-
American Mineralogist, Volume 67, pages 1144-1154, 1982
Techniques for using iron crucibles in experimental igneous
petrology
Cenl R. TnoRNssn exo J. SrepHr,N HuBsNEn
U.S. Geological Survey959 National Center
Reston, Virginia 22092
Abstract
Some iron metal used to fabricate crucibles contains impurities
comprising manganese,titanium and other elements. Under
closed-system conditions these impurities impose an
fo, that can differ from that of a silicate charge in
equilibrium with pure metallic iron.Equilibration of the impurities
in the crucible with the charge can reduce the iron oxidecomponent
of the charge and contaminate it with manganese and titanium.
Pretreatment ofimpure iron crucibles in a CO2-CO atmosphere at
1050'C, under conditions slightly morereducing than Fe-Fer-*O,
minimizes undesirable changes in the bulk composition of
thecharge.
Introduction
Bowen and Schairer (1932), in their study of theiron-oxide-SiO2
system, pioneered the use of ironcrucibles to "control and define"
iron oxide in itsferrous state at high temperature. However,
Bowenand coworkers, and others using metallic iron tocontain iron
oxide-bearing materials, found appar-ently unpredictable variations
in the ferrous-ironcontent of the run products. Iron-bearing
silicatecharges in iron crucibles open to an atmosphere ofpurified
nitrogen are gradually oxidized, as reflect-ed by the progressive
oxidation of the metallic ironcapsule and a steady increase in
ferrous iron con-tent of the charge with time. This change
wasascribed to the small but significant proportion ofoxygen
present in the "inert" gas and was avoidedby keeping run times to
"not more than fifteenminutes" (Bowen and Schairer,1932, p. 179
1935;Bowen et al., 1933), a run duration supposedlysufficient for
achievement of crystal/melt equilibri-um in their system, but one
which is probably notadequate for all systems. In systems open to
anatmosphere of mixtures of CO2 and H2 or CO2 andCO gases,
accumulation or depletion of ferrous ironin the charge is the
result of either oxidation ofmetallic iron in contact with the
charge or reductionof iron oxide within the charge. The extent
ofchange is primarily controlled by run duration andthe oxygen
fugacity (/o,) of the gas mixture (e.g.,Roeder, 1974, Fig. 1). An
additional problem withthe open-system technique is volatilization
and lossof alkalis from the charge, resulting in changes in
bulk composition which affect changes in the oxida-tion state of
iron (Thornber et al. 1980).
Experimental research on the petrogenesis oflunar samples
necessitated devising methods tocontain silicate melts at low/6,
that are in equilibri-um with metallic iron (disseminated
throughoutsome lunar rocks) while maintaining the bulk com-position
of the charge for a time sufficient to reachequilibrium-a period of
hours to days or evenweeks. Toward this end, Muan and Schairer
(1970)revived a method, originally explored (and discard-ed) by
Bowen and Schairer (1932), of sealingcharges in iron crucibles,
which in turn are sealed inevacuated silica-glass tubes. Muan and
Schairerreported significant ferrous-iron gain in charges
andattributed it to a reaction between the metal cruci-ble and
ferric iron in the starting mixture.
Subsequent experimental work on natural andsynthetic lunar
materials, in similar closed systems,has been sporadically plagued
by analogous prob-lems of iron gain or loss. The implications of
suchundesired, perhaps unrecognized, bulk-composi-tion changes and
explanations for their cause arediscussed by Kesson (1975) and
Walker et al. (1975,1976) and are summarized by O'Hara and
Hum-phries (1977). Because of the successful use
ofultra-high-purity iron crucibles (Johnson-MatheyGrade 1,99.998Vo
Fe)t by Kesson (1975), Walker el
rAny use of trade names in this publication is for
descriptivepurposes only and does not constitute endorsement by the
U.S.Geological Survey.
0003-004x/82/l I l2-l 144$02.00
-
THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL PETROLOGY l
145
al. (1977), Longhi et al. (1974), and Green et al.(1975), recent
workers have generally assumed thatless pure iron crucible
material, such as "ingot"grade iron (Grove et al., 1973; Walker et
al., 1972)or electromagnet-grade iron (Muan and Schairer,1970;
Thornber and Huebner, 1980), may impartundesirable changes in the
bulk composition of thecharge.
Problems of ferrous-iron loss encountered by thepresent authors
in their experimental study of thethermal history of lunar
fragment-laden basaltTTll5(Thornber and Huebner, 1980) prompted a
system-atic investigation of the use of iron crucibles inclosed
systems. We wanted to understand the na-ture of impurities in
crucible materials, their role ingoverning redox equilibria, and
the effect of theimpurities upon the bulk composition of
thecharges. Despite the obvious disadvantages of usingimpure iron
crucibles, this material is, by virtue ofbeing less expensive, more
expendable. Polishedthin sections can be cut through the crucible
with-out concern for its reclamation, allowing observa-tion of an
entire cross-section through the chargeand thus a more complete
evaluation of its chemicaland textural variability. In particular,
observationof the crucible-charge interface textures is
oftencritical for the recognition of crystal nucleation oncrucible
walls. For this reason, we sought methodsof using relatively impure
iron crucibles by mini-mizing or eliminating undesirable effects of
cruci-ble-charge interaction.
Characterization of iron metals investigated
Previous investigators considered the relevantimpurities in iron
capsules to be carbon (Biggar eral., 1974; Longhi et al., 1974) and
phosphorus(Walker et al.,1976,1977).We also initially consid-ered
the possibility that sulfur, arsenic and hydro-gen (the last
occluded during refining and manufac-turing of the iron rod stock)
might be undesirableimpurities. However, typical mill analyses of
im-pure iron do not report hydrogen and show carbon,arsenic,
phosphorus, and sulfur at levels too low toaccount for the observed
iron gain or loss.
Petrographic examination of iron from differentsources reveals
characteristics that might be used toselect the most suitable iron
for different experi-ments. We can distinguish different lots of
impureiron on the basis of their inclusions (Figs. 1a, b, andTable
1). For example, different lots of electromag-net-grade iron
contain at least I volume percent ofimpurities in the form of I to
-10 micrometer
medium- and dark-grey blebs, yellow prisms andminor plum-pink
and tan-colored inclusions. Quali-tative chemical analysis by
energy-dispersive elec-tron microprobe and
scanning-electron-microscopetechniques reveals that the inclusions
contain vari-ous combinations of manganese, titanium, iron,nickel,
chromium, aluminum, silicon and sulfur(Table 1). The yellow
prismatic phase has the colorand morphological characteristics of
titanous oxide(TiO). Materials Research Corp. uenz grade
andJohnson-Mathey specpure foil do not show anyinclusions or
precipitates when examined with apetrographic microscope at 800x
magnification.
The bulk chemistry of some lots of iron metal isnot known.
Clearly, however, at least some analy-ses supplied by manufacturers
fail to report poten-tial reducing agents such as hydrogen, and
showtitanium and manganese values (Table 2) that aretoo low to
account for the observed inclusions(much less any additional
titanium and manganesealloyed with the iron). Analyses of
high-purity ironshow negligible amounts of manganese and
titanium(Table 2), consistent with the absence of inclusions(Table
1) in iron.
Experimental methods
Our experiments used crucibles of iron (CoreySteel Corp. lot
#92474, Armco electromagnet-gradeand Materials Research Corp. lot
#2611926 VP-grade, Table 2) and a sintered glass having
thecomposition of lunar basalt 77115 (Thornber andHuebner. 1980).
The crucibles were 0.250 inch indiameter and 0.30 inch long. The
inner bore was0.205 inch in diameter and finished with a
conicalbottom, giving a maximum depth of 0.26 inch. Atypical
capsule weighed -1 gm. The crucibles weremachined and stored in low
sulfur oil, a procedureproven effective by Walker et al. (1977),
cleaned inspectrographic grade N-Heptane or Freon, andthen stored
in a vacuum desiccator.
The sintered glass was prepared by thrice fusingat -1300'C (in
air) a manganese-free reagent-gradeoxide-and-carbonate mix, in a
platinum cruciblepretreated with the mix under similar
conditions.To assure homogeneity, the oxidized glass wascrushed and
ground in reagent-grade isopropyl alco-hol before each remelting.
The composition of theglass measured after the third fusion is
presented inTable 3. The oxidized glass was then placed in
apresaturated AgTsPd3e alloy container and reducedat 1050"C and I
bar pressure in a CO2 and H2 gasmixture with an fo, of 1g-l: s bar.
Particular care
-
I l,16 THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL
PETROLOGY
Fig. I . Reflected-light photomicrographs (oil immersion)
showing inclusions in different lots of Armco electromagnet-grade
iron; (a)yellow prisms (y) and plum-colored blebs (p) in Corey
Steel Corp. loI #92474, (b) medium-grey elongate blebs m-g in
LipiniMuanRod. Bar scale in each photograph represents 25 pm.
was taken to preserve the composition of the re-duced and
sintered glass by quenching it in liquidnitrogen, then storing it
in a vacuum desiccator.The sintered product was tested with a
magnet andcharacterized by X-ray diffraction and optical
tech-niques, all of which showed that it contained anegligible
amount of metallic iron. The sinteredstarting material was ground
in alcohol, dried in airand loaded into iron crucibles. The same
batch ofreduced starting material was used in all experi-ments
reported in this investigation.
Hydrogen treatment of crucibles was accom-plished by heating
them at -825"C in a stream ofhydrogen which flowed through a silica
tube in ahorizontal furnace. Crucibles in this assembly werebrought
up to temperature (- l/2 hour), heated for Ihour, and then cooled
to room temperature (-1l2hour) before removal. In the presence of
the hydro-gen stream, iron, manganese, and titanium are allstable
as metals. Several such capsules were bakedin a vacuum before being
filled with silicate charge.Our intention in including this extra
step was to
determine whether or not H2 could be removedfrom a-iron (at
800'C) or "y-iron (at 1100'C).
Some crucibles were annealed at 1050"C and Ibar in a flowing gas
mixture of COz and H2 or CO2and CO. The latter mixture was used
when ahydrogen-free atmosphere was needed. Ideally, themixing
ratios 9-21 for CO2-H2 or 7-23 for CO2-COdefine the same furnace
/6, values (10-146 bar,compared with l0-ra'0 for Fe-Fe1-*O).
Practically,the temperature may have been too low for
theachievement of furnace-gas equilibrium (Huebner,1975); if so,
the prevailing"fo, would have been morereducing. Nevertheless, at
all .fo, values believedencountered, iron metal, MnO, and TiO2 are
stablephases. The crucibles were quenched in a mannerthat did not
cause oxidation or otherwise alter theircomposition. Drop-quenching
risks contaminationwith carbon, a stable and ever-present phase in
thecool, upstream portion of the flowing COz-CO gasmixture. The
best procedure proved to be rapidremoval from the top (downstream)
end of thefurnace and movement through a N2-rich atmo-
-
THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL PETROLOGY
rt47
Table l. Description and qualitative analyses ofinclusions in
iron-crucible materials
I r on Ma te r i a l Inclusion Type
Yel I ow( pr i sms )
Pl um(b l ebs )
Tan Medium-grey l {edium-grey Dark-grey(b l ebs ) ( b l ebs ) (
e l onqa te b l ebs ) { b l ebs )
Armcot rod (Corey Steellot #92474)
Armcot rod (Corey Steell o t # 9 1 9 1 3 )
Armcot rod (Corey Steellot f91 368)
Batten cruci b l es0 . 2 5 " d i a m .
Ba t t en c ruc i b l es0 . 1 0 " d i a m .
Armcot rod (L ip in/Muan )
Armcot crucib leRoede r ( 1974 )
MRC VP rod('tot #26/1926)
I'IRC Marz rod(lot #26/2226)
Johnson -Mathey(specpure fo i I )
Armcot rod 92474:C02lH2 t reatedC02lC0 t reatedHZ treated
Anncot rod (L ip in/ l {uan)H2 t reated
Elements presentl n t n c t u s l o n
AI
si
J
Ca
t l
Cr
Mn
Fe*
Ni
t race
no i nc l us l ons obse rved
no i nc l us i ons obse rved
T Xxx
no l nc l us l ons obse rved
no l nc l us i ons obse rved
X
x
x
I
x
x
X
x
)(xx
x
x
x' €2ec.ttormg net- gna.de .iltt na
Fe deteded no.q be &n to uttvuudi.ttg ilon latt.
sphere (eminating from the dewer with liquid nitro-gen) and into
the liquid nitrogen to minimize contactwith air.
All capsules were filled with sintered glass suchthat the
crucible-to-sample weight-ratio was 16.0t
0.5. After the capsules were loaded, their lids wereloosely
fitted, and charge and crucible were heated(825'C) in a vacuum for
10 minutes to dry the chargeand remove any hydrocarbons introduced
duringcharge or crucible preparation. Iron sponge, previ-
-
I 148
Table 2. Chemical analyses of iron crucible materials (ppm)
Armco el ectro-l Batten2 MRC vP3 MRC Marz4r c r r o n c r u c i
b l e s
THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL
PETROLOGY
1 0 0
NA
80
c
s i
P
AI
S
T i 7 0 0 N A < 1 0 . 0 1 . 4 0
Cr NA NA 30.0 1 .60
Ii'ln 800 28O NA NA
the development of the manganese-titanium richphases (Fig. 2);
conversely, reduction of the cap-sule material in hydrogen gas
diminishes the occur-rence of inclusions as they are reduced and
alloywith the metallic iron. This result can be explainedby
considering the values of/6, used in the capsulepretreatment. At
relatively high fo,the manganeseand titanium, originally present in
iron-rich alloy,are oxidized and form included precipitates. At
theextremely low /s, values characteristic of the hy-drogen-rich
atmosphere, the metallic state is stableand both manganese and
titanium tend to alloy withthe iron. We have no evidence that the
pretreatmentchanges the bulk composition with respect to man-ganese
and titanium; rather, these elements aresimply redistributed among
the phases present.
Oxidation halos formed when impure crucibleswere annealed in the
CO2-H1 or COZIO gasmixtures. On oxidation, the prismatic, yellow
TiOprecipitates are altered to grey manganese-titaniumoxide. The
width of the halo is defined as the depthof this oxidation from the
crucible surface, and isdistinctly different in capsules treated in
COz-Hzand COrCO gas mixtures, but at the same nominalvalues of
oxygen fugacity and temperatures. Incrucibles treated in COrHz for
143 hours, rindwidth is -200 microns, yet CO2-CO treatments for
Table 3. Composition *tt.T,:t:tit material (fused three
times
Average o f l0 ana lysesox ide @
Si 02
I i02
Al 203
Fe0*
oz**Mn0
Mgo
Ca0
NaZ0
Keo
Tota 1
2 . 8 0 . 1 3
I
M2 iton neealula,ted a,a Fe1i l
"ExcoAltt oxqgen alunvlng all iaan il (etuic
l 8 .0 12 .0
50.0
-
THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL
PETROLOGY
;::i::!q93i;i,:
Fig. 2. ReflectedJight photomicrograph (oil immersion) showing
oxidation halo in CO2-Co-treated electromagnet-grade ironcrucible.
Arrow points toward edge of crucible. Note medium-grey oxidation
rim around yellow prisms (y) and development ofmedium and dark-grey
blebs (m-g and d-g). Bar scale represents 25 /rm.
ir::!'d:t;!;:r;:,
only 96 hours resulted in a rind which is -270microns wide. This
difference could be due to thepreferential permeation of hydrogen
from the COz-H2 furnance gas deep into the metal, where it
wouldtend to retard oxidation. (As we will see later,
thisconclusion is supported by the relative difference iniron loss
with time in runs using CO2-H2 pretreatedcrucibles as compared with
similar experiments inCO2-CO treated crucibles.)
The effect of H2 treatment on impure iron cruci-bles is most
readily observed in the electromagnet-grade material which contains
abundant grey man-ganiferous inclusions (Lipin/Muan Rod),
presum-ably a manganous oxide. Exposure to the H2atmosphere at high
temperature results in develop-ment of numerous vugs, some of which
surroundremnant oxides at the outer extremities of the rod.
In contrast to the behavior of manganese andtitanium, crucible
pretreatment appears to changethe bulk volatile content of the
metal. Treatment atlow /e, effectively removes oxide tarnish,
leavingthe crucibles shiny and bright. We also found thatannealing
in hydrogen reduces the carbon contentof electromagnet-grade iron
from - 100 ppm (Table2) to
-
I 150 THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL
PETROLOGY
Table 4. Run summary
RunnumDer
Cruc i b l e*materi al Run data
tem!---imeResul tswtl I ronas FeO
4 .82 . 0I . 30 . 7
1 . 84 .8z . l
' l I
1 . 6
6 . 7E 2
3 . 2'10 .7
1 0 . 79 . 39 . 5
9 . 41 0 . 3I 0 . 0
9 . 71 1 . 4
1 0 . 3
l l . 41 l . 9
46 EM22 EM
EMI O E M6 E M7 E M8 E M
1 4 E M1 3 E M1 5 E M
34 EM
36 EM
Cruc ib le p re t rea tmentatmo3p-h-erTffi !f Tf TTfi E(hrs)
untreateduntreated
6 Z f
82s8?5825
82582582s
825
825
| 050I 050I 050
C0z-C0 1 050c0;-c0 r I 00cot-co 1o5oC02-C0 1100
untreatedunt reatedunt rea ted
no yesno yes
nononon o
yesyesyes
I 100"c /l / 4 h rI 1 00 .c /
l h rn 0n0no
I 250t252
1250125012501250
1250I 25312 50
1250
1250
1252t25lI 250
I 2501 250I 2501250
t252t2s2t252
I 250I 250
t252
1 250I 250
2 3 . 548
1 2 0 . 5
48
48
2448
r 4 3
4868
I 2 0t 2 ? . 5
?47 7
l s l
24I 6 2 . 5
24t 5 l
I2448
t42
96
2492
t . 02 . 1
H2H2H2H2
H2H2H2
H2
H 2
C0rHzcoi-HiC02-H2
yesyesyesyes
yesyesyes
yes
yes
yesyesyes
yesyesyesyes
yesyesyes
yesyes
n0
yesyes
non on ono
n onono
nono
no
nono
?2
2
H2 825Hi 82s
2I
22
I
2?
?
2
1 6 3r 6 01 6 0
96489648
t 742l 843
EMEMEM
EMEMEMEM
VPVPVP
VPVP
VP
VPVP
q
' l o
2021
4l38
44
3940
H2 825
Clt-Hz I I00coi-Hi I loo
1 1 61 1 6
I
EItrt Cory Steel Lot 192474, laneo eleotronagnet-grsdt.ihontWr
Moteti.aLt Ruet*Jt Cotponabion Lot 126/1926, VP-gadde 'iton
the FeO content of the melt decreases (Fig. 3). Webelieve that
the oxidation of manganese (and per-haps titanium) alloyed in the
iron, and the conver-sion of reduced titanium oxide to higher
oxides, canreduce FeO initially present in the silicate melt.
Iron loss from melts contained in hydrogen-treat-ed
electromagnet-grade crucibles, annealed in avacuum before being
loaded, is sporadic and com-parable to that observed in runs using
untreatedelectromagnet-grade crucibles (Table 4). Thus, hy-drogen
and vacuum heat-treatment of impure iron,despite removing carbon,
is unsatisfactory. Therewere no inclusions observed in crucibles of
VP ironpretreated in hydrogen or in a COr}Iz gas mixture,and no
substantial changes in FeO concentration
were observed in runs using either treated or un-treated
high-purity iron containers. Furthermore,no inclusions formed in
the crucibles during theseruns.
Electron-microprobe analyses of charges run inimpure crucibles
demonstrate that transition metalimpurities diffuse from the
capsule and into thesilicate melt. The concentrations of MnO and
TiO2within the charge are shown to increase with time(Fig. 5),
indicating movement of manganese andtitanium from the crucible and
into the charge. Thisobservation, in conjunction with that of an
in-creased abundance of oxide impurities at the cruci-ble-charge
interface, is suggestive of mutual diffu-sion of manganese and
titanium into the charge and
-
Eo s o E
o
0 4 0 coo
0 3 0 5q)
u0 2 0 f
o
0 r 0 a0)
q)
9 z oo
o 4 0
o
oor 6 0
s
THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL PETROLOGY l
t 5 l
t-J
t 0
l _ -l ' - - - - - l '
. t t l6 7
- 0 0 0! 5 0
Run t ime (hours)
Fig. 3. Percent FeO loss in charge (solid line/left ordinate)
and depth ofcrucible oxidation halo (dashed line/right ordinate)
ys. timein charges run in H2-treated electromagnet-grade iron
crucibles. Length of bars represents total variation of l0 depth
measurements.Run numbers correspond to those given in Table 4,
1 4 0r 3 01 2 0I l 0t 0
of oxygen into the crucible. Thus, it appears thatthese
impurities must in part be responsible for theobserved ferrous iron
reduction.
That substantial manganese accumulated in runs
reported by Roeder (1974) is shown by a compari-son of his
original starting compositions with thoseof the run products
(Roeder, personal communica-tion, 1981). We find that polished
sections of the
Fig. 4. ReflectedJight photomicrographs (air) showing oxidation
halo developed in H2-treated electromagnet-grade iron
cruciblescontaining charges and run for 8 hours (left) and 24 hours
(right) (runs l0 and 6 in Table 4). The crucible-charge interface
is at thebottom of each photograph. Bar scale represents 50 pm.
-
tt52 THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL
PETROLOGY
crucibles used by Roeder contain a substantialamount of
manganese-rich inclusions (Table 1). Wehave also noted that in long
runs at subsolidustemperatures, manganese-bearing capsules
contam-inate pyroxene by causing an anomalously highMnO content (as
much as 5.5 wt.%) at the surfaceof originally manganese-poor (0.2%
MnO) orthopy-roxene plates.
Both FeO depletion and MnO and TiO2 accumu-lation in silicate
charges are alleviated by pretreat-ing the electromagnet-grade
crucibles in atmos-pheres more oxidizing than pure hydrogen (Figs.
5and 6). Contamination of the charge by manganeseand titanium and
FeO loss was observed in runsusing H2-treated crucibles but was
less notable inCO'r}Iztreated crucibles. Increased concentrationof
manganese and titanium was barely detectable incharges contained in
CO2-CO-treated electromag-net-grade crucibles and FeO loss did not
occur inruns of up to 68 hours duration. Minor FeO lossafter
heating for -120 hours may have resultedfrom run times that
exceeded those of crucible CO2lCO pretreatment (this observation
should be born
in mind when making long runs in pretreated cruci-bles).
Comparison of the COz-Ifz and the CO2-COresults indicates that
hydrogen occluded in theCoz-H2treated crucibles may be partly
responsiblefor bulk-composition changes of the charges,
aspreviously suggested from the relative differencesin crucible
halo development as a function of pre-treatment. Also, in runs
using H2-treated crucibles,minor devitrification of the inner wall
of the silicatube is observed and may be attributed to thepresence
of minor H2O generated through crucible-charge interaction.
Pretreatment in CO2-CO gasappears to affect immobilization of the
manganeseand titanium by converting these elements from themetallic
state (in which they would diffuse rapidly)to oxides (which are
relatively immobile in theiron). These observations are also
consistent withresults of open system experiments using
electro-magnet-grade iron by Schwerdtfeger and Muan(1966) using
CO2-CO gas mixtures and those ofRoeder (1974) using COz-H2
mixtures. Schwerdt-feger and Muan reported no appreciable gain
ofMnO or TiOz by Mn-Fe-Si-O compositions equili-
4 0
3 0
oO)
N
oF
s=
0 6
o 4
o 2
o d
os)o
c=s3
2 0
0 8
, _ 8
22ltar-' --- 'a7 coz /Hz Pret'eotment -aU
ot^ 4 2\r- coz /9ofiGot'n"nt
Run t ime (hours)
Fig. 5. Wt.Vo TiO2 (above) and wt.Vo MnO (below) vs. time in
charges run in electromagnet-grade iron crucibles pretreated in
H2(squares), COZ-H2 (triangles), COrCO (circles) and untreated
(diamonds). The composition of the starting mixture (Table 3)
isrepresented by a hexagon. Each point represents an average of
three to five analyses and error bars define one standard
deviationabout the mean. Run numbers correspond to those in Table
4.
r 5 01 4 0r 3 01 2 0I t 0
H 2 Preireotmenr_ 8
A'ra6rx;_ 7t a -
ri) 43v
22
#----X:
-
brated in atmospheres of CO-CO whereas somemanganese
accumulation is evident in basaltic-melt-ing experiments of Roeder
(1974).
The lack of serious iron loss problems in experi-ments of Lipin
and Muan (1975) and Lipin (1978)using electromagnet-grade iron
crucibles in evacu-ated silica tubes could be explained by the
intrinsicfo, of the specific lot of iron used. As shown inFigure lb
and Table l, the "Lipin/Muan" materialcontains abundant elongate
grey oxide blebs whichappar to have a composition of
mangano-wiistite.Crucibles of this material should have an
intrinsic,fo, relatively close to that of the silicate charge
inequilibrium with metallic iron. Thus, there would beno tendency
for substantial redox interaction be-tween the charge and its
container.
Conclusions
It is apparent that the ferrous oxide content ofcharges in iron
crucibles varies in response to thelocal redox potential of the
system. This redoxpotential can be defined by the "fo" of the
ambientatmosphere (in open systems), the incipient fo. ofthe
starting material, and the,fo, imposed by theiron crucible and its
impurities. It is also dependentupon the temperature and duration
of the run, aswell as the presence of other components in thesystem
which affect changes in the oxidation stateof iron dependent of
"fo, (Thornber et al., 1980;Roeder. 1974).
I 153
As suggested by previous investigators and asindicated by our
results, high-purity iron cruciblescan be used to contain
iron-bearing silicate in closedsystems without causing substantial
bulk-composi-tion changes. However, it is also apparent that
thedegree of charge-crucible redox interaction usingimpure iron
crucibles can be highly variable. Itappears that oxidation of
crucible impurities islargely responsible for ferrous iron loss in
iron-bearing oxide materials heated in impure iron cruci-bles and
that the extent of iron-loss is dependentupon the intrinsic fo, of
the crucible material asregulated by the nature of its impurities.
The mostprominent of the complement of reduction reac-tions,
written to emphasize the direction of masstransfer, appear to
be:
l. Mnlslucible) * FeO1slr..g.;-+ MnO * F€lcharge+crucibte)
2a. Tilcrucu"; * 2FeO1"ha1gs;-+ TiOz * 2F€1"hr.g.+crucible)
or2b. TiO1".'"iore) * F€O1"6u.*",
--+ TiOz * Felgharge+crucibte)
3. Hz(crucibte) * FeOlgharge)--; F€(charee) + HzO
The technique for pretreatment of electromagnet-grade iron
crucibles is a critical factor affectingsubsequent redox equilibria
in these experiments.
THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL
PETROLOGY
6 0
,t.0
2 0
0.0
oo,oI
c
oor
s; n J
2 2 ,
H 2 prelreotmenl- 8
r501 4 01 2 0
Run i ime (hours)
Fig. 6. Wt.Vo FeO vs. time in charges run in electromagnet-grade
iron crucibles pretreated in H2 (squares), C.Or-Hr (triangles)
andCOr-CO (circles) and untreated (diamonds). The composition of
the starting mixture (Table 3) is represented by a hexagon.
Eachpoint represents an average of three to five analyses and
symbol size is considerably larger than one standard deviation
about themean for each data point. Run numbers correspond to those
in Table 4.
-
THORNBER AND HUEBNER: IRON CRUCIBLES IN EXPERIMENTAL
PETROLOGY
Results obtained using crucibles treated at 1050"Cin atmospheres
of variable hydrogen content sug-gest that the amount of occluded
hydrogen affectsthe degree of exchange of the reducing
impuritiesand oxygen between crucible and charge and ispartly
responsible for charge reduction. The rateand magnitude of
interdiffusion of titanium, manga-nese, and oxygen between crucible
and charge isdependent upon the "fo, of capsule pretreatmentand, to
a lesser extent, the presence or absence ofhydrogen in the
system.
It appears that pretreatment of electromagnet-grade iron
crucibles in CO7-CO gas satisfactorilyminimizes ferrous-iron loss
by immobilizing themanganese and titanium impurities as oxide
blebs(thereby hindering movement of manganese and/ortitanium into
the charge and of oxygen into thecrucible) and by removing carbon
(and hydrogen)present in untreated crucible material. Such a
treat-ment may not be necessary for impure crucibleshaving an
intrinsic,fo, which is relatively close tothe Fe-Fe1-*O buffer.
AcknowledgmentsWe want to thank Mary Woodrutr for making many of
the
polished thin sections of the crucibles and James McGee
formasterful maintenance of our moribund microprobe. Gordon L.Nord,
Jr. provided the Mn analysis ofthe surface ofthe annealedpyroxene
plate. Donald Lindsley (SUNY-Stony Brook) taughtone of us (JSH) the
usage of iron sponge. We appreciate theconstructive reviews of this
paper by David Walker (HarvardUniv), Ian Duncan (SMU), Peter Roeder
(Queen's Univ) and I-Ming Chou and Bruce Lipin (U.S. Geol. Survey).
This effortnucleated from a study of the crystallization of a lunar
highlandsfragmental basalt. We are grateful to the National
Aeronauticsand Space Administration for providing partial support
throughContract T-781H.
References
Biggar, G. M., Humphries, D. J., and O'Hara, M. J.
(1974)Experimental crystallization and geochemical interpretation
oflow titanium basalts. Lunar Science V,60-62.
Bowen, N. L., and Schairer, J. F. (1932) The system
FeO-SiO2.American Journal of Science. 24. 177-213.
Bowen, N. L., Schairer, J. F. and Posnjak, E. (1933) Thesystem,
CaO-FeO-SiO2. American Journal of Science, 26,193-284.
Bowen, N. L. and Schairer, J. F. (1935) The system,
MgO-FeO-SiOz. American Journal of Science, 29, l5l-217.
Green, D. H., Ringwood, A. 8., Ware, N. G., and Hibberson,W. O.
(1975) Experimental petrology and petrogenesis ofApollo 17 mare
basalts (abstract). Lunar Science VI, 3ll-313.
Grove, T. L., Walker, D., Longhi, J., Stolper, E. and Hays,J. F.
(1973) Petrology of rock 12002 and origin of picriticbasalts at
Oceanus Procellarum. Proceedings of the LunarScience Conference
4th, 995-l0ll.
Huebner, J. S. (1975) Oxygen fugacity values of furnace
gasmixtures. American Mineralogist, 60, 815-823.
Jarosewich, E., Nelen, J. A., and Norberg, J. A. (1979)
Electronmicroprobe reference samples for mineral analyses.
Smithso-nian Contributions to the Earth Sciences no. 22 (R. F.
Fudali,ed.) 68-72.
Kesson, S. E. (1975) Mare basalts: Melting experiments
andpetrogenetic interpretations. Proceedings of the Lunar
ScienceConference 6th, 921 -9M.
Lipin, B. R. and Muan, A. (1975) Equilibrium relations
amongiron-titanium oxides in silicate melts: The system
CaMgSizOe-"FeO"-TiOz in equilibrium with metallic iron. Proceedings
ofthe Lunar Science Conference 6th, 945-95E.
Lipin, B. R. (1978) The system Mg2SiOa-Fe2SiOa{aAl2Si2Os-SiO2
and the origin of Fra Mauro basalts. American Mineral-ogist, 63,
350-364.
Longhi, J., Walker, D., Grove, T. L., Stolper, E. M., and
Hays,I.F. (1974) The petrology of the Apollo l7 mare
basalts.Proceedings of the Lunar Science Conference 5th,447469.
Muan, A. and Schairer, J. F. (1970) Melting relations of
materi-als of lunar composition. Carnegie Institute of
WashingtonYearbook, 69, 243-245.
O'Hara, M. J. and Humphries, D. I. (1977) Problems of iron
gainand loss during experimentation on natural rocks: the
experi-mental crystallization of five lunar basalts at low
pressures.Royal Society of London Philosophical Transactions, A
286,3 13-330.
Roeder, P.L. (1974) Activity of iron and olivine solubility
inbasaltic liquids. Earth and Planetary Science Letters,
23,397-410.
Schwerdtfeger, K. and Muan, A. (1966) Activities in olivine
andpyroxenoid solid solution of the system Fe-Mn-Si-O atll50'C.
Metallurgical Society AIME Transactions, 236, 201-210.
Thornber, C. R., Roeder, P. L., and Foster, J. R. (1980)
Theeffect of composition on the ferric-ferrous ratio in
basalticliquids at atmospheric pressure. Geochimica et
CosmochimicaActa. 44. 525-532.
Thornber, C. R. and Huebner, J. S. (1980) An experimentalstudy
ofthe thermal history offragmentladen "basalt" 77115.Proceedings of
the Conference of the Lunar Highlands Crust(J. J. Papike and R. B.
Merrill, eds.) p. 233-252.
Walker, D., Longhi, J., and Hayes, J. F. (1972)
Experimentalpetrology and origin ofFra Mauro rocks and soil.
Proceedingsof the Third Lunar Science Conference, |, 797-81'1 .
Walker, D., Longhi, J., Stolper, E. M., Grove, T. L. and Hays,J.
F. (1975) Origin of titaniferous lunar basalts. Geochimica
etCosmochimica Acta. 39. 1219-1235.
Walker, D., Kirkpatrick, R. J., Longhi, J., and Hays, J.
F.(1976) Crystallization history of lunar picritic basalt
sample12002: phase equilibria and cooling rate studies.
GeologicalSociety of America Bulletin, 87, 646-656.
Walker, D., Longhi, J., Lasaga, A. C., Stolper, E. M., Grove,T.
L. and Hays, J. F. (1977) Slowly cooled microgabbros15555 and
15065. Proceedings of the Lunar Science Confer-ence.8th.
152l-1547.
Wiggins, L. B. and Huebner, J. S. (1981) Combined
X-raywavelength and energy dispersive analysis to increase
micro-probe eftciency. U.S. Ceological Survey Open File
Report#8t-104r.
Manuscript received, January 28, 1982;acceptedfor publication,
July 22, 1982.