crustal concentration.
Sources: 1. Hawkes and Webb (1962); 2. Krejci-Graf (1972); 3. Henderson (1982); 4. Thornton(1983); 5. Salomons and Förstner (1984); 6. Shacklette and Boerngen (1984); 7. Pettygrove andAsano (1985); 8. Canadian Environmental Health Directorate (1986); 9. Hem (1989); 10. Sposito(1989); 11. Finkelman (1993); 12. Manning and Gize (1993); 13. Leventhall (1993); 14. Allard(1995); 15. Donat and Bruland (1995); 16. WHO (1996); 17. Emsley (1999); 18. Rice (1999);19. California Air Resources Board (2001a); 20. California Air Resources Board (2001b); 21.Winter: WebElements (2001); 22. Firestone (2002).
L1608_C02.fm Page 44 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 45
Efforts have been made to define what is meant by the terms “natural,”“normal,” and “background” and why such distinctions are important.Fergusson (1990) notes the following:
An important reason for knowing the natural concentrations of the heavyelements, is that they provide a true reference point for estimating theextent of pollution from the elements. This is of particular importancewhen assessing the toxic effects of the elements. Natural levels allowcontemporary levels to be seen in perspective, i.e., whether they areexcessive or not.
Element concentrations currently found in the biosphere are often callednormal, which may be misinterpreted for natural. Fergusson notes that the term“typical” is more applicable to contemporary trace element concentrations. Asfor natural element concentrations, distinctions must be made between:
1. Ancient concentrations as determined in prehistoric human remainssuch as bone, teeth, and hair recovered from skeletal and mummifiedremains. True prehuman background concentrations in naturalmaterials may also be found in deep ice, sediment and peat cores,and in deep ocean waters as opposed to surface sea water which hasbeen contaminated by human inputs (e.g., streams, rivers, and theatmosphere).
2. Remote places that include the Arctic, Antarctic, Greenland, mid-ocean water, and in mountainous or alpine areas such as the Hima-laya Mountains of Tibet. These areas have had little human impact,although with increasing atmospheric pollution even remote areascan be significantly affected.
3. Mineralized areas where element concentrations are higher than thesurrounding region. Such element concentrations often surroundknown ore deposits or areas having undergone element enrichmentfrom hydrothermal (hot spring) activity. The difference in mineral-ized areas from natural background is often called “threshold con-centrations” (Hawkes and Webb, 1962). The determination of naturalbackground or threshold concentrations of metals in surface waterin mineralized areas has become a significant problem because manyformerly mined areas have been designated as Superfund sitesresulting from metals contamination produced by waste rock dumpsand tailings at mines and smelter sites. Therefore, understandingbackground concentrations derived from mineralized areas becomesimportant in distinguishing natural threshold background concen-trations from mined areas (Runnels et al., 1992).
4. Rural settings, that is, those areas away from urban environments.However, some rural areas such as farms may have element con-centrations that have been influenced by the use of fertilizers andpesticides.
Au: Figure citations in text were missing, we have sup-plied them. Please check.
L1608_C02.fm Page 45 Thursday, July 15, 2004 6:57 PM
46 Chromium(VI) Handbook
5. Urban areas, such as those within cities, which generally have ele-ment concentrations higher than natural background. This is partic-ularly true of surface soils in parks, greenbelts, and alongside roadsand highways that are impacted by vehicles using and burningpetroleum products and fuels (lubricating oil, gasoline, and diesel).
6. Industrial areas and zones where element concentrations may be con-siderably above those found in the other settings but still belowregulatory agency levels which might be considered toxic to humans.
For a detailed explanation of the above, the reader is referred to Fergusson(1990, Chap 6, pp. 166–176).
2.2.1 Mantle
In the earth’s mantle, chromium occurring as Cr2O3 may range in concentra-tions from 0.41 to 0.55% depending on the model used (Henderson, 1982).That chromium originated in the upper mantle has been fairly well establishedin the study of podiform chromites, which are believed to have almost exclu-sively originated in the upper mantle or in crust-mantle transition that occursin suprasubduction zones (Li et al., 2002).
2.2.2 Chromium Minerals
The International Minerals Association (IMA) recognized, as of mid-2002,the existence of 82 chromium minerals occurring in the natural environment(Table 2.8a); most are rather rare with some minerals unique to one minerallocality or deposit. Several chromium minerals are unique to meteorites. Themajor chromium ore mineral is chromite, a magnesium–iron–chromium–aluminum oxide, [(Mg,Fe)(Al,Cr,Fe)2O4] in which the chromic oxide contentvaries from approximately 15 to 65% due to isomorphous substitution ofchromium for iron or aluminum. Chromium concentrations in chromitegenerally average 46.46% (Sposito, 1989; Darrie, 2001; Barthelmy, 2002).
Of the 82 known chromium minerals, 23 (approximately 30%) are Cr(VI)-bearing minerals; these are in the Dana mineral classes of anhydrous chro-mates; compound chromates; compound phosphates; compound borates;compound iodates, hydroxides, and oxides; and multiple oxides.
Many chromium minerals are quite colorful (Table 2.8b); for example,uvarovite (a chromium-bearing garnet) is bright green and crocoite (an anhy-drous chromate) forms bright-reddish orange acicular crystals. Mineral col-lectors covet such chromium minerals.
2.2.3 Chromium Ore Deposits
Chromite ore is not currently commercially mined in the United States, Can-ada, or Mexico. In the Western Hemisphere it is mined only in Brazil andCuba. U.S. mining ceased in 1961, when the United States Defense Production
Au: Ok?
L1608_C02.fm Page 46 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 47
TAB
LE 2
.8A
Chr
omiu
m M
iner
als:
Che
mic
al F
orm
ula,
Cla
ssifi
cati
on, a
nd L
ocal
itie
s
Min
eral
Nam
eFo
rmu
laC
r C
onc.
(%
)
Dan
a C
lass
ifica
tion
Typ
e L
ocal
ity/
Loc
alit
ies
En
viro
nm
ent/
Rem
ark
sM
iner
al C
lass
No.
Ank
angi
teB
a(Ti
, V+
3 , C
+3 )
8O16
1.29
Mul
tipl
e ox
ides
7.9.
4.2
Ank
ang
Co.
, Sha
anxi
pr
ovin
ce, C
hina
Bar
ber
ton
ite
Mg 6
Cr 2
(CO
3)(O
H) 1
6⋅4(
H2O
)15
.90
Car
bona
tes—
hy
dro
xyl o
r ha
loge
n
16b.
6.1.
2K
aaps
che
Hoo
p, B
arbe
rton
, Tr
ansv
aal,
Rep
ublic
of S
outh
A
fric
aB
ento
rite
Ca 6
(Cr,
Al)
2(SO
4)3
(OH
) 12⋅2
6 (H
2O)
6
.03
Hyd
rate
d s
ulfa
tes
31.1
0.2.
2H
atru
rim
For
mat
ion,
Dea
d
Sea,
Isr
ael
Bra
cew
ellit
eC
r+3 O
(OH
)61
.17
Hyd
roxi
des
and
ox
ides
con
tain
ing
hyd
roxy
l
6.1.
1.5
Mer
ume
Riv
er, K
amak
usa,
M
arar
uni d
istr
ict,
Guy
ana
Bre
zina
ite
Cr 3
S 454
.88
Sulfi
des
— in
clud
ing
sele
nid
es a
nd
tellu
rid
es
2.10
.2.2
Tucs
on M
eteo
rite
, Pim
a C
o.,
AZ
Cal
sber
gite
CrN
78.7
8N
ativ
e el
emen
ts1.
1.20
.1A
gpal
ilik
frag
men
t, C
ape
York
met
eori
te, G
reen
land
Car
mic
hael
ite
(Ti,
Cr,
Fe)[
O2x
(OH
) x]
15.8
1M
ulti
ple
oxid
esw
ith
Nb,
Ta,
and
Ti
8.7.
13.1
Gar
net
Rid
ge U
ltra
mafi
cD
iatr
eme,
Col
orad
oPl
atea
u, A
Z
Mic
rosc
opic
incl
usio
ns in
m
antl
e-d
eriv
ed p
yrob
e ga
rnet
.C
asse
nd
ann
eite
Pb5(
VO
4)2(
CrO
4)2
(H2O
)
6.8
6C
ompo
und
ph
osph
ates
43.3
.2.2
Ber
esov
, Sve
rdlo
vsk
(Eka
teri
nbur
g), U
ral
Mou
ntai
ns, R
ussi
a
Ass
ocia
ted
wit
h em
brey
ite.
Cas
wel
lsilv
erit
eN
aCrS
237
.38
Sulfi
des
—in
clud
ing
sele
nid
es a
nd
tellu
rid
es
2.9.
17.1
Nor
ton
Cou
nty
met
eori
te, K
S
(Con
tinu
ed)
L1608_C02.fm Page 47 Thursday, July 15, 2004 6:57 PM
48 Chromium(VI) Handbook
TAB
LE 2
.8A
(Con
tinu
ed)
Min
eral
Nam
eFo
rmu
laC
r C
onc.
(%
)
Dan
a C
lass
ifica
tion
Typ
e L
ocal
ity/
Loc
alit
ies
En
viro
nm
ent/
Rem
ark
sM
iner
al C
lass
No.
Ch
rom
atit
eC
aCrO
433
.32
Anh
ydro
us
chro
mat
es35
.3.2
.1Je
rusa
lem
-Jer
icho
Hig
hway
, Jo
rdan
Chr
ombi
smit
eB
i 16C
rO27
1.
36O
xid
e m
iner
als
4.0.
0.0
Jial
u m
ine,
Luo
nan,
Sha
anxu
pr
ovin
ce, C
hina
Chr
omce
lad
onit
eK
CrM
g[Si
4O10
](O
H) 2
12.3
3M
ica
grou
p71
.2.2
a.15
Sred
nyay
a Pa
dm
a U
-V
dep
osit
, Sou
ther
n K
arel
ia,
Rus
sia
Foun
d in
met
asom
atiz
ed-
hyd
roth
erm
ally
alt
ered
U-V
dep
osit
s. C
r-d
omin
ant
anal
og 1
of
alum
inoc
elad
onit
e.
Form
erly
IM
A 1
999-
024.
Chr
omd
ravi
teN
aMg 3
(Cr,
Fe+
3 )6(
BO
) 33S
i 6O18
(OH
) 420
.99
Cyc
osili
cate
s:
tour
mal
ines
61.3
e.1.
11O
nega
dep
ress
ion,
Kar
elia
, R
ussi
aC
hrom
feri
de
Fe3C
r 1−x
(x
= 0
.6)
11.0
4N
ativ
e el
emen
ts1.
1.12
.2K
umak
reg
ion,
Ura
l M
ount
ains
, Rus
sia
Chr
omit
e (c
hrom
e ir
on o
re; c
hrom
ic
iron
)
Fe+
2 Cr 2
O4
46.4
6M
ulti
ple
oxid
es7.
2.3.
3St
illw
ater
, MT;
Bas
tid
e d
e la
C
arra
de,
Gas
sin,
Var
, Fra
nce
Prim
ary
ore
of c
hrom
ium
; oc
curs
in g
rani
tes
and
ot
her
igne
ous
rock
s;
occu
rs in
pla
cer d
epos
its.
Chr
omiu
mC
r10
0.00
Nat
ive
elem
ents
1.1.
12.1
Sich
uan
(Sze
chua
n), C
hina
Chr
omph
yllit
e(K
, Ba)
(Cr,
Al)
2[A
lSi 3
O10
](O
H, F
) 2Tr
ace
Mic
as—
mus
covi
te
subg
roup
71.2
.21.
11Pe
reva
l mar
ble
quar
ry; K
aper
pi
t Pok
habi
kha
Riv
er V
alle
y,
Sout
hern
Lak
e Ba
ikal
regi
on,
Sibe
ria,
Rus
sia
Slyu
dyan
ka c
ompl
ex,
Sibe
ria,
Rus
sia:
Cr-
enri
ched
laye
rs in
qua
rtz-
diop
side
-bea
ring
roc
ks.
Coc
hrom
ite
(Co,
Ni,
Fe+
2 )(C
r, A
l)2O
4
trac
eM
ulti
ple
oxid
es—
sp
inel
gro
up7.
2.3.
5B
on A
ccor
d n
icke
l dep
osit
, B
arbe
rton
, Tra
nsva
al,
Rep
ublic
of
Sout
h A
fric
a
L1608_C02.fm Page 48 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 49
Coc
roit
ePb
CrO
416
.09
Anh
ydro
us
chro
mat
es35
.3.1
.1A
del
aid
e M
ine,
Dun
das
, Ta
sman
iaSe
cond
ary
Cr
min
eral
inox
idiz
ed P
b ve
ins
infi
ltra
ted
by
Cr-
bear
ing
flui
ds
Dau
bree
lite
Fe+
2 Ce 2
S 436
.10
Sulfi
des
— in
clud
ing
sele
nid
es a
nd
tellu
rid
es
2.10
.1.1
1B
olso
nde
Map
imi m
eteo
rite
, M
exic
o
Dea
nes
mit
hit
eH
g+2H
g+2 3C
r+6 O
5S2
4.
34A
nhyd
rous
ch
rom
ates
35.4
.3.1
Cle
ar C
reek
mer
cury
min
e,
New
Id
ria
dis
tric
t, Sa
n B
enit
o C
o., C
AD
ietz
eite
Ca 2
(IO
3)2(
CrO
4)9.
52C
ompo
und
iod
ates
23.1
.1.1
Lau
taro
, Ata
cam
a D
eser
t, A
ntof
agas
ta, C
hile
Don
athi
te(F
e+2 ,
Mg)
(Cr,
Fe+
3 )2O
423
.65
Oxi
de
min
eral
s4.
0.0.
0H
estm
and
ö Is
land
, Nor
way
Mix
ture
of
two
spin
els:
on
e cu
bic
and
one
te
trag
onal
Du
kei
teB
i 24C
r+6 8O
57(O
H) 6
(H2O
) 3
6.40
Hyd
roxi
des
and
ox
ides
con
tain
ing
hyd
roxy
l
6.4.
12.1
Lav
ra d
a Po
sse,
San
Jos
e d
eB
reja
uba,
Con
ceic
ao d
oM
ato
Den
tro
Cou
nty,
Bra
zil
Foun
d o
n a
mus
eum
sp
ecim
en o
f puc
heri
te a
t D
uke
Uni
vers
ity,
D
urha
m, N
CE
doy
leri
teH
g+2 3C
r+6 O
4S2
6.
65A
nhyd
rous
ch
rom
ates
35.4
.4.1
Cle
ar C
reek
mer
cury
min
e,
New
Id
ria
dis
tric
t, Sa
n B
enit
o C
ount
y, C
AE
mb
reyi
tePb
5(C
rO4)
2(PO
4)2 ⋅
H2O
7.
05C
ompo
und
ph
osph
ates
43.4
.3.1
Cum
bria
, Eng
land
; Ber
ezov
, E
kate
rinb
urg
(Sve
rdlo
vsk)
, U
ral M
ount
ains
., R
ussi
aE
skol
aite
Cr 2
O3
68.4
2Si
mpl
e ox
ides
4.3.
1.3
Out
okum
pu, K
arel
ia, F
inla
ndFe
rchr
omid
eC
r 3Fe
1−x
(x =
0.6
)87
.47
Nat
ive
elem
ents
1.1.
12.3
Kum
ak r
egio
n, U
ral
Mou
ntai
ns, R
ussi
aFl
oren
sovi
teC
u(C
r 1.5Sb
0.5)
S 423
.59
Sulfi
des
—in
clud
ing
sele
nid
es a
nd
tellu
rid
es
2.10
.1.1
4Sl
yud
yank
a co
mpl
ex, L
ake
Bai
kal r
egio
n, Z
abai
kaly
e (T
rans
baik
al) S
iber
ia, R
ussi
a
(Con
tinu
ed)
L1608_C02.fm Page 49 Thursday, July 15, 2004 6:57 PM
50 Chromium(VI) Handbook
TAB
LE 2
.8A
(Con
tinu
ed)
Min
eral
Nam
eFo
rmu
laC
r C
onc.
(%
)
Dan
a C
lass
ifica
tion
Typ
e L
ocal
ity/
Loc
alit
ies
En
viro
nm
ent/
Rem
ark
sM
iner
al C
lass
No.
Forn
acit
e(P
b, C
u)3[
(Cr,
As)
O4]
2
(OH
)6.
93C
ompo
und
ph
osph
ates
43.4
.3.2
El K
hun
min
e, A
nara
k, I
ran
Geo
rgee
rick
sen
ite
Na 6
CaM
g(IO
3)6
(CrO
4)2⋅
12(H
2O)
6.
12C
ompo
und
iod
ates
23.1
.2.3
Ofic
ina
Cha
cabu
co, C
hile
Ari
d c
limat
e m
iner
al
asso
ciat
ed w
ith
Chi
lean
ni
trat
e ca
liche
dep
osit
s.
Coe
xist
s w
ith
halit
e,
nitr
atin
e, a
nd n
iter
Gri
mal
diit
eC
r+3 O
(OH
)61
.17
Hyd
roxi
des
and
ox
ides
con
tain
ing
hyd
roxy
l
6.1.
5.1
Mer
ume
Riv
er, K
amak
usa,
M
arar
uni d
istr
ict,
Guy
ana
Trig
onal
—he
xago
nal
scal
enoh
edra
l
Guy
anai
teC
r+3 O
(OH
)61
.17
Hyd
roxi
des
and
ox
ides
con
tain
ing
hyd
roxy
l
6.1.
2.3
Mer
ume
Riv
er, K
amak
usa,
M
arar
uni d
istr
ict,
Guy
ana
Ort
horh
ombi
c—
dip
yram
idal
Has
hem
ite
Ba(
Cr,
S)O
415
.70
Anh
ydro
us
chro
mat
es35
.3.3
.1L
isd
an-S
iwag
a Fa
ult,
Has
hem
reg
ion,
Am
man
, Jo
rdan
Haw
thor
init
eB
a[Ti
3Cr 4
Fe4M
g]O
1919
.99
Mul
tipl
e ox
ides
7.4.
1.3
Bul
tfon
tein
, Kim
berl
ey,
Rep
ublic
of
Sout
h A
fric
aH
eid
eite
(Fe,
Cr)
1+x(
Ti, F
e)2S
43.
55Su
lfid
es —
incl
udin
g se
leni
des
and
te
lluri
des
2.10
.2.3
Bus
tee
met
eori
te, G
orak
hpur
, B
asti
dis
tric
t, U
ttar
Pra
des
h,
Ind
ia
Hem
ihed
rite
Pb10
Zn(
CrO
4)6
(SiO
4)2F
2
10.2
1C
ompo
und
ch
rom
ates
36.1
.1.2
Flor
ence
Pb-
Ag
min
e,
Whi
cken
burg
, Mar
icop
a C
o., A
Z
L1608_C02.fm Page 50 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 51
IMA
199
8-02
9(C
e, R
EE
,Ca)
4
(Mg,
Fe2+
)(M
g, F
e3+) 2
(Ti,
Nb)
2 Si
4O22
6.26
Soro
silic
ates
56.2
.8.7
Cr a
nalo
g of
che
vkin
ite
—
(Ce)
str
uctu
re
IMA
199
9-01
8C
a 0.2(H
2O) 2
CrS
232
.46
Sulfi
des
— in
clud
ing
sele
nid
es a
nd
tellu
rid
es
32.4
6C
lose
to
scho
llhor
nite
.
IMA
1999
-034
PbC
r+3 2(
CO
3)2(
OH
) 4H
2O20
.10
Car
bona
tes
—
hyd
roxy
l or
halo
gen
16b.
2.1.
4C
r an
alog
of
dun
das
ite
IMA
200
0-04
2M
g 6C
r 2(O
H) 1
6C12
⋅ 4H
2O15
.64
Car
bona
tes
—
hyd
roxy
l or
halo
gen
16b.
6.2.
5
Iqu
iqu
eite
K3N
a 4M
g(C
r+6 O
4)B
24
O39
(OH
) ⋅ 12
(H2O
)
3.55
Com
poun
d b
orat
es27
.1.8
.1Z
apig
a, I
quiq
ue, T
arap
acá
prov
ince
, Chi
leIr
anit
ePb
10C
u(C
rO4)
6(Si
O4)
2
(F, O
H) 2
10.2
2C
ompo
und
ch
rom
ates
36.1
.1.1
Seba
rz m
ine,
Ana
rak,
Ira
n
Isov
ite
(Cr,
Fe) 2
3C6
67.4
1N
ativ
e el
emen
ts1.
1.16
.3Is
Riv
er, I
sovk
y D
istr
ict,
mid
dle
Ura
ls, R
ussi
aC
r an
alog
of
haxo
nite
; fo
und
in A
u-Pt
pla
cers
al
ong
the
Is R
iver
.K
alin
init
eZ
nCr 2
S 434
.94
Sulfi
des
— in
clud
ing
sele
nid
es a
nd
tellu
rid
es
2.10
.1.1
3Sl
yud
yank
a d
epos
it, S
outh
B
aika
l, Z
abai
kaly
e (T
rans
baik
al),
Sibe
ria,
R
ussi
aK
norr
ingi
teM
g 3C
r 2(S
iO4)
322
.95
Nes
osili
cate
s —
ga
rnet
gro
up65
.1.3
c.4
Kao
kim
berl
ite
pipe
, Les
otho
Kos
moc
hlor
NaC
r+3 S
i 2O6
22.8
9In
osili
cate
s (c
linop
yrox
enes
)51
.4.3
a.4
Tolu
ca m
eteo
rite
, Xiq
uipi
lco,
M
exic
oK
rino
vite
NaM
g 2C
rSi 3O
1014
.14
Inos
ilica
tes
(aen
igm
atit
es)
69.2
.1a.
4C
anyo
n D
iabl
o m
eteo
rite
, M
eteo
r C
rate
r, C
ocon
ino
Co.
, AZ
Subh
edra
l gra
ins
in
grap
hite
nod
ules
in
octa
hed
rite
s
(Con
tinu
ed)
L1608_C02.fm Page 51 Thursday, July 15, 2004 6:57 PM
52 Chromium(VI) Handbook
TAB
LE 2
.8A
(Con
tinu
ed)
Min
eral
Nam
eFo
rmu
laC
r C
onc.
(%
)
Dan
a C
lass
ifica
tion
Typ
e L
ocal
ity/
Loc
alit
ies
En
viro
nm
ent/
Rem
ark
sM
iner
al C
lass
No.
Lin
dsl
eyit
e(B
a, S
r)(T
i, C
r, Fe
,M
g)21
O38
17.6
9M
ulti
ple
oxid
es w
ith
Nb,
Ta, a
nd T
i8.
5.1.
8D
eBee
rs m
ine,
Kim
berl
y,
Rep
ublic
of
Sout
h A
fric
aFo
und
in k
imbe
rlit
es a
nd
peri
dot
ite
nod
ules
Lop
ezit
eK
2Cr 2
O7
35.3
5A
nhyd
rous
ch
rom
ates
35.2
.1.1
Ofic
ina
Mar
ia E
lena
, To
pcop
illa
and
Ofi
cina
R
osar
io, I
quiq
uw P
ampa
, Ta
rapa
cá, C
hile
Ata
cam
a D
eser
t, C
hile
Lov
erin
gite
(C
a ,C
e)(T
i, Fe
+3 ,
Cr,
Mg)
21O
38
6.81
Mul
tipl
e ox
ides
wit
h N
b, T
a, a
nd T
i8.
5.1.
2Ji
mbe
rlan
a in
trus
ion,
N
orse
man
, Wes
tern
A
ustr
alia
, Aus
tral
ia
Syno
nym
: chr
ompi
coti
te
Mac
qu
arti
tePb
3Cu(
CrO
4)(S
iO3)
(OH
) 4 ⋅ 2
H2O
5.
30C
ompo
und
ch
rom
ates
36.1
.2.1
Mam
mot
h m
ine,
Tig
er, P
inal
C
ount
y, A
ZE
ncru
sts
dio
ptas
e
Mag
nesi
ochr
omit
eM
gCr 2
O4
54.0
8M
ulti
ple
oxid
es—
sp
inel
gro
up7.
2.3.
1Sc
hwar
zenb
erg,
Sile
sia,
G
erm
any
Man
gano
chro
mit
e(M
n, F
e+2 )
(Cr,
V) 2
O4
43.8
0M
ulti
ple
oxid
es—
sp
inel
gro
up7.
2.3.
2N
airn
e d
epos
it, B
ruku
nga,
A
del
aid
e, S
outh
Aus
tral
ia,
Aus
tral
ia
Syno
nym
: chr
ompi
coti
te
Mat
hias
ite
(K, C
a, S
r)(T
i, C
rFe,
Mg)
21O
38
18.5
4M
ulti
ple
oxid
es w
ith
Nb,
Ta,
and
Ti
8.5.
1.7
Jage
rsfo
ntei
n d
iam
ond
min
e,
Ora
nge
Free
Sta
te, R
epub
lic
of S
outh
Afr
ica
Occ
urs
in k
imbe
rlit
e
Mcc
onne
llite
CuC
rO2
35.2
4M
ulti
ple
oxid
es7.
1.1.
2M
erum
e R
iver
, Kam
akus
a,
Mar
arun
i dis
tric
t, G
uyan
aM
olyb
dof
orn
acit
ePb
2Cu[
(As,
P)O
4][M
o,C
r)O
4](O
H)
1.32
Com
poun
d
phos
phat
es43
.4.3
.3Ts
umeb
, Nam
ibia
L1608_C02.fm Page 52 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 53
Mon
gsha
nite
(Mg,
Cr,
Fe2+
) 2(T
i, Z
r)5
O12
5.24
Mul
tipl
e ox
ides
7.11
.11.
1Y
imen
guan
are
a (?
), Sh
and
ong,
Chi
na
Con
sid
ered
a “
dou
btfu
l”
spec
ies.
Phy
sica
l pr
oper
ties
are
sim
ilar
to
ilmen
ite
(ilm
); fo
und
as
incl
usio
ns in
ilm
. Not
an
IM
A a
ppro
ved
nam
e.M
ount
keit
hite
(Mg,
Ni)
11(F
e3+, C
r)3
(SO
4, C
O3)
3.5(
OH
) 24 ⋅
11
H2O
3.
34C
ompo
und
sul
fate
s32
.4.3
.1M
t. K
eith
Ni d
epos
it, 4
00 k
m
nort
h-no
rthw
est
of
Kal
gooo
lie, A
ustr
alia
Occ
urs
in b
leac
hed
se
rpen
tini
te
Nat
alyi
teN
a(V
+3 ,C
r+3 )
Si2O
65.
74In
osili
cate
s (c
linop
yrox
enes
)65
.1.3
c.5
Slyu
dyan
ka c
ompl
ex, L
ake
Baik
al r
egio
n, Z
abai
kaly
e (T
rans
baik
al)
Sibe
ria,
Rus
sia
Occ
urs
in q
uart
z-be
arin
g sc
hist
s
Nic
hrom
ite
(Ni,
Co,
Fe+
2 )(C
r, Fe
+3 ,
Al)
2O4
30.6
5M
ulti
ple
oxid
es —
sp
inel
gro
up7.
2.3.
4B
on A
ccor
d n
icke
l dep
osit
, Tr
ansv
aal,
Rep
ublic
of S
outh
A
fric
aO
lkho
nski
te(C
r+3 ,
V+
3 )2T
i 3O9
19.9
4M
ulti
ple
oxid
es w
ith
Nb,
Ta,
and
Ti
8.4.
1.2
Wes
tern
sho
re o
f Lak
e B
aika
l, 4.
5 km
sou
th o
f O
lkho
n Is
land
, Rus
sia
Pett
erit
ePb
Cr3+
(CO
3)2
(OH
) 4 ⋅ H
2O20
.10
Car
bona
tes
—
hyd
roxy
l or
halo
gen
16b.
2.1
Red
Lea
d m
ine,
Zee
han-
Dun
das
min
ing
fiel
d,
nort
hwes
tern
Tas
man
ia,
Aus
tral
ia
Foun
d in
oxi
diz
ed z
one
Cr-
anal
og o
f d
und
asit
e Fo
rmer
ly I
MA
199
9-03
4
Ph
oen
icoc
hro
ite
Pb2(
CrO
4)O
9.
52A
nhyd
rous
ch
rom
ates
35.1
.2.1
Ber
ezov
, Eka
teri
nbur
g (S
verd
lovs
k), U
ral
Mou
ntai
ns, R
ussi
aR
edin
gton
ite
(Fe2+
, Mg,
Ni)
(Cr,
Al)
2
(SO
4)4 ⋅
22H
2O
8.49
Hyd
rate
d a
cid
and
su
lfat
es29
.7.3
.6R
edin
gton
min
e, K
noxv
ille,
N
apa
Co.
, CA
Hg
exha
lati
ve a
nd
sand
ston
e-ho
sted
U-V
d
epos
its
Red
led
geit
eB
aTi 6C
r 2O
16(H
2O)
12.9
6M
ulti
ple
oxid
es7.
9.5.
2R
ed L
edge
min
e so
uth
of
Was
hing
ton,
Nev
ada
Cou
nty,
CA
(Con
tinu
ed)
Au
: Ple
ase
chec
k fo
r th
e m
issi
ng
info
rmat
ion
.
L1608_C02.fm Page 53 Thursday, July 15, 2004 6:57 PM
54 Chromium(VI) Handbook
TAB
LE 2
.8A
(Con
tinu
ed)
Min
eral
Nam
eFo
rmu
laC
r C
onc.
(%
)
Dan
a C
lass
ifica
tion
Typ
e L
ocal
ity/
Loc
alit
ies
En
viro
nm
ent/
Rem
ark
sM
iner
al C
lass
No.
Rila
ndit
e(C
r,Al)
6SiO
11⋅5
H2O
(?)
41.1
5M
ulti
ple
oxid
es7.
11.1
4.1
Rila
nd c
arno
tite
cla
ims
20 k
m E
NE
of
Mee
ker,
Coa
l Cre
ek, C
O
Sand
ston
e-ho
sted
U-V
d
epos
its
San
tan
aite
Pb11
CrO
62.
01A
nhyd
rous
ch
rom
ates
35.4
.1.1
Sant
a A
na m
ine,
Car
acol
es,
Sier
ra G
ord
a, C
hile
Scho
llhor
nite
Na 0
.3C
rS2⋅H
2O36
.87
Sulfi
des
— in
clud
ing
sele
nid
es a
nd
tellu
rid
es
2.9.
17.2
Ens
tati
te a
chon
dri
te
met
eori
te, N
orto
n C
ount
y,
KS
Wea
ther
ing
prod
uct
of
casw
ells
ilver
ite
Shui
skit
eC
a 2(M
g,A
l)(C
r,Al)
2
(SiO
4)(S
i 2O7)
(OH
) 2 ⋅
H2O
Trac
eSo
rosi
licat
es58
.2.2
.9B
iser
sk d
epos
it, U
ral
Mou
ntai
ns, R
ussi
a
Stic
htit
eM
g 6C
r 2(O
H) 1
6(C
O3)
⋅4H
2O15
.90
Car
bona
tes
—
hyd
roxy
l or
halo
gen
16b.
6.2.
2A
del
aid
e m
ine,
Sti
chti
te H
ill,
Dun
das
, Tas
man
ia,
Aus
tral
ia
Alt
erat
ion
prod
uct
of
serp
enti
ne
Tara
pac
aite
K2C
rO4
26.7
8A
nhyd
rous
ch
rom
ates
35.1
.1.1
Sant
a A
na m
ine,
Car
acol
es,
Sier
ra G
ord
a, C
hile
Ata
cam
a D
eser
t, C
hile
Ton
gbai
teC
r 3C
286
.66
Nat
ive
elem
ents
1.1.
17.1
Liu
Zhu
ang,
Ton
bai C
ount
y,
Hen
an, C
hina
Uva
rovi
teC
a 3C
r 2(S
iO4)
320
.78
Nes
osili
cate
s —
ga
rnet
gro
up51
.4.3
b.3
Bis
sers
k, S
aran
sk,
Mor
dov
skay
a, R
ussi
aM
etam
orph
osed
chr
omit
ed
epos
its
Vau
qu
elin
ite
Pb2C
u(C
rO4)
(PO
4)(O
H)
7.
37C
ompo
und
ph
osph
ates
43.4
.3.1
Ber
ezov
, Eka
teri
nber
g (S
verd
lovs
k), U
ral
Mou
ntai
ns, R
ussi
aV
olko
nsko
ite
Ca 0
.3(C
r+3 ,M
g,Fe
+3 )
2
(Si,A
l)4O
10(O
H) 2
⋅4H
2O
13.1
2C
lays
— s
mec
tite
gr
oup
71.3
.1a.
4M
ount
Efi
mya
tsk,
Ura
l M
ount
ains
, Rus
sia
Syno
nym
s: V
olch
onsk
oite
an
d W
olch
onsk
oite
L1608_C02.fm Page 54 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 55
Vuo
rela
inen
ite(M
n+3 ,F
e+2 )
(V+
3 ,Cr+
3 )2
O4
11.7
3M
ulti
ple
oxid
es —
spin
el g
roup
7.2.
4.1
Stät
a, D
over
stop
, Ber
gsla
gen,
Sw
eden
Wat
ters
ite
Hg+
4Hg+
2 Cr+
6 O6
9.47
Anh
ydro
us
chro
mat
es35
.4.2
.1C
lear
Cre
ek m
ercu
ry m
ine,
N
ew I
dri
a d
istr
ict,
San
Ben
ito
Cou
nty,
CA
Oxi
diz
ed z
one
of
exha
lati
ve H
g d
epos
its
Yed
linit
ePb
6CrC
l 6(O
,OH
) 8
3.17
Oxy
halid
es a
nd
hyd
roxy
halid
es10
.6.3
.1M
amm
oth
min
e, T
iger
, Pin
al
Co.
, AZ
Yim
engi
teK
(Cr,T
i,FeM
g)12
O19
29.1
9M
ulti
ple
oxid
es7.
4.1.
2Y
imen
gsha
n ar
ea, S
hand
ong,
C
hina
Zha
nghe
ngit
e(C
u,Z
n,Fe
,Al,C
r)4.
28N
ativ
e el
emen
ts1.
1.6.
2X
iaoy
anzh
uang
, Box
ian
Co.
, A
nhui
, Chi
naZ
inco
chro
mit
eZ
nCr 2
O4
44.5
6M
ulti
ple
oxid
es -
sp
inel
gro
up7.
2.3.
6O
nega
dep
ress
ion,
Kar
elia
, R
ussi
a
Not
e:M
iner
als
in b
old
typ
e ar
e C
r(V
I) m
iner
als.
Sour
ce:
Hur
lbut
(19
63);
Mar
tin
and
Bla
ckbu
rn (
1999
and
200
1); P
erro
ud (
2001
); W
ebm
iner
al (
2002
).
L1608_C02.fm Page 55 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 56
TAB
LE 2
.8B
Chr
omiu
m M
iner
als:
Cry
stal
logr
aphy
and
Phy
sica
l Pro
pert
ies
Min
eral
Nam
eC
ryst
al
Sys
tem
Ph
ysic
al P
rop
erti
es
Cle
avag
eC
olor
Den
sity
(S
.G.)
Dia
ph
anie
tyH
abit
Moh
sH
ard
nes
sL
ust
erS
trea
k
Ank
angi
teTe
trag
onal
-d
ipyr
amid
alno
neB
lack
4.44
Opa
que
—6.
5V
itre
ous:
adam
anti
neG
rayi
sh-
blac
kB
arb
erto
nit
e H
exag
onal
-d
ihex
agon
al
dip
yram
idal
[001
]-pe
rfec
tV
iole
t; pi
nk; p
ink
viol
et2.
1Tr
ansp
aren
t to
tran
sluc
ent
—1.
5−2.
0Pe
arly
Whi
te
Ben
tori
teH
exag
onal
-d
ihex
agon
al
dip
yram
idal
[101
0]-p
erfe
ctV
iole
t; lig
ht v
iole
t2.
025
Tran
spar
ent
—2.
0V
itre
ous:
gla
ssy
Vio
let
Bra
cew
ellit
eO
rtho
rhom
bic-
dip
yram
idal
—D
arkr
edd
ish-
brow
n;re
dd
ish-
brow
n;bl
ack
4.45
–4.4
8(a
v =
4.4
6)Tr
ansl
ucen
t to
op
aque
—5.
5−6.
5A
dam
anti
ne;
met
allic
Dar
k br
own
Bre
zina
ite
Ort
horh
ombi
c-d
ipyr
amid
al—
Bro
wni
sh-g
ray
to
gray
4.12
Opa
que
—3.
5–4.
5M
etal
lic: d
ull
—
Cal
sber
gite
Isom
etri
c-he
xoct
ahed
ral
—G
ray
5.9
Opa
que
—7.
0M
etal
lic—
Car
mic
hael
ite
Mon
oclin
ic-
pris
mat
icno
neC
inna
mon
to
blac
k—
Tran
sluc
ent
to
opaq
ue—
6.0
Met
allic
—
Cas
send
anne
ite
Mon
oclin
ic-
pris
mat
ic—
Red
dis
h-or
ange
——
mic
rosc
opic
cr
ysta
ls; p
laty
: sh
eets
3.5
Res
inou
s:
grea
syYe
llow
ish-
oran
ge
Cas
wel
lsil
veri
teTr
igon
al-
hexa
gona
l sc
alen
ohed
ral
—Ye
llow
ish-
gray
3.21
Opa
que
—1.
2M
etal
lic—
Ch
rom
atit
eTe
trag
onal
-d
itet
rago
nal
dip
yram
idal
none
Lem
on y
ello
w3.
142
——
——
Chr
ombi
smit
eTe
trag
onal
none
Bro
wn;
ora
nge;
ye
llow
9.8
Tran
sluc
ent
—3.
0–3.
5A
dam
anti
neB
row
nish
-ye
llow
Chr
omce
lad
onit
eM
onoc
linic
-sp
hero
idal
—E
mer
ald
-gre
en t
o d
ark
gree
n—
Tran
spar
ent
——
Vit
reou
s: d
ull
—
Au
: Ple
ase
chec
k sp
ellin
g.
"Dia
ph
anie
ty"
or "
Dia
ph
anei
ty"?
L1608_C02.fm Page 56 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 57
Chr
omd
ravi
teTr
igon
al-
dit
rigo
nal
pyra
mid
al
Ind
isti
nct
Gre
enis
h-bl
ack
or
dar
k gr
een
3.4
Tran
sluc
ent
to
opaq
ue—
7.0–
7.4
Vit
reou
s:
resi
nous
Gre
enis
h-gr
ay
Chr
omfe
rid
eIs
omet
ric-
hexo
ctah
edra
lN
one
Gra
yish
-whi
te—
Opa
que
—4.
0M
etal
lic—
Chr
omit
e(C
hrom
e ir
on o
re;
chro
mic
iron
)
Isom
etri
c-he
xoct
ahed
ral
Non
eB
lack
or
brow
nish
-bl
ack
4.5–
5.09
(av
= 4
.79)
Opa
que
Mas
sive
gr
anul
ar5.
5M
etal
licB
row
n
Chr
omiu
mIs
omet
ric-
hexo
ctah
edra
l—
Whi
te7.
2O
paqu
e—
4.0
Met
allic
—
Chr
omph
yllit
eM
onoc
linic
-pr
ism
atic
[001
]-pe
rfec
tE
mer
ald
gre
en2.
88Tr
ansp
aren
tPl
aty:
she
ets
3.0
Vit
reou
s: g
lass
yW
hiti
sh-
gree
nC
ochr
omit
eIs
omet
ric-
hexo
ctah
edra
lIn
dis
tinc
tB
lack
5.22
Opa
que
—7.
0M
etal
licG
reen
Coc
roit
eM
mon
oclin
ic-
pris
mat
ic[1
10]-
dis
tinc
t[0
01]-
ind
isti
nct
[100
]-in
dis
tinc
t
Yello
w; o
rang
ish-
red
; red
dis
h-or
ange
5.9–
6.1
(av
= 6)
Tran
sluc
ent
Cry
stal
line:
ac
icul
ar2.
5–3.
0A
dam
anti
neYe
llow
ish-
oran
ge
Dau
bree
lite
Isom
etri
c-he
xoct
ahed
ral
—B
lack
3.81
Opa
que
—4.
5–5.
0M
etal
licB
row
nish
-bl
ack
Dea
nes
mit
hit
eTr
iclin
ic-
pina
coid
alG
ood
Ora
nge-
red
—Tr
ansp
aren
t—
4.5–
5.0
Ad
aman
tine
—
Die
tzei
teM
onoc
linic
-pr
ism
atic
[100
]-in
dis
tinc
tG
old
en y
ello
w3.
617
Tran
spar
ent
—3.
5V
itre
ous:
gla
ssy
Lig
ht y
ello
w
Don
ath
ite
Tetr
agon
al-
dit
etra
gona
l d
ipyr
amid
al
Non
eB
lack
5.0
——
6.5–
7.0
Met
allic
Bla
ckis
h-br
own
Du
kei
teTr
igon
al-
dit
rigo
nal
pyra
mid
al
—Ye
llow
; yel
low
ish-
brow
n—
Tran
spar
ent
——
——
Ed
oyle
rite
Mon
oclin
ic-
pris
mat
icG
ood
Gre
enis
h-ye
llow
; or
ange
—Tr
ansp
aren
t to
op
aque
——
Res
inou
s—
Em
bre
yite
Mon
oclin
ic-
pris
mat
icN
one
Ora
nge
6.45
Tran
spar
ent
to
tran
sluc
ent
—3.
5E
arth
y: d
ull
Yello
w
Esk
olai
teTr
igon
al-
hexa
gona
l sc
alen
ohed
ral
Non
eB
lack
5.18
Opa
que
—8.
0–8.
5M
etal
licG
ray
(Con
tinu
ed)
L1608_C02.fm Page 57 Thursday, July 15, 2004 6:57 PM
58 Chromium(VI) Handbook
TAB
LE 2
.8B
(Con
tinu
ed)
Min
eral
Nam
eC
ryst
al
Sys
tem
Ph
ysic
al P
rop
erti
es
Cle
avag
eC
olor
Den
sity
(S
.G.)
Dia
ph
anie
tyH
abit
Moh
sH
ard
nes
sL
ust
erS
trea
k
Ferc
hrom
ide
Isom
etri
c-he
xoca
tahe
dr
al
Non
eG
rayi
sh-w
hite
—O
paqu
e—
6.5
Met
allic
—
Flor
enso
vite
Isom
etri
c-he
xoct
ahed
ral
Non
eB
lack
—O
paqu
e—
5.0
Ad
aman
tine
: m
etal
licB
lack
Forn
acit
eM
onoc
linic
-pr
ism
atic
Non
eG
reen
; yel
low
; oliv
e br
own
6.27
Tran
spar
ent
—2.
0–3.
0A
dam
anti
neG
reen
ish-
yello
wG
eorg
eeri
ckse
nit
eM
onoc
linic
-pr
ism
atic
Non
ePa
le y
ello
w; b
righ
t le
mon
yel
low
3.03
5Tr
ansp
aren
t to
tr
ansl
ucen
tPr
ism
atic
cr
ysta
ls3.
0–4.
0V
itre
ous:
gla
ssy
Pale
yel
low
Gri
mal
diit
eTr
igon
al-
hexa
gona
l sc
alen
ohed
ral
—D
ark
red
or
red
dis
h-br
own
4.11
Opa
que
—3.
5–4.
5M
etal
licR
ed
Guy
anai
teO
rtho
rhom
bic-
dip
yram
idal
—G
rayi
sh-b
row
n;
gold
en b
row
n;
red
dis
h-br
own
4.53
——
——
Bro
wn
Has
hem
ite
Ort
horh
ombi
c-d
ipyr
amid
alG
ood
Bro
wn;
gre
enis
h-br
own;
yel
low
br
own;
ligh
t yel
low
br
own;
dar
k gr
eeni
sh-b
row
n
4.59
Tran
spar
ent
to
tran
sluc
ent
—3.
5A
dam
anti
neB
row
nish
-w
hite
Haw
thor
init
eH
exag
onal
-d
ihex
agon
al
dip
yram
idal
Non
eB
lack
6.0–
6.5
——
—M
etal
lic—
Hei
dei
teM
onoc
linic
-pr
ism
atic
Non
eSt
eel g
ray
4.1
Opa
que
—3.
5–4.
5M
etal
lic—
Hem
ihed
rite
Tric
linic
-pi
naco
idal
—O
rang
e; b
row
n;
blac
k6.
42—
—3.
0V
itre
ous:
gla
ssy
Saff
ron
IMA
199
8-02
9M
onoc
linic
-pr
ism
atic
—B
lack
—O
paqu
e to
tr
ansl
ucen
t—
—R
esin
ous
—
IMA
199
9-01
8Tr
igon
al—
Coa
l bla
ck—
Opa
que
——
Subm
etal
lic—
L1608_C02.fm Page 58 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 59
IMA
199
9-03
4O
rtho
rhom
bic
—Pa
le g
ray;
pin
kish
-vi
olet
—Tr
ansl
ucen
t—
—Pe
arly
—
IMA
200
0-04
2Tr
igon
al-
hexa
gona
l sc
alen
ohed
ral
—M
agen
ta; p
urpl
e—
Tran
spar
ent
——
Vit
reou
s: g
lass
y—
Iqu
iqu
eite
Trig
onal
-d
itri
gona
l[0
001]
-per
fect
; [0
110]
-ind
isti
nct
Yello
w2.
05Tr
ansp
aren
t—
2.0
Vit
reou
s: g
lass
yYe
llow
ish-
whi
teIr
anit
eTr
iclin
ic-p
edia
l—
Bro
wni
sh-y
ello
w;
yello
w; h
oney
br
own
5.8
——
3.0
Vit
reou
s: g
lass
yYe
llow
Isov
ite
Isom
etri
c-he
xoct
ahed
ral
—Ir
on g
ray
—O
paqu
eG
ranu
lar
8.0
Met
allic
—
Kal
inin
ite
Isom
etri
c-he
xoct
ahed
ral
—B
lack
—O
paqu
e—
5.0
Ad
aman
tine
—
Kno
rrin
gite
Isom
etri
c-he
xoct
ahed
ral
Non
eB
lue
gree
n; g
reen
3.75
6Tr
ansp
aren
t to
tr
ansl
ucen
t—
6.0–
7.0
Vit
reou
s: g
lass
y
Kos
moc
hlor
Mon
oclin
ic-
pris
mat
ic—
Bri
ght
gree
n—
Tran
spar
ent
to
tran
sluc
ent
—6.
0–7.
0V
itre
ous:
gla
ssy
Lig
ht g
reen
Kri
novi
teTr
iclin
ic-
pina
coid
alN
one
Dar
k gr
een;
em
eral
d
gree
n3.
38Su
btra
nslu
cent
to o
paqu
eM
icro
scop
ic
crys
tals
6.0–
7.0
Suba
dam
anti
neG
reen
ish-
whi
teL
ind
sley
ite
Trig
onal
-rh
ombo
hed
ral
Non
eB
lack
4.63
Opa
que
Mic
rosc
opic
cr
ysta
ls7.
5M
etal
licG
ray
Lop
ezit
eTr
iclin
ic-
pina
coid
al[0
10]-
perf
ect;
[100
]-d
isti
nct;
[001
]-d
isti
nct
Ora
nge
red
; red
2.69
Tran
spar
ent
—2.
5V
itre
ous:
gla
ssy
Lig
ht y
ello
w
Lov
erin
git
e(C
hrom
pico
tite
)Tr
igon
al-
rhom
bohe
dra
lN
one
Bla
ck4.
41O
paqu
e—
7.5
Met
allic
Gra
yish
-bl
ack
Mac
qu
arti
teM
onoc
linic
-pr
ism
atic
[100
]-go
odO
rang
e5.
49—
Euh
edra
l cr
ysta
ls3.
5A
dam
anti
neL
ight
ora
nge
Mag
nesi
o-ch
rom
ite
Isom
etri
c-he
xoct
ahed
ral
Non
eB
lack
4.2
Opa
que
—5.
5M
etal
licD
ark
gray
Man
gano
-chr
omit
eIs
omet
ric-
hexo
ctah
edra
l—
Gra
yish
-bla
ck4.
86O
paqu
e—
5.5
Met
allic
—
Mat
hias
ite
Trig
onal
-rh
ombo
hed
ral
Non
eB
lack
4.6
Opa
que
—7.
5M
etal
licG
ray
(Con
tinu
ed)
L1608_C02.fm Page 59 Thursday, July 15, 2004 6:57 PM
60 Chromium(VI) Handbook
TAB
LE 2
.8B
(Con
tinu
ed)
Min
eral
Nam
eC
ryst
al
Sys
tem
Ph
ysic
al P
rop
erti
es
Cle
avag
eC
olor
Den
sity
(S
.G.)
Dia
ph
anie
tyH
abit
Moh
sH
ard
nes
sL
ust
erS
trea
k
Mcc
onne
llite
Trig
onal
-he
xago
nal
scal
end
ohed
ral
—D
ark
red
5.49
——
—M
etal
lic—
Mol
ybd
ofor
naci
teM
onoc
linc-
pris
mat
icN
one
Lig
ht g
reen
6.66
——
2.0–
3.0
Ad
aman
tine
—
Mon
gsh
anit
eH
exag
onal
—B
lack
—O
paqu
e—
5.0–
6.0
Met
allic
—M
ount
keit
hite
Hex
agon
al[0
001]
-per
fect
Whi
te o
r lig
ht p
ink
2.12
—Fl
akes
: flat
thin
cr
ysta
ls2.
0Pe
arly
W
hite
Nat
alyi
teM
onoc
linic
-pr
ism
atic
[110
]-d
isti
nct
Yello
wis
h-gr
een;
lig
ht g
reen
3.55
Tran
spar
ent
—7.
0V
itre
ous:
silk
yG
reen
Nic
hrom
ite
Isom
etri
c-he
xoct
ahed
ral
Ind
isti
nct
Bla
ck5.
24O
paqu
e—
6.0–
6.5
Met
allic
Gre
enis
h-gr
ayO
lkho
nski
teM
onoc
linic
Non
eB
lack
4.48
Opa
que
Plat
y: s
heet
s8.
0M
etal
licB
lack
Pett
erit
eO
rtho
rhom
bic
—Pa
le g
ray;
pin
kish
vi
olet
—Tr
ansl
ucen
t—
—Pe
arly
—
Ph
oen
icoc
hro
ite
Mon
oclin
ic-
pris
mat
ic—
Dar
k re
d7.
01Tr
ansl
ucen
t—
2.5
Ad
aman
tine
: re
sino
usYe
llow
or
ange
Red
ingt
onit
eM
onoc
linic
—W
hite
; lig
ht v
iole
t1.
761
Tran
spar
ent
Fibr
ous
crys
tals
2.0
Silk
yW
hite
Red
led
geit
eTe
trag
onal
-d
ipyr
amid
alG
ood
Yello
w g
reen
;bl
ack
3.72
Opa
que
—6.
7A
dam
anti
neG
rayi
sh-
whi
teR
iland
ite
Unk
now
n—
Bla
ckO
paqu
eO
paqu
ePl
aty:
she
ets
2.0–
3.0
Res
inou
sG
rayi
sh-
brow
nS
anta
nai
teH
exag
onal
-tr
apez
ohed
ral
[000
1]-p
erfe
ctSt
raw
yel
low
9.15
5—
—4.
0—
—
Scho
llhor
nite
Trig
onal
-d
itri
gona
l py
ram
idal
[000
1]-p
erfe
ctG
ray
2.7
Opa
que
Mic
rosc
opic
cr
ysta
ls1.
5–2.
0M
etal
lic—
L1608_C02.fm Page 60 Thursday, July 15, 2004 6:57 PM
Chemistry, Geochemistry, and Geology of Chromium 61
Shui
skit
eM
onoc
linic
-pr
ism
atic
[001
]-pe
rfec
tD
ark
brow
n3.
24—
—6.
0—
Gre
enis
h-br
own
Stic
htit
eTr
igon
al-
hexa
gona
l sc
alen
ohed
ral
[000
1]-p
erfe
ctL
ilac;
ligh
t vi
olet
pi
nk2.
2Tr
ansl
ucen
t to
tr
ansp
aren
tE
ncru
stat
ions
no
dul
ar;
mic
aceo
us
1.5–
2.0
Vit
reou
s: g
lass
yPa
le v
iole
t bl
ue
Tara
pac
aite
Ort
horh
ombi
c-d
ipyr
amid
al[0
01]-
dis
tinc
t;[0
10]-
dis
tinc
tL
ight
yel
low
2.74
Tran
spar
ent
——
——
Ton
gbai
teU
varo
vite
Isom
etri
c-he
xoct
ahed
ral
Non
eG
reen
3.4–
3.8
Tran
spar
ent
to
tran
sluc
ent
Cry
stal
line:
fin
e la
mel
lar
6.5–
7.0
Vit
reou
s: g
lass
yW
hite
Vau
qu
elin
ite
Mon
oclin
ic-
pris
mat
icIn
dis
tinc
tB
row
n; b
row
nish
-gr
een;
bro
wni
sh
blac
k; g
reen
; bla
ck
6.0
Tran
spar
ent
to
tran
sluc
ent
Ren
ifor
m:
mam
mill
ary-
botr
yoid
al
2.5–
3.0
Ad
aman
tine
: re
sino
usB
row
nish
Vol
kons
koit
e (V
olch
onsk
oite
, W
olch
onsk
oite
)
mon
oclin
ic-
pris
mat
ic[?
]-pe
rfec
tYe
llow
; oliv
e gr
een;
gr
een
2.0–
2.5
(av
= 2
.25)
Subo
paqu
e—
1.5–
2.0
Res
inou
sB
lue
gree
n
Vuo
rela
incn
ite
Isom
etri
c-he
xoct
ahed
ral
—B
row
nish
-gra
y;
gray
ish-
blac
k4.
64O
paqu
e—
6.5
Met
allic
—
Wat
ters
ite
Mon
oclin
ic-
pris
mat
icN
one
Bro
nze;
red
dis
h-br
own;
bla
ck8.
91O
paqu
e to
tr
ansl
ucen
tC
ryst
allin
e:
encr
usta
tion
s4.
5Su
bmet
allic
Dar
k br
ick
red
Yed
linit
eTr
igon
al-
rhom
bohe
dra
l[1
120]
-dis
tinc
tR
ed v
iole
t5.
85Tr
ansp
aren
t to
tr
ansl
ucen
t—
2.5
Vit
reou
s: g
lass
yW
hite
Yim
engi
teH
exag
onal
-d
ihex
agon
al
dip
yram
idal
[000
1]-p
erfe
ctB
lack
4.34
Opa
que
—4.
0M
etal
licB
row
n
Zha
nghe
ngit
eIs
omet
ric-
hexo
ctah
edra
l—
Non
e—
Gol
den
yel
low
—3.
5M
etal
licB
ronz
e
Zin
coch
rom
ite
Isom
etri
c-he
xoct
ahed
ral
—B
row
nish
-bla
ck—
Tran
sluc
ent
to
opaq
ue—
5.8
Subm
etal
lic—
Sour
ces:
Bla
ckbu
rn a
nd D
enne
n (1
997)
; Mar
lin (
1999
); M
and
arin
o (2
001)
; Mar
tin
and
Bla
ckbu
rn (
2001
); Pe
rrou
d (
2001
); B
arth
elm
y (2
002)
.
Not
e:av
= a
vera
ge I
MA
= I
nter
nati
onal
Min
eral
Nam
es A
ssoc
iati
on.
Au
: Ple
ase
chec
k fo
r th
e m
issi
ng
valu
e.
L1608_C02.fm Page 61 Thursday, July 15, 2004 6:57 PM
62 Chromium(VI) Handbook
Act was phased out, which had subsidized many chromite mines. The UnitedStates currently has known chromite deposits in California, Maryland,Montana, North Carolina, Oregon, Pennsylvania, Texas, Washington, andWyoming, but the ore’s low chromium content makes these deposits uneco-nomical for mining. There are essentially three grades of chromite ore: (1)chemical-grade, which averages 28.6% chromium; (2) metallurgical grade,which also averages 28.6% chromium; and (3) refractory grade, which aver-ages 23.9% chromium. In 2001, chromite ore and chromium ferroalloys andmetal were imported from the Republic of South Africa (48%), Kazakhstan(16%), Russia (9%), Turkey (9%), Zimbabwe (9%), and other nations (9%).Current world resources exceed 11 billion tons of shipping-grade chromite(USEPA, 1984; Papp, 2002). In 2001, 22% of chromium metal was recycledmostly in the form of stainless steel scrap. In 2002, 63% of chromium oreswere imported, primarily from South Africa, Kazakhstan, Zimbabwe, Turkey,and Russia (McCartan et al., 2003).
Primary chromite deposits only exist in certain types of ultramafic orclosely related anorthositic rocks, of which there are two main types: (1) thestatiform or layered intrusion and (2) the pod shaped (podiform) type(Thayer, 1973). The main ore mineral is chromite [(Mg, Fe2+) (Cr, Al, Fe3+)2O4],which has a variable composition. The Cr2O3 content of most chromite oreranges from about 15 to 64%; however, in mixtures of chromite and othersilicate minerals, Cr2O3 contents range from 7 to 55% (Thayer, 1973; Lipinand Page, 1982). Thicknesses of ore grade zones vary from 0.5 to 10 m. Thesedeposits are described in the following sections.
2.2.3.1 Stratiform Mafic-Ultramafic Chromite Deposits
These deposits are also known as chromium-platinum mafic-ultramafic com-plexes. They occur as igneous plutons intruded into tectonically stable por-tions of the crust or craton during the Achaean period (Precambrian) morethan 1.9 billion years ago. The elongated character of many of these complexessuggests possible intrusion during crustal rifting. As the magma cooled, itcrystallized with different minerals crystallizing sequentially at differentrates. Minerals thus preferentially settled out by the process known as mag-matic segregation (Bates and Jackson, 1987), producing a layered igneouscomplex. Such layered complexes generally cover thousands of square kilo-meters. Layering has resulted in more felsic (less dense) rock types in theupper portion of the complex and ultramafic (more dense) rocks near thebottom. However, the complexes’ overall composition is that of a gabbro.Secondary alteration in the form of serpentinization may occur in olivine-richwall rocks (Hughes, 1982; Hatch et al., 1972; Lipin and Page, 1982).
The stratiform mafic-ultramafic association includes world-class chromitedeposits, which also produce significant amounts of platinum group metals(PGMs); two examples are:
1. The Precambrian Stillwater Complex in Montana, which was orig-inally intruded as a sill. This basic layered intrusion composed of
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Chemistry, Geochemistry, and Geology of Chromium 63
approximately 15 repetitious layers of hartzburgites, chromatite,olivine chromatite, and bronzitite is exposed only over a 48.28 kmlength because it is bounded by faults at either end. The complexes’thickness has been measured from 4876.8 to 5486.4 m. Thirteenchromatite zones have been recognized but only one zone has beenmined. PGMs and disseminated nickel–copper sulfide mineralsalso occur within the chromite ore zones (Ridge, 1972; Lipin andPage, 1982; Page 1992a).
2. The Bushveld Igneous Complex of the Republic of South Africawhich covers an area that averages 280 km long by 160 km wide(with the greatest width approximately 400 km) and a thickness of9 km centered on the town of Rustenburg in the Transvaal State.The Bushveld complex contains cumulus ferrogabbro to diorite,which has vanadium–titanium magnetite layers. Chromite layersoccur in cumulate hartzburgite, dunite, and pyroxinite. The orescontain varying amounts of the minerals chromite, ilmenite, mag-netite, pyrrhotite, pentlandite, chalcopyrite, and PGMs. Chromiteore thicknesses increase in basal depressions within the layers(Page, 1992b). Ore reserves may be several billion metric tons.
2.2.3.2 Podiform- or Alpine-Type Chromite Deposits
Podiform-type deposits occur as podlike masses in the ultramafic portionsof ophiolite complexes. Local rock types include highly deformed duniteand hartzburgite, which may be locally serpentinized. The deposits generallyare highly deformed, having formed throughout the Phanerozoic in thelower part of the oceanic lithosphere along spreading plate boundaries. Theycan be divided into minor podiform chromites, most of which are found inCalifornia and Oregon, and major podiform chromite, occurring mostly inIran, Turkey, and Cuba (Singer and Page, 1992; Singer et al., 1992; Ash, 1996).
Ore mineralogy generally consists of chromite with possible ferrichromite,magmetites, and PGMs (ruthenium, osmium, and iridium) (Albers, 1992).Minor podiform deposits range from about 0.16 to 10,000 metric tons witha median of about 100 metric tons. Ore grades range from 10 to 56% Cr2O3
with a median grade of 44% Cr2O3. Major podiform deposits range fromabout 500 to 2.0 million metric tons with a median of about 2000 metric tons.Ore grades range from 22 to 56% Cr2O3 with a median grade of 46% Cr2O3
(Singer and Page, 1992; Singer et al., 1992).
2.2.4 Crude Oil, Tars and Pitch, Asphalts, and Coal
Crude oils (Table 2.9) contain many different, mostly metallic, elements;cobalt (Co), chromium (Cr), copper (Cu), molybdenum (Mo), nickel (Ni),lead (Pb), vanadium (V), and zinc (Zn) are commonly found. Ni and V aregenerally enriched with respect to most other elements. Crude oils are poorly
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64 Chromium(VI) Handbook
enriched with six valence group metals: Cr, Mo, W, and uranium (U). Theratio of V/Cr is generally equal to or greater than 10 in most crude oils(Krejci-Graf, 1972).
Coal contains more than 50% carbonaceous material with the remainderoccurring as clay minerals, discrete mineral grains, and organically boundassociated elements. Seventy-nine such elements have been detected in coal.Chromium may be associated with the clays; the average chromium concen-tration for U.S. coals is 15 mg/kg (Finkelman, 1993).
2.2.5 Rock
Elemental chromium concentrations in crustal rocks range from 20 mg/kg infelsic igneous rocks such as granites (Table 2.10) to more than 2000 mg/kg inultramafic igneous rocks (and their metamorphosed equivalents). The crustalaverage (CA) is reported at approximately 100 mg/kg. The CA is the basis for
TABLE 2.9
Chromium Concentrations in Some Crude Oils, Tars and Pitch, Asphalts, and Coal
Organic Material Location Cr Concentration (ppm) Reference
Crude oil
Western Canada Basin
Average 0.05
Manning andGize (1993)
Maximum 1.68Saudi Arabia Average 0.65
Maximum 1.18Central European Mollase Basin, Germany
Average 0.11Maximum 0.45
Italy Average 3.15Maximum 10.7
Gösting II, Austria
Range 30–70
Krejei-Graf(1972)
N.E. Caucasus, Russia (13 samples)
Average 100
Urals, Russia (27 samples)
Average 100
Tars and pitch
Messel, Germany
Average 80
Kohtla, Estonia Range 70–350Kohtla, Estonia Range 35–70
Asphalts Matitza, Romania
Range 70–350
Selenia, Albania Range 35–70Coal United States
(7,847 samples)Average 15 Finkelman
(1993)Maximum 250
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Chemistry, Geochemistry, and Geology of Chromium 65
defining the chromium enrichment factor (EF) in rocks and soils, which rangesfrom 2 for felsic rock types, primarily granites, to 20 for ultramafic rocks.
Cr(III) is also a trace element constituent of igneous rocks, probably occur-ring as separate phase minerals such as chromite, chromium-bearing mag-netite, and/or ilmentite. It ranges from 100 mg/kg in amphiboles, pyroxenes(where it is actually dispersed in augite), biotite, magnetite, and olivine to1.0 mg/kg in plagioclase and potassium feldspars (Hughes, 1982; Brickerand Jones, 1995). Therefore, the largest source of chromium in minerals inthe crustal rocks is in the rock-forming minerals (Table 2.11).
Black shales (also known as metalliferous shales) form in anoxic (anaerobic)environments in fresh, brackish, marine, or hypersaline water. Black shales areenriched in arsenic (As), Cu, Cr, Mo, Ni, and U in concentrations ranging fromn × 101 mg/kg to n × 102 mg/kg (where n = 1 to > 9). Metals such as V and Znare further enriched to concentrations ranging to n × 103 mg/kg. These metalsgenerally are not visible with the naked eye or ordinary light microscopybecause they do not occur as discrete mineral phases; rather, they are micro-scopically dispersed in organic matter, clay, or sulfides. Chromium concentra-tions in black shale (Table 2.12) range from about 20 to 3000 mg/kg generally
TABLE 2.10
Total Chromium Concentrations in 1.0 km3 of the Crust for Different Rock Type
Rock Type
AverageDensity(g/cm3)
109 Metric Tons (Mg)/km3
Average CrConcentration
(mg/kg = g/Mg)1010 g-Cr/
km3
104 Metric Tons-
Cr/km3
Igneous:Granite 2.667 2.667 20 5.33 5.33Granodiorite 2.716 2.716 20 5.43 5.43Quartz diorite 2.806 2.806 20 5.61 5.61Diorite 2.700 2.700 20 5.40 5.40Gabbro 2.976 2.976 2000 595.20 595.20Peridotite 3.324 3.324 2000 664.80 664.80Dunite 3.277 3.277 2000 655.40 655.40Pyroxenite 3.231 3.231 2000 646.20 646.20
Volcanic:Basalt 2.965 2.965 220 65.23 65.23
Sedimentary:Limestone 2.400 2.400
2.3002.600
10 2.40 2.40
Shale 2.300 35 8.05 8.05Sandstone 2.600 100 26.00 26.00
Notes: There are 1.0 × 1015 cm3 in 1.0 km3; 106 g = 1.0 Mg.
Source: Rock densities: Daly et al. (1966).
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66 Chromium(VI) Handbook
averaging 100 to 111 mg/kg (Holland, 1979; Thornton, 1983; Leventhal, 1993;Quinby-Hunt et al., 1997).
2.2.6 Soil
Cr(III) coprecipitates with secondary soil clay minerals such as illites andsmectites (Sposito, 1983). For residual soils, chromium concentrations gen-erally reflect underlying bedrock concentrations; however, under tropicalweathering conditions, chromium in soil may be considerably enriched overbedrock, particularly in the B horizon (Hawkes and Webb, 1962; Mattigodand Page, 1983 and Table 2.7).
Worldwide chromium concentrations in soil average about 200 mg/kg.Scottish surficial soils range from 5.0 to 3000 mg/kg. In Canada, backgroundchromium concentrations (from 173 soil samples) range from 10 to 100 mg/kg with an average of 43 mg/kg (CCME, 1996). U.S. soils range from 1.0 to2000 mg/kg, averaging 54 mg/kg (Shacklette and Boerngen, 1984). Ninesamples of California residual soils had chromium concentrations rangingfrom 4.0 to 32 mg/kg, averaging 15.4 mg/kg (Pettygrove and Asano, 1985).
TABLE 2.11
Cr Concentration in Rock Forming Minerals in a Typical Granite
Mineral
AverageMineral
Fraction (%)
Cr Concentration in Mineral
(mg/kg)
Total Cr Concentration in Rock
mg/kg %
Quartz 35.0 Trace (approx. ~0.1) 0.035 0.32Feldspar 60.0 10 6.0 54.37Biotite 4.0 100 4.0 36.35Magnetite 1.0 100 1.0 9.06Total 100.0 11.035 100.10
TABLE 2.12
Chromium Concentrations for Various Shales
Shale TypeCr Concentration
(ppm)
Average shale 90Average black shale 100–111Atlantic (Cretaceous) 200Green River Formation, WY (Eocene) 40Black Sea (layer C) 150Appalachian (Devonian) 60Condar, Australia 55Alum (Cambrian) 94Mecca Quarry, PA 400New Albany (Devonian) 70Falling Run /Henryville 100Associated phosphates 20
Source: Leventhal (1993).
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Chemistry, Geochemistry, and Geology of Chromium 67
2.2.7 Precipitation (Rain Water) and Surface Water
The average chromium concentration in rain water ranges from 0.2 to1.0 µg/l (Keiber and Heiz, 1992). Surface fresh water has total chromiumranging from approximately 0.5 to 2.0 µg/l with dissolved chromiumranging from approximately 0.02 to 0.3 µg/l. Chromium in remote areas,such as Antarctic lakes, ranges from less than 0.6 to 30 µg/l with con-centrations increasing with depth. Canadian surface waters range from0.2 to 44 µg/l. Surface water concentrations in industrialized regionsrange from 1.0 to 10 µg/l. U.S. surface water concentrations range to 84µg/l (WHO, 1996). Cr(VI) may be the dominant dissolved form in surfacewaters, particularly in oxygenated environments (CCME, 1996).
2.2.8 Groundwater
Background chromium groundwater concentrations generally follow themedia that it occurs in. Most chromium concentrations are low, averagingless than 1.0 µg/l (WHO, 1996). Typical ranges are from 0.02 to 6.0 µg/l witha median concentration of 0.2 µg/l (Allard, 1995).
2.2.9 Sea Water
Chromium in ocean or seawater averages about 0.3 µg/l with chromiumprobably occurring as Cr(OH)4
−and CrO42−. These species have an average
residence time of 6000 years (Henderson, 1982; Firestone, 2002). However,concentration variations occur in different oceans and seas. In the North Sea,a chromium concentration of 0.17 µg/l was detected (WHO, 1996). Chro-mium concentrations also vary with depth: in the North Pacific, chromiumconcentrations range from 0.156 µg/l at the surface to 0.26 µg/l in deeperwaters. In the North Atlantic, chromium ranges from 0.182 µg/l at the surfaceto 0.234 µg/l in deeper water (Donat and Bruland, 1995).
2.2.10 Air
In remote areas, chromium ambient air concentrations (Table 2.7) are rela-tively low: in Arctic air, concentrations range from 0.005 to 0.07 ng/m3. Inthe Canadian Arctic, air samples collected in the early 1980s had chromiumconcentrations at 0.26 ng/m3 (CCME, 1996). Most nonindustrialized areashave chromium air concentrations below 10 ng/m3.
In the United States, average chromium concentrations are generally below300 ng/m3 with median concentrations less than 20 ng/m3 (WHO, 1996). InCalifornia, the California Air Resources Board (CARB) routinely measuresatmospheric chromium [including Cr(VI)]. In 1986, CARB estimated that theambient chromium population-weighted annual concentration ranged from8.9 to 17.8 ng/m3. They also estimated that Cr(VI) comprised 3 to 8% of the
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68 Chromium(VI) Handbook
reported ambient chromium in air; this occurred in the form of respirableparticulates with a median diameter of approximately 1.5 to 1.9 µm.
Ambient chromium concentrations derived from soil particles are believedto be primarily composed of Cr(III) compounds. In the Regensburg area inthe Federal Republic of Germany, Nusko and Heumann (1997) found that thesurface layers of forest soils had a Cr(III)/Cr(VI) ratio of 1.47 as opposed toCr(III)/Cr(VI) in aerosol particles that ranged from 0.27 to 0.35. This sug-gested that Cr(III) predominates in organic-rich soil and that oxidation ofCr(III) to Cr(VI) occurs in the atmosphere. Cr(III) concentrations in aerosolparticles (measured in three seasons from August 1994 to September 1995)ranged from 0.05 to 36 ng/m3, and Cr(VI) concentrations in aerosol particles(for samples analyzed for the same period) ranged from 0.16 to 1.22 ng/m3.
Worldwide, most chromium atmospheric source emissions are from wind-borne soil particles (62.4%) and volcanoes (34.67%) with the remaining emis-sions from sea salt spray, forest fires, and biogenic sources (Papp, 1994 andTable 2.13). Atmospheric persistence or residence time is estimated at anexperimental half-life (t1/2) of 13 h with total residence time estimated at lessthan 14 d. Chromium is removed from the atmosphere by dry and wet (rainout) deposition with most chromium deposition occurring through wet dep-osition although chromium particles less than 5.0 µm diameter may remainairborne for extended periods, allowing extended transport over large dis-tances by wind (CCME, 1996; CARB, 2002).
2.2.11 Biogeochemical Cycling
The determination of the natural biogeochemical cycling of chromiumbecomes somewhat difficult because of the large input of anthropogenic chro-mium, which has considerably perturbed the natural cycle. This is also typicalin the natural biogeochemical cycling of other metals such as aluminum, iron,copper, zinc, and lead in which atmospheric emissions from mining, smelting,
TABLE 2.13
Chromium Natural Atmospheric World Emissions
Source103 Metric Tons per Year
Median Percent Range
Wind-borne soil particles 27 62.40 3.6– 50Sea salt spray 0.07 0.16 0.03–1.4Volcanoes 15 34.67 0.81– 29Wild forest fires 0.09 0.21 0–0.18Biogenic: total 1.11 2.57 0.1–2.22Continental particulates 1.0 2.31 0.1–2.0Continental volatiles 0.05 0.12 0–0.10Marine 0.06 0.14 0–0.12Total 43.27 100.01 4.5–83
Source: Papp (1994).
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Chemistry, Geochemistry, and Geology of Chromium 69
and the burning of fossil fuels have substantially increased atmospheric, sur-face water, sediment, soil, plant and animal tissue levels (Schlesinger, 1991).
Dobrovolsky (1994) noted that most of the chromium total metal mass turn-over (calculated in millions of metric tons per year) occurs in stream loss to theoceans of suspended sediment input from streams and rivers. The amount ofsuspended load transported by this process is 2.460 million metric tons per year(Table 2.14b). This probably is correct in that most of the chromium metal mass,some 278,000 million metric tons, is concentrated in granites in the continentalcrust and another 132,000 million metric tons is concentrated in sediments eitheroverlying the granite crust or laying on the ocean floor. Recycling of this mate-rial is by subduction and reintroduction by magmatic activity (plutonic andvolcanic) back into the crust. This occurs only over a wide geologic time scale.
TABLE 2.14A
Chromium Concentrations inContinental Vegetation Annual Growth
ParameterCr Concentration
(mg/kg)
Ash 35Dry Phytomass 1.8Live Phytomass 0.7
Source: Dobrovolsky (1994).
TABLE 2.14B
Land Surface Chromium Fluxes
ParameterMetal Mass Turnover(106 Metric Tons/year)
World biological cycle on Land 0.309Stream loss:In solution 0.041In suspension 2.460Continental dust loss 0.19Transport from ocean to land MinorOceanic photosynthetic organismsbiological cycling
0.16
Source: Dobrovolsky (1994).
TABLE 2.14C
Chromium Mass Distribution in the Biosphere
ParameterMetals Mass
(106 Metric Tons)
Land vegetation 4.5Oceans (dissolved) 274Sedimentary 132,000Granites in continental crust 278,000
Source: Dobrovolsky (1994).
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70 Chromium(VI) Handbook
2.3 Chromium Geochemistry
2.3.1 Cr(III) Geochemistry
In its Cr(III) form, the Cr3+ has an ionic radius of 0.064 nm and it readilysubstitutes in crystal lattices for trivalent iron, which has an ionic radius of0.060 nm and trivalent aluminum, with an ionic radius of 0.050 nm (Smith,1972). Iron oxides and clays in soil and saturated zone alluvium thereforereadily sorb Cr(III).
Under standard conditions (at 25°C and 1.0 bar atmospheric pressure), inthe chromium–oxygen–hydrogen system (Figure 2.1), the Cr(III) stabilityzone occurs over a wide Eh and pH field under both reducing to oxidizingand acid to alkaline conditions. Cr(III) generally forms insoluble chromicoxide (Cr2O3), from approximately pH 5.0 to 13.5 and from an approximateEh ranging from +0.8 to −0.75 V (volts). At slightly less than pH 5.0, Cr2O3
dissolves to form soluble chromium hydroxide (CrOH2+). At a pH of approx-imately greater than 13.5 and an Eh ranging from 0.05 to −0.8 V, solubleCr(III) anion (CrO2
−) forms (Brookins, 1987). In aqueous environments underlow Eh conditions, the main Cr(III) species are the Cr(III) cations (Cr3+) andCrOH2+ (Richard and Bourg, 1991).
Under standard conditions, in the chromium–water–oxygen system, theCr(III) stability zone also occurs over a wide Eh and pH field under bothreducing to oxidizing and acid to alkaline conditions. Cr(III) generally formssoluble Cr3+ from about pH 0 to about pH 8 and at an Eh from approximately−0.4 to −1.2 V at the upper stability line. At a pH greater than 4 to aboutpH 7.5, Cr3+ dissolves to form soluble chromium hydroxide cations: CrOH2+
and Cr(OH)2+. At a pH of approximately 8.0, insoluble and amorphous
Cr(OH)3 forms, although small quantities of Cr(III) may be solubilizedwithin this stability zone. At extreme pH and reducing conditions (abovepH 12.0 and below Eh 0.0), soluble chromium hydroxide anions [Cr(OH)4
−
] form (Hem, 1977). In aqueous environments under low Eh conditions, themain Cr(III) species are the chromic cations Cr3+ and Cr(OH)2+ (Richard andBourg, 1991).
In the chromium–water–oxygen system (Figure 2.2), under standard con-ditions (predominant in groundwater), the governing reactions are:
Cr3+ + H2O ↔ CrOH2+ + H+ (2.1)
CrOH2+ + H2O ↔ Cr(OH)2+ + H+ (2.2)
Cr(OH)2+ + H2O ↔ Cr(OH)3
0 + H+ (2.3)
Cr(OH)30 + H2O ↔ Cr(OH)4
− + H+ (2.4)
Therefore, soluble chromium cations and anions are produced from insolubleCr(III) hydroxide. However, under most natural groundwater conditions, Cr(III)is relatively insoluble and rarely occurs above concentrations exceeding thedrinking water maximum contaminant level (MCL) of 50 ppb (Calder, 1988).
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Chemistry, Geochemistry, and Geology of Chromium 71
2.3.2 Cr(VI) Geochemistry
Under standard conditions, in the chromium–water–oxygen system(Figure 2.2), the Cr(VI) stability zone or field occurs over a much narrowerrange than the Cr(III) stability field. Cr(VI) species primarily occur underoxidizing (+Eh) and alkaline conditions (pH >6.0). In this field, Cr(VI) gen-erally forms soluble chromate (CrO4
2−) anions from approximately pH 6.0 to14.0 and at an Eh from approximately −0.1 to +0.9 V. At slightly below pH5.0, Cr2O3 dissolves to form soluble CrOH2+ (Brookins, 1987).
FIGURE 2.1Eh-pH diagram for the chromium–oxygen–hydrogen system. Eh values in volts (V). (Diagramfrom Brookins, D.G., 1987, Eh-pH Diagrams for Geochemistry, Springer-Verlag, New York, 176 p.With permission.)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
–0.2
–0.4
–0.6
–0.80 2 4 6
pH
Eh
(V)
8 10 12 14
CrOH2+
Cr2O3
HCrO4–
SYSTEM Cr–O–H25°C, 1 bar
PO2 = 1 bar
PH2 = 1 bar
CrO2–
CrO42–
10–4
10–6
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72 Chromium(VI) Handbook
In aqueous environments, under oxidizing conditions, Cr(VI) is extensivelyhydrolyzed; therefore, it is present as an anion, generally forming CrO4
2− anddichromate (HCrO4
−) anions (Calder, 1988; Richard and Bourg, 1991). Accord-ing to Calder (1988), in the chromium–water–oxygen system (Cr + H2O + O2)and under standard conditions, the reaction that occurs is:
HCrO4− ↔ CrO4
2− + H+ (2.5)
FIGURE 2.2Eh-pH diagram for the chromium–oxygen–water system. Eh values in volts (V). (Diagrammodified from Hem, J.D., 1989, U.S. Geological Survey Water Supply Paper 2254, U.S. Govern-ment Printing Office, Washington, D.C., 263 p.)
1.2
1.0
0.8Dichromate
Cr2O72–
ChromateCrO4
2–
0.6
0.4
0.2
0
Eh
(vol
ts)
OX
IDIZ
ING
RE
DU
CIN
G
–0.2
–0.4
–0.6
–0.80 2 4 6 8
pH
10 12 14
Cr3
+
Cr(
OH
)2+
Cr(
OH
) 2+
Cr(
OH
)– 4
Cr(OH)3 (am)Insoluble Hydroxide
5 mg/L Dissolved Cr
5 mg/L Dissolved Cr
0.5 mg/L Dissolved Cr
WATER REDUCED
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Chemistry, Geochemistry, and Geology of Chromium 73
Cr(VI) substitutes for S(VI) because their ionic radii are similar: Cr(VI) anionshave ionic radii between 0.0325 and 0.052 nm and S(VI) anions have ionicradii between 0.029 to 0.034 nm (Robertson, 1976). This becomes importantbecause sulfate anions can replace chromate and dichromate anions.
In soils and aqueous environments, Cr(III) adsorption by manganeseoxides is the first step toward its oxidation to Cr(VI) by manganese oxides.These typically accumulate on the surface of iron oxides and clay minerals(Bartlett and James, 1979). Mn(III) and Mn(IV) oxyhydroxides and Mn(IV)oxides (such as the mineral pyrolusite) oxidize Cr(III) to Cr(VI); oxidationrates tend to be higher with increasing pH (Eary and Rai, 1986; Hug et al.,1997). Fendorf and Zasoski (1992) noted the following overall reaction:
Cr3+ + 1.5δ−MnO2 + H2O → HCrO4− + 1.5Mn2+ + H+ (2.6)
A similar equation for this reaction was determined from experiments byPalmer and Puls (1994). Soil organic matter quickly adsorbs and reducesCr(VI) to Cr(III). It generally remains mobile only if its concentration exceedsthe adsorbing and reducing capacity of the soil (Bartlett and Kimble, 1979).Organic compounds also reduce Cr(VI) to Cr(III). Richard and Bourg (1991)noted that simple amino, humic, and fulvic acids produce intermediate Cr(V)species that changes to Cr(III) within a few days.
Cr(VI) reduction can also occur from reaction with ferrous [Fe(II)] iron.This generally involves a three-step process (Sedlak and Chan, 1997):
Fe2+ + Cr6+ → Fe3+ + Cr5+ (2.7)
Fe2+ + Cr5+ → Fe3+ + Cr4+ (2.8)
Fe2+ + Cr4+ → Fe3+ + Cr3+ (2.9)
2.3.3 Chromium Reaction Rates (Kinetics)
Chromium reaction rates or kinetics (how fast a reaction will occur), fromCr(III) to Cr(VI) by oxidation and back to Cr(III) by reduction, have beenextensively studied in the laboratory (Lin, 2000). As we have seen, Cr(III)can oxidize to Cr(VI) generally under the catalytic influence of Mn(IV)oxides. Eary and Rai (1986) noted that aqueous Cr(III) oxidation was notcaused by surface catalyzed reactions but by direct reactions of β -MnO2.
Kinetic experiments by Saleh et al. (1989) in lake water, sediment, and soilindicated that reaction rates for the oxidation of Cr(III) to Cr(VI) are relativelyslow with t1/2 ranging from 0.58 to 37.2 yr. Reaction rates for the reductionof Cr(VI) to Cr(III) tend to be very rapid with t1/2 ranging from instantaneousto 53 d under anaerobic or reducing conditions. For aerobic conditions, thet1/2 was measured from 15 min to 21.5 d.
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74 Chromium(VI) Handbook
2.4 Chromium Distribution in Primary Environments
2.4.1 Possible Sources of Natural Cr(VI) in Rocks
In the California Coast Ranges, the Franciscan complex contains ultramaficrocks that host chromite ore deposits. Davis (1966) described these depositsas magmatic segregations of chromite in peridotite and peridotite altered toserpentinite. Chromium minerals in these rocks, however, remain largelyunaltered, mostly occurring in the form of the mineral chromatite (FeCr2O3).Other chromium-bearing minerals include eskolaite (Cr2O3) and the chro-mium-bearing spinel minerals include rutile, ilmenite, and magnetite; theseare also contained within other silicate minerals such as the hornblendesand pyroxenes. Many ultramafic rock types contain significant amountsof chromium. Matzat and Shiraki (1974) reported that dunite, peridotite,pyroxentite, and serpentinite carry chromium concentrations to a maximumof 2400 mg/kg. Soils and sediments derived from these rocks also containchromium. Scott (1995) noted from the analysis of 158 soil samples, collectedfrom 25 locations in Sunnyvale and Mountain View (California), that theaverage chromium concentration was 51.28 mg/kg with a range from 30.5to 72.0 mg/kg.
Serpentinized and altered ultramafic rocks have been described in the liter-ature; these include the New Idria serpentinite body in San Benito County,which covers an approximate 124.3 km2 area along the ridge of the DiabloRange (Figure 2.3). About 18.1 km2 of the unit is located within the ArroyoPasajero’s Los Gatos Creek drainage basin. The serpentinite body contains anasbestos deposit of highly sheared and fractured, soft, friable, and powderyserpentinite containing blocks and fragments of harder rocks. The alteredmatrix includes plates and flakes of chrysotile, antigortite, magnesite, brucite,and talc. Asbestos, chromite, and mercury ore have been mined from thisportion of the upper Los Gatos Creek drainage area (Jones, 1988). However,much of the chromium contained in these minerals probably is Cr(III), althoughCr(VI)-bearing minerals may occur due to hydrothermal alteration of the sourcerocks (see the following paragraph; the next section; Steinpress, 2001).
The possible geochemical processes that transform Cr(III) to Cr(VI)include deuteric and hydrothermal alteration of the country rock containingchromium and chromium-bearing minerals. Hydrothermal alteration ofultramafic rocks, including serpentinite, could mobilize relatively insolubleCr(III) minerals by altering them into different mineral species. Numerousexhalative mercury-gold deposits, with associated carbonate-silicate alter-ation (producing a quartz, opal, and a carbonate-rich rock), occur through-out the California Coast Ranges. Examples include the Sulphur Bank minein Lake County, the New Almaden mine in Santa Clara County, and theNew Idria mining district in San Benito County, California (Figure 2.4).Mercury-gold deposits occur mostly in serpentinite associated with Fran-ciscan complex and associated ultramafic rocks. These deposits were formed
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Chemistry, Geochemistry, and Geology of Chromium 75
by hot-spring systems, from alkaline sulfide solutions at relatively lowtemperature and shallow depths, remnants of which are still present. [Inthe central and southern Coast Ranges, four hot springs with water tem-peratures 16.7 to 48.9°C are listed on the San Jose 1:250,000 scale map sheet.The Santa Cruz 1:250,000 scale map sheet lists 12 hot springs with watertemperatures ranging from 23.9 to 62.2°C (Jennings, 1985).] Widespreadalteration of country rock is in the vicinity of these hot-spring systems, withadvanced argillic alteration occurring in adjacent volcanic rocks at theMcLaughlin gold-mercury deposit in Lake County. That hydrothermal alter-ation has transformed chromium oxide minerals is evidenced by the pres-ence of redingtonite (a hydrous chromium sulfate) at the Manhattan minein the Knoxville mining district of Lake County, California (Davis, 1966;Albers, 1981; Vredenburgh, 1982; Peters, 1991; King and Rytuba, 1999).
The geochemical process of transforming Cr(III) to Cr(VI) in soil, sediments,and groundwater also would include oxidation by manganese dioxide(MnO2), and Franciscan complex rocks in the California Coast Ranges hostnumerous manganese mineral deposits. These occur as thin elliptical chertbodies containing the manganese minerals psilomelane, pyrolusite, rhodoch-rosite, hausmannite, and braunite. Such mineral assemblages occur in 70% of
FIGURE 2.3Ultramafic rock bodies in the central Coast Range of northwest-central California. (Map modifiedfrom Churchill, R.K. and Hill, R.K, 2000, California Division of Mines and Geology, Sacramento,California, Open File Report 2000–19, one map sheet, 1:1,000,000 scale. With permission.)
- ultramafic rock
LEGEND
MontereyBay
Monterey
SantaCruz
SanFrancisco
SanJose
NewIdria
101
101
101
580
5
5
99
99
Modified from: Churchil and Hill, 2000.
P a c i f i c
O c e a n
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76 Chromium(VI) Handbook
manganese deposits (Albers, 1981; Mosier and Page, 1988). In groundwater,manganese(IV) oxide occurs in the same Eh and pH ranges as the chromateand dichromate anions (Figure 2.5). Insoluble manganese dioxide coating sed-iments in the saturated zone may oxidize Cr(III) to Cr(VI).
FIGURE 2.4Location of mercury-gold ore deposits in the central Coast Range of California. (Map modifiedfrom Bailey et al., 1973, U.S. Geological Survey Professional Paper 820, U.S. GovernmentPrinting Office, Washington, D.C., pp. 401–414.)
Rocks younger thanCoast Range thrust
Rocks of Great Valley sequencelying above Coast Range thrust
Eugeosynclinal (Franciscan) rockslying beneath Coast Range thrust
Older, generally metamorphosedrocks and late Mesozoic granlticrocks
CO
AS
T
SIE
RR
A
PA
CIFIC
NEVADA
RANGESOCEAN
VALLEY
GR
EA
T
New Idria
SulphurBank
SanFrancisco
NewAlmaden
Santa Barbara
Sacramanto
LEGEND
Contact(Dashed where inferred, dotted where
concealed, hachures on overthrust side)
- Coast Range thrust- Major strike slip fault or fault zone
- Mercury - gold deposit- Less productive mercury deposit
Modified from: Bailey and others, 1973.
-
-
-
-
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Chemistry, Geochemistry, and Geology of Chromium 77
2.4.2 Known Sources of Natural Cr(VI) in Rocks
As of 2002, there were at least 24 known Cr(VI) minerals (Barthelmy, 2002);these occur as:
1. Cr(VI)-bearing minerals that are formed in the oxidized zones oflead (Pb) deposits, generally as the mineral crocoite (PbCrO4). In the
FIGURE 2.5Eh-pH diagram for the system manganese-oxygen-hydrogen. Eh values in volts (V). (Diagramfrom Brookins, D.G., 1987, Eh-pH Diagrams for Geochemistry, Springer-Verlag, New York, 176 p.With permission.)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
–0.2
–0.4
–0.6
–0.80 2 4 6
pH
Eh
(V)
8 10 12 14
SYSTEM Mn–O–H25°C, 1 bar
PO2 = 1 bar
PH2 =1 bar
MnO2
Mn2+
Mn(
OH
) 2
Mn(
OH
) 3–
Mn3O4
MnO
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78 Chromium(VI) Handbook
United States, crocoite occurs in the Vulture mining district of Arizona(Hurlbut, 1963). Hemihedrite [Pb10Zn(CrO4)6(SiO4)2F2] occurs at theFlorence lead-silver mine near Whickenburg, in Maricopa County,Arizona; deanesmithite (Hg2
+Hg32+Cr6+O5S2) and edoylerite
(Hg32+Cr6+O4S2) occur at the Clear Creek mercury mine in the New
Idria mining district, in the Diablo range of San Benito County,California (Perroud, 2001; Barthelmy, 2002).
2. Cr(VI) minerals in nitrate-rich evaporite deposits in arid or desertenvironments such as the Chilean nitrate deposits in the AtacamaDesert. Chromate minerals are largely confined to iodine-bearingnitrate deposits where the annual precipitation is very low (approx-imately 50 mm per year). The ores consist of caliche-containingsodium nitrate (NaNO3), lautarite [Ca(IO3)2], other iodates, anddietzeite [Ca2(IO3)2(CrO4)] (Williams-Stroud, 1991; Barthelmy, 2002).The Peru-Chile Desert, which includes the Atacama Desert, formsone of a series of apparently long-lived, west coastal-type, subtrop-ical deserts such as those found in Australia and the Namib Desertof Africa. These are also known as hyperarid deserts and they mayhave begun forming as much as 14 million years ago. The Peru-ChileDesert is between 10° and 30° south latitude and 70° and 80° westlongitude. The Atacama Desert is centered at approximately 25°south latitude and 70° west longitude (Hartley and Chong, 2002).Similar climatic conditions may have occurred in the recent geologicpast (Cenozoic) in the Mojave Desert of California and Arizona.
3. Sheared, altered, and serpentinized ultramafic rocks, such as theFranciscan complex, containing Cr(III) minerals. Steinpress (2001;Section 3.1, this volume) reported the presence of 0.06 to 0.46 mg/kgCr(VI) in serpentinite. However, no specific mineral identificationwas made and the actual Cr(VI) mineral assemblages are not known.
2.5 Chromium Distribution In Secondary Environments
2.5.1 Known Natural Cr(VI) Occurrences in Surface Water and Groundwater
In surface water, Cr(III) generally is the predominant species; however,Cr(VI) has been reported in oxygenated fresh water lakes and estuaries.Kaczynski and Kleber (1993) noted natural Cr(VI) maximum concentrationsof 0.026 µg/l in Masonboron Inlet, North Carolina.
In the Paradise Valley, north of Phoenix, Arizona, groundwater collectedfrom wells with the pH approaching 9.0 contained 100 to 200 µg/l Cr(VI)(Robertson, 1976; Hem, 1989). Robertson believed that hydrolysis of feld-spars and common mafic minerals, such as augite and biotite comprising
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Au: Ok?
Au: InsertionOk?
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Chemistry, Geochemistry, and Geology of Chromium 79
the alluvium, caused alkaline groundwater conditions in the middle of thevalley. This coupled with calcite produced highly alkaline conditions. Inaddition, there was an absence of ferrous iron, organic matter, and reducingorganisms, which allowed the groundwater to retain its oxidizing and alka-line character, transforming Cr(III) into Cr(VI).
Godgul and Sahu (1995) noted that in the Sukinda chromite belt of Orissa,India, serpentinization and magnesium ion releases during deuteric alter-ation of ultramafic rocks (peridotites) and associated extensive oxidation(laterization) created alkaline (high pH) pore water. This resulted in theproduction of Cr(VI) detected in stream, well, and quarry water. Cr(VI)concentrations ranged from 58 to 64 µg/l in stream water, not detected (ND)to 17 µg/l in well water, and ND to 1791 µg/l in quarry water. These watershad relatively high total manganese concentrations (7230 to 1,540,720 ppb)and relatively low total iron (ND to 12,950 µg/l) concentrations.
In the Presidio in San Francisco, Cr(VI) was present in background-oxidized groundwater with high pH’s, up gradient from any known con-tamination, with concentrations ranging from 52 to 98 µg/l (Steinpress,2001; Section 3.1, this volume). Possible natural background concentrationsof Cr(VI) occur in groundwater near Davis, California with concentrationsranging from 1.0 to 180 µg/l. It is believed that the overall regional Cr(VI)distribution in groundwater is random and therefore a natural condition(Davis, 1995).
In studies by the U.S. Geological Survey (USGS), Cr(VI) has been foundin western Mojave Desert, California groundwater (Ball and Izbicki, 2002).The USGS evaluated Cr(VI) in water sources from public water supplies,domestic and observation wells in alluvial aquifers. Total Cr concentrationsranged from 0.8 µg/l (the detection limit) to 60 µg/l; almost all of the totalCr was Cr(VI).
In 2001, routine sampling of water wells in the Soquel Creek Water District(SCWD) south of Santa Cruz, California determined that Cr(VI) occurred ingroundwater within the Aromas Red Sands (Aromas) aquifer at concentrationsranging from 6 to 38 parts µg/l. While these concentrations were below theMCL for total chromium of 50 µg/l, concerns about public health impactsfrom Cr(VI) in drinking water had been raised during public meetings. ToddEngineers (2002) conducted a focused Cr(VI) groundwater study to determinewhether the detected Cr(VI) was from anthropogenic (man-made) releases orfrom naturally occurring chromium in the Aromas Red Sands Formation. Thestudy found abundant evidence confirming that naturally occurring chromium-bearing minerals found in the Aromas Red Sands Formation were the possiblesources of the Cr(VI) detected in the Aromas aquifer. Environmental conditionsin Aromas aquifer water favor Cr(VI) production from dissolved Cr(III)because the Aromas aquifer waters are well oxygenated, generally have alka-line pH (greater than 7.0), and sediments are predominantly quartz-rich andlow in ferrous iron and aluminum oxides. No anthropogenic sources of Cr(VI)were identified in the inventory of possible contaminating activities in areassurrounding public water supply wells.
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80 Chromium(VI) Handbook
2.6 Forensic Geochemistry
Following the definition of forensic geology (Bates and Jackson, 1987) as“The application of the Earth Sciences to the law,” and of environmentalforensics by Stout et al. (1998): “… the systematic investigation of a contam-inated site(s) or events(s) that has impacted the environment which focuseson defensibly allocating liability for the contamination.”
Forensic geochemistry is the application of geochemical principles in thenatural environment to the law. Therefore, forensic geochemical investiga-tions require chemical and isotopic analysis of physical materials such as air,water, soil, and sediment. For metals, this usually involves the analysis oftheir stable isotopes and at times their radioactive isotopes. For example,Hurst et al. (1996) and Hurst (2002) have successfully used stable lead iso-topes to identify gasoline release sources and even provided an age for thesereleases. A complete explanation of environmental forensic and forensicgeochemical techniques is beyond the scope of this section and the reader isreferred to recent texts by Morrison (2000) and Murphy and Morrison (2002).
2.6.1 Soil
Very few forensic geochemical investigations involving Cr(VI) from contami-nated sites are cited in recent literature. This is largely because such forensicinvestigations are rather new. Recent experimental work by Prokisch et al.(2000) in soil showed that anthropogenic chromium could be distinguishedfrom geological chromium by analysis of yttrium (Y) and total chromium; theresults are plotted on an Y–Cr diagram (Figure 2.6). From experimental data,they found that soil collected and analyzed from noncontaminated areas (geo-logic chromium) had very close Cr–Y linear correlation. However, for areascontaminated by anthropogenic chromium, the Cr–Y values were scattered.
2.6.2 Groundwater
Ellis et al. (2001) reported the first measurements of chromium isotope frac-tionation of 53Cr and 52Cr during Cr(VI) reduction to Cr(III). Their studyshowed that dissolved Cr(VI) from groundwater at three contaminated sitesthat had delta (δ) 53Cr values ranging from 1.1%(per mil) to 5.8%. Thisindicated that reduction of Cr(VI) had occurred because the δ 53Cr valuesfrom plating bath solutions showed little or no fractionation for platingoperations ranging up to five years of use.
In a similar chromium isotope study, completed in the western MojaveDesert of California, Ball et al. (2001) noted that stable chromium isotopescould be used to determine the reduction of Cr(VI) to Cr(III) in a loweraquifer; they found that 53Cr was enriched in groundwater relative to 52Cr.This suggested that the Cr(VI) was from a natural source because anthropo-genic sources of chromium typically have δ53Cr values very near zero.
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81 Chromium(VI) Handbook
2.6.3 Air
As described in Section 2.2.10, Nusko and Heumann (1997) found that theCr(III)/Cr(IV) ratio in dust from soil erosion ranged from 0.27 to 0.35,whereas the same ratio in the atmosphere aerosol particles was much higher,ranging from 0.39 to 0.63, indicating oxidation of Cr(III) to Cr(VI) in theatmosphere. They also found that the soil surface layer at the forest’s edgewhich was high in organic material had a Cr(III)/Cr(VI) ratio of 1.47, indi-cating that Cr(VI) was preferentially reduced by the soil organic matter.Therefore, stable chromium isotopes may be used to determine the source(s)of Cr(VI) in atmosphere aerosol particles.
2.7 Acknowledgments
The author appreciates the editorial comments and suggestions made byDavid W. Abbott, R.G., C.H.G., Senior Geologist with Todd Engineers;Cynthia P. Avakian, Senior Project Manager with Hydro-EnvironmentalTechnologies, Inc.; and Dr. Jacques Guertin, Consulting EnvironmentalScientist. Alain E. Boutefeu, Graphics Coordinator/Technician with ToddEngineers, created and placed section figures into the required format.
FIGURE 2.6Plot of total chromium versus yttrium for soilscontaining chromium from anthropogenic andgeologic (geogenic) sources. (Diagram modified from Prokisch, J., Kovcs, Palencsr, A.J., Szegvri, I.,and Gyori, Z., 2000, Environ. Geochem. Health, 22, 317–323. With permission.)
60
50
40
30
20
10
07 8 9 10 11 12 13
Y (mg/kg)
Cr
(mg/
kg)
anthropogenicchromium
geologicalchromium
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82 Chromium(VI) Handbook
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