Canadian MineralogistYol. 29, pp. 791-802 (1991)

ABSTRACT

The equilibrium andalusite + calcite + quafiz =anorthite + CO2 has been experimentally reversed insupercritical H2O-CO2 fluids at P(total) = P(COJ +P(H2O) = I and 2 kbar. The following half-bracketsdefine the location of this equilibrium:P (kbar) Temperature X(COz) Stable Assemblage2 399 + 5"C 0.53 high T2 412 + 3"C 0.68 high T2 4W + 3'C 0.82 high T2 392 + 4"C 0.68 low T2 379 * 2'C 0.21 high T2 364 + 3'C 0.17 low TI 366 + 3'C 0.14 high T1 338 * 8'C 0.74 low TReaction direction was determined by comparing XRDpatterns of reactant with those of product assemblages,by observing textures of the products with an SEM, andby monitoring the mass of COz consumed or producedduring an experiment. The composition of the fluid phaseat the conclusion of an experiment was determined byweighing the masses of HzO and COz. These newexperimental data indicate that the position of thisequilibrium is approximately 40"C lower at P(total) = 2kbar and X(COz\ = 0.6 than the results of Kerrick &Ghent (1979), who monitored weight change in a singlecrystal, and of Jacobs & Kerrick (1981), who monitoredthe change in the mass of COz to determine reaction inexperiments of considerably shorter duration (seven days)than ours (129-238 days). Our results, which indicate thatthe position of this equilibrium is slightly lower intemperature than calculated with the thermodynamic dataoi Berman (1988), offer good support for the Kerrick &Jacobs (1981) equation of state for HzO-COz fluids inthis P-T region.

Keywords; experimental, equilibrium, thermodynamic,andalusite, anorthite, H2o-co2 mixing.

Son,tnaarnE

EXPERIMENTAL REVERSAL OF THE EOUILIBRIUMANDALUSITE + GALCITE + OUARTZ = ANORTHITE + GO,

JOSEPH V. CHERNOSKY, JN.Department of Geological Sciences, University of Maine, Orono, Maine 04469' U.S.A.

ROBERT G. BERMANGeological Survey of Canoda, 601 Booth Street, Ottawa, Ontario KIA 088

ir des pressions totales [= P(cO2) + P(H2o)] de I et 2kilobars. Les demi fourchettes suivantes d6finissent laposition de l'6quilibre:P (kbar) Temp6rature X(CO) Assemblage stable2 399 + 5'C 0.53 haute T2 412 *. 3"C 0.68 haute T2 409 + 3"C 0.82 haute T22

1I

392 + 4oC 0.68 basse T379 * 2'C 0.21 haute T364 + 3'C 0.17 basse T366 * 3'C 0.74 haute T338 + 8'C 0.74 basse T

La direction de l'6quilibre a 6t6 d6termin6e en comparantles diffractogrammes des r6actifs avec ceux des produits,en examinant l'6vidence texturale avec un microscope6lectronique ir balayage, et en 6valuant la masse de COzproduite ou absorb6e au cours de la r6action. Nousobtenons la composition de la phase fluide au tetme d'uneexp6rience en d6terminant la masse d'eau et de gazcarbonique. Ces donn6es expdrimentales nouvelles indi-quent une position de l'6quilibre, i une pression totale de2 kbar et pour X(COz) = 0.6, environ 40oC au dessousde la d6termination de Kerrick et Ghent (1979),'obtenuepar changement de poids d'un cristal unique, et celle deJacobs et Kerrick (1981), obtenue par mesure de la massede COz dans des expdriences consid6rablement pluscourtes (sept jours) que les n6tres (129-238 jours). Nosr6sultats, qui indiquent une position de l'dquilibre i unetemp6rature l€gbrement plus faible que ce que pr€voit labanque de donn6es thermodynamiques de Berman (1988),€taye 1'6quation d'6tat pour les fluides HzO-COz dans cetintervalle de pression et de temp6rature qu'avaientpropos6e Kerrick et Jacobs (1981).

(Traduit par la R6daction)

Mots-clds: approche exp6rimentale, dquilibre, ther-modynamique, andalousile, anorthite, m6lange H2O-coz.

INTRoDUCTIoN

Nous avons ddtermin6 les conditions d'6quilibre de la For quantitative modeling of a variety of

r6acrion andalousite + calcite + qvartz = anorrhire + petrological problems in both metamorphic andC02 au moyen d'exp6riences avec phase fluide H2O-CO2 igneous systems, accurate description of the

791

THE CANADIAN MINERALOGIST

thermodynamic properties of HrO-CO2 mixtures isrequired. Despite this importance, direct measure-ments of P-V-T properties of these mixtures arelimited in number, and most petrologists rely onthermodynamic properties predicted with equationsof state incorporating a variety of assumed rulesof mixing (e.g., Kerrick & Jacobs 1981, Holloway1977). Owing to the considerable difficulty inmaking experimental measurements of P-V-Tproperties, the phase-equilibrium approach canprovide an important source of data by which thethermodynamic properties of H2O-CO2 mixturesmay be constrained.

In deriving a thermodynamic data-base forminerals, Berman (1988) found that the equation

of state of Kerrick & Jacobs (1981) permittedexcellent agreement (generally within 5'C) amongmost phase-equilibrium data involving an H2O-CO2 vapor and the constraints imposed by otherphase-equilibrium data. One major exception,illustrated in Figure 1, was noted between thecalculated position for the equilibrium:

Calcite + Andalusite + Quartz =Anorthite + CO2 (l)

and the experimental data reported for thisequilibrium by Jacobs & Kerrick (1981) and Kerrick& Ghent (1979). Well-characterized minerals wereused in both of these studies. and brackets were

5m

480

r l

Bernan (1988) database+ Iacobs & Kerrick (1981)

X Kerrick & Ghent (1979)

O Thompson (19?6) \

P,,o,n = 2 kbar

\ \

An + C0'L

\

And + Cal + Qtz

C)o\r 460

Cj)

t {W5

iJ

s€ 424F{c;)9.{ 400

Ea) 380

F{360

MoleFrc. 1. Comparison of experimental data at 2 kbar and the position of equilibrium (1) predicted with the thermo-

dynamic data of Berman (1988) using the H2O-CO2 equation of state of Kerrick & Jacobs (1981). Symbolsmark the location of experimental data after adjustment away from the equilibrium curve for quoted uncertainties(connecting lines) in temperature and fluid composition.

r r l l l t l t t t l

010n30405060?08090Im

percent c02

ANDALUSITE + CALCITE + QUARTZ = ANORTHITE + CO2 793

located by carefully monitoring the weight changeof either a quafiz sphere (Kerrick & Ghent 1979)or the mass of CO2 (Jacobs & Kerrick 1981) at theconclusion of an experiment. Brackets determinedin both studies for the related equilibrium

Kyanite + Calcite + Quartz : Anorthite + COz A)

are consistent with Berman's (1988) database.The present study was initiated in order to

resolve the inconsistency noted above and therebyprovide more rigid constraints for the refinemenlof thermodynamic data. This project is part of abroader study aimed at providing constraints onH2O-CO2 mixing models at low temperatures andpressures (see also Miider 1991).

ExpsnrMsNraI- METHoDS

Procedure

Experiments were conducted in 30.5-cm-longhorizontally mounted cold-seal hydrothermal ves-sels machined from Haynes Alloy #25 (stellite) orRen6 #41. Each vessel was connected to its own15.2-cm Astra pressure gauge. At the initiation ofan experiment, the pressure reading on the Astragauge was referenced to one of two factory-calibrated, 40.6-cm Heise bourdon-tube gaugescertified by the manufacturer as being accurate to+ 0.190 of full scale (0-4000 bars and 0-7000bars). The Heise gauges were maintained at I atmexcept when calibrating the Astra gauges. Minorfluctuations in pressure resulting from temperaturedrift did occur; however, results of experiments thatsuffered significant pressure changes were dis-carded. Pressures are believed accurate to within+ 50 bars of the stated value.

The temperature of each experiment wasmeasured with a sheathed thermocouple positionedadjacent to the charge in a well drilled into the baseof the pressure vessel and parallel to the vessel bore,and was recorded daily. The calibration of everythermocouple was checked after each experimentby placing a previously calibrated "standard"thermocouple within the pressure vessel at atmos-pheric pressure and noting the difference intemperature between the external and standardthermocouple. Temperature corrections were usual-ly less than 5oC. This procedure insures internallyconsistent temperatures among all pressure vesselsand, most importantly, allows one to monitorchanges in the calibration of the external ther-mocouples with time. Measurements made atatmospheric pres$ure indicate that temperaturegradients in the pressure vessels were less than loCover a working distance of 3.0 cm. Hence, error

resulting from a temperature gradient along thegold sample containers, which were 1.25 cm long,was assumed negligible. We believe that theprincipal source of uncefiainty in the temperaturesreported below is due to daily fluctuations. Theseuncertainties are reported as +2 standard devia-tions about the mean temperature in Table l. Forthe thermodynamic computations, 3oC was arbitra-rily added to the temperature uncertainties reportedin Table I to account for calibration uncertainties.

Charges were prepared by sealing 7 to 11 mg ofstarting material together with silver oxalate(Ag2CrO) + H2O in l.25-cm-long gold capsules.The mass of fluid at the start of an experimentvaried from 6.4to7,6 mg, with the solid:fluid ratioranging from 0.9 to 1.8. Sufficient fluid was presentto ensure that Plu1a : Psolids.

The technique used to determine fluid composition was pioneered by Johannes (1969). The fluidcomposition at the conclusion of an experiment wasdetermined by first weighing the mass of CO2 lostafter puncturing the sealed capsule in which theexperiment was conducted; the punctured capsulewas heated and subsequently weighed to determinethe mass of HrO evaporated. The puncturingprocess was observed with a binocular microscopein order to detect the inadvertent loss of H2O,which appear as bubbles around the puncture oras droplets on the puncturing needle. The escapeof H2O together with CO2 was averted bycentrifuging the H2O to one end of the capsuleprior to puncturing the opposite end of the capsule.

Values of X(COJ final are more precise thanvalues of X(CO) initial reported in Table I becausethe masses of the constituents loaded into the goldcapsules were determined with a Mettler HrObalance, which can be read to 0.01 mg, whereasmasses of CO2 and HrO lost upon puncture andsubsequent drying of the charge were determinedwith a Cahn 4700 Electrobalance, which can beread to 0.001 mg. Because the precision of weighingon any balance is inversely proportional to the load,the relatively heavy gold sample containers canlimit the precision in weighing of H2O and CO2 atthe conclusion of an experiment. To increaseprecision, the gold capsules were counterweighedto mechanically balance out most of the sampleweight, allowing use of a finer electrical range todetermine the masses of HrO and CO2.

It is difficult to assess the ultimate precision withwhich the masses of CO2 and HrO can bedetermined. Only one attempt at determining themass of CO2 is possible because the capsule slowlyloses weight after puncture, presumably owing tothe evaporation of H2O. The mass of CO2 at theconclusion of an experiment probably is overes-timated somewhat because it requires about 30

794 THE CANADIAN MINERALOGIST

TABLE 1. EXPERIMENTAL DATA FOR THE EoUILIBRIUM: AND + CAL + QTZ = AN + CO'

Experlnent Pttotd T Duration

# (kbar ) ( ' c ) (hours )

Xco,

In i t la l F inal

Exlen! ofMrO AC02 St.abLe Reactlon

(mg) (mg) Assemblage (xRD)

L L

1 0

L q

1 8

L

7

U

J

4

5

0 . 7 2

0 . 7 2

0 . 7 0

0 . 1 9

A t 1

n 4 c

0 . 5 2

0 . 7 9

0 . 6 6

0 . ? 0

0 . 7 1

0 . 7 4

0 . 7 4

0 . 7 4

0 . 2 t

0 . 6 8

0 . 8 2

0 . 6 8

0 . 7 3

0 . 7 3

1

1

1

2

2

2

3 3 8 ( 8 ) 3 0 9 6

366 (3 ) 5707

392 (7 ) 5?05

364 (3 ) 5802

379 (2 t 4338

392 (4) 4294

3 9 9 ( 5 ) 5 7 0 6

409 (3 ) s70 ' 7

412 (3 ' , t 4104

4 2 4 ( 7 ) 1 7 2 2

461 (6 ) 4294

- 0 . 0 9 - 0 . 1 6 A n d + C a I + Q t z w

- 0 . 0 7 + 0 . 0 3 A n

- 0 . 1 3 + 0 . 5 5 A n S

-0 ,23 -0 .45 And + Ca1 + Q l z S

- 0 . 0 7 + 0 . 0 2 A n w

- 0 . 0 6 - 0 . 1 L A n d + C a l + Q t z W

+ 0 . 0 L + 0 . 2 2 A n w

- 0 . 1 1 + 0 . 1 1 A n w

- 0 . 0 4 + 0 . 3 2 A n M

- 0 . 0 7 + 0 . 3 9 A n S

- 0 . 0 4 + 0 . 5 6 A n S

Numbers in paxentheses represent two st.andard deviations in terms of least uniLs cj-ted for themean temperatures 1-o Lheir imnediate lef t . AHrO and ACO2 are def lned by (H2O products-H2o

reactants) and (CO2 products-CO2 react .ants) , respect ively. S(strong), M(moderate) , and W(weak)

represent qual-ilative estimaLes of the exlent of reacLion based on visual examinatlon of XRD

pat lerns. * lndLcates react lon di rect . ion determlned on the basis of SEM and changes in f lu id

composi t lons.

seconds for the digital readout on the balance todampen. The mass of water that is determined afterdrying the capsule to constant weight probably isunderntimated somewhat, owing to evaporationwhile the mass of CO2 is being determined as wellas to possible ad$orption on the surfaces of thefine-grained solids. Neglecting the possible effectof adsorption of H2O, masses of H2O and CO2 areprobably precise to within 0.01 mg. Blank experi-ments with capsules containing silver oxalate +H2O suggest that the reproducibility achieved inmeasuring X(CO) is better than * 0.02 nearX(COr) = 6.5.

Storting material

In addition to a supercritical H2O-CO2 fluidphase, the starting material for each experimentconsisted of a mixture of both natural (andalusite,anorthite and quartz) and synthetic (calcite) phasesdescribed below. Stoichiometric proportions ofandalusite, calcite and quaftz were mixed with anequal amount by weight of anorthite andhomogenized by grinding under methanol with anautomatic mortar and pestle for 15 minutes to

insure a fine grain-size and consequent reactivity.After the conclusion of an experiment, solids weregently disaggregated under methanol with an agatemortar and pestle.

Unit-cell parameters of phases used in thestarting materials were calculated by refiningpowder-diffraction data obtained with an Enraf-Nonius FR552 Guinier camera and CuKcvl radiation.BaF (Baker Lot 308, q : 6.1971 t 0.0002 A)slandardized against gem diamond (a = 3.56703A, Robie et al. 1966), and Si (NBS StandardReference Material 640) were used as internalstandards. Least-squares unit-cell refinements wereperformed using the computer program of Ap-pleman & Evans (1973).

Andalusite [Al2SiOj] from Minas Gerais, Brazil,was obtained from M. Holdaway. Results of anatomic absorption analysis (Holdaway 1971), withtotal Fe calculated as Fe2O3, indicate that theandalusite contains 0.38 wt.Vo Fe2O3 and 0.04 wt.VoMnO. Unit-cell parameters are given in Table 2 andcompare favorably with those of another andalusitefrom Minas Gerais (PDF 13-122).

Anorthite [CaAl2Si2Os] from Miakejima, Japan,was obtained from M. Holdaway, who cites a

ANDALUSITE + CALCITE + QLIARTZ = ANORTHITE + CO2

TABTE 2. UNIT-CELL PARAMETERS OF PIIASES USED IN TIIE STARTING MATERIA.I,S

a (A) b (A) c (A) v (A:1

795

Anoltblte

Cal-cIte

Quartz*

8 . 1 8 s ( 3 ) 1 2 . 8 9 9 ( s ) 1 4 . 1 9 1 ( 5 ) L 3 4 4 . 4 4 1 6 7 ' S l

4 .991 (1 ) r 7 . 07214 ) 368 .25 (10 ) Bdz

4 . 9 L 2 4 ( L t s . 4 0 s 2 ( 2 ' , 1 1 2 . 9 5 0 ( 6 ) s l

2 L

13

rlgures In parentheses lepresent lhe estlnateci slandard clevlaElons lnterms of Least unj-ts cj.ted for the value to their lnnedlate leftt theunceltalnties were calculated uslng a unlt-cell refllemen! Program andrep leaent p rec ls ion onLy . For anor th l te , o = 93or p = 1150 and l= 91o.Abbreviatlons: s = x-ray standardi N = nurnber of leflectlons used lnlhe reflneaent. r values delennined by S. Huebner e K. Shalt. S vaLuesreported by ttoldaway (L977').

composition of Ane8. Clear, inclusion-free cleavagefragments were hand-picked and ground. Theunit-cell parameters given in Table 2 were refinedassuming space group Pl and the indices given byBorg & Smith (1969). The refined unit-cellparameters agree with those cited by Borg & smith(1969). Unfortunately, it is not possible to deter-mine the Al-Si distribution in a calcic plasioclasewith a c = 14 i\ cell and space group Pl (Kroll &Ribbe 1983).

Calcite [CaCOr] was obtained from FisherScientific (lot 771134). The powder pattern andunit-cell parameters (Table 2) compare favorablywith those of synthetic calcite (PDF 5-586).

Quartz [SiOz] from Minas Gerais, Brazil, hasunit-cell parameters (Table 2) that comparefavorably with those of quartz from LakeToxaway, North Carolina (PDF 5-0490).

Exqminqtion of experimental products

The products of each experiment were examinedwith polarized light microscopy (PLM), XRD, andscanning electron microscopy (SEM). PLM proveduseful for observing gross morphological changesthat occurred during hydrothermal treatment.Although we did not detect an inhomogeneousdistribution of phases, we did observe that theinterior of a charge generally is finer grained thanthat portion of the charge in contact with thecapsule wall. Unfortunately, the experimentalproducts consist of an intimate mixture of fine-grained phases, which precluded the establishmentof reliable criteria for judging reaction directionwith PLM.

An Amray 1000 SEM operated at 20 kV andequipped with an ORTEC energy-dispersionspectrometer (EDS) was used to examine theexperimental products. Lack of appropriatesoftware precluded quantitative analysis with the

EDS but did allow unambiguous identification ofeach phase.

Inirially, a small amount of the charge wassprinkled on a double-sticky tape for examination.Unfortunately, several problems, including tightclusters of fine grains and charging due to thepresence of too much sample, hampered observa-tion. To disaggregate clusters of grains, ap-prodmately 2 mg of solids were combined withseveral cm3 of methanol in a test tube and dispersedin an ultrasonic bath. The suspended finer-grainedfraction (typically less than 3 pm) was aspiratedonto a Cu tape glued to an Al SEM stub. Althoughthis procedure greatly enhanced observation of finegrains, grains coarser than about 3 pm tended tosettle and consequently were omitted from themount. After allowing the coarser-grained fractionto settle, several drops of methanol containing thisfraction were placed on Cu tape and allowed todry. This procedure resulted in a mount containingcoarse grains relatively free of adhering finer-grained particles. The edges of the Cu tape and Alstub were coated with silver paint and sputteredwith gold to prevent "charging" of the sample.Approximately 30-50 grains in both the coarse-grained and fine-grained fractions of each samplewere examined.

REsulrs

Results of experiments on equilibrium (l) arelisted in Table I and plotted on Figure 2. Althoughtwo tight brackets were obtained at 2 kbar, thebracket at I kbar is fairly broad as a result of verysluggish reaction rates. Because little reactionoccurred, even in experiments lasting over 5700hours, several techniques, including XRD, SEM,and measurements of the composition and directionof evolution of the H2O-CO2 supercritical fluid,were used to determine direction of reaction.

Berman (1988) da tabase

t r I kbar da ta

A 2 kbar da ta

2 kbll

I kbar

'796THE CANADIAN MINERALOGIST

460

\r/ 440

MO(Dt-r= 400

q€

l-{ 380(I)

e 360

40

MoleFtc.2. Temperature - composition diagram showing experimental data for equilibrium (1) at fluid pressures of I and

2 kbar. Symbols indicate the nominal location of experiments, with lines connecting to the initial composition ofthe fluid in the experimental charges. Solid and open symbols depict growth of the lorv-temperature andhigh-temperature assemblages, respectively. The solid curves show the positions of the equilibrium calculated withthe thermodynamic data of Berman (1988).

(D

F 340

50 60

percont c0,

Initially, XRD was the technique of choice fordetermining direction of reaction, but it provedrelatively insensitive in the detection of the smallamount of reaction occurring close to the equi-librium curve. For those experiments where XRDproved useful, a number of non-overlapping X-rayreflections of product as well as reactant phaseswere compared. An experiment was consideredsuccessful if an estimated 20Vo change could beobserved in the intensities of selected reflections ofan experimental product relative to those of thestarting mriterial.

For equilibrium (l), the mass of CO2 shouldincrease for experiments in which the high-tempera-ture assemblage is stable and decrease where thelow-temperature assemblage is stable. Furthermore,as the reaction proceeds, the composition of thefluid phase should evolve in such a way that X(CO2)of the fluid migrates toward the value defined by

the equilibrium curve (Greenwood 1975). SinceH2O does not participate in the equilibrium, themass of water should remain constant.

Compositional changes occurring in the fluidphase are listed in Table 1. Rather than remainingconstant, the water content in the majority of theexperiments decreased slightly. The water contentat the conclusion of an experiment may have beenunderestimated for reasons suggested earlier. Alter-natively, a small quantity of an undetected hydrousquench phase might have nucleated and grown.Although changes in the CO2 content, ACO2, arerelatively small, especially in the most constrainingexperiments, they are consistent with othermonitors of reaction direction and are consideredreliable.

SEM was not used to determine the relativeproportions of the phases but proved useful toexamine surfaces of phases for signs of dissolution

ANDALUSITE + CALCITE + QUARTZ = ANORTHITE + CO2 791

or growth. We expected that grains of the stableassemblage would show euhedral boundaries,whereas grains of the unstable assemblage wouldbe rounded. Because the examination of only a fewgrains in a given experiment did not prove to bedefinitive for an unambiguous determination ofreaction direction, the dimensions and morphologyof 30-50 grains in both the coarse- and fine-grainedfractions of each experiment were recorded.

Anorthite growth and concomitant dissolution

of the low-temperature assemblage are more readilydiscerned with the SEM than the converse. Fortemperatures well above the equilibrium curve, fewgrains of anorthite having smooth, discrete grain-boundaries (abundant in the starting material) areobserved. Rather, anofihite grains havingnumerous "growth" steps, some with euhedraloutlines (Fig. 3a), euhedral anorthite grains "strungtogether" (Fig. 3b), and intergrown clusters ofanofthite grains (Fig. 3c) are observed. As the

Frc. 3. SEM photomicrographs showing growth features on anonhite. (a) Subhedral to euhedral growth-steps'

experiment-s. (b) Euhedrai growth, expeiiment 7. (c) Coalescing grains ofanonhite, with growth steps, experiment7; note small subhedral to euhedral giains of anorthite scattered on growth surfaces. (d) Nucleation and growth

of euhedral anorthite. experiment 3. Scale bar is 1 pm in a and d, 5 pm in b, and 10 pm in c.

798 THE CANADIAN MINERALOGIST

equilibrium curve is approached from the high-temperature side, the proportion of anorthite grainshaving discrete grain-boundaries increases over thatof grains having $owth steps with irregularoutlines. Occasionally, we observed small euhedralcrystals of anorthite in isolation (Fig. 3d) ornucleating on larger anorthite (Fig. 4a) or onandalusite (Fig. ab) grains. Andalusite grains tend

to be rounded, whereas calcite and quartz grainsare very rounded. Etch pits are not common, butdo occur on calcite and andalusite; andalusiteoccasionally displays a subparallel alignment ofwhat we interpret to be etched zones (Fig. 4c).

On the low-temperature side of the equilibriumcurve, coarse andalusite grains display sharpgrain-boundaries, and some have "growth" steps.

b

Ftc. 4. SEM photomicrographs showing dissolution and growth features. (a) Nucleation and growth of small subhedralto euhedral grains of anorthite on a large seed of anorthite, experiment 10. (b) Nucleation and growth ol subhedralto euhedral anorthite on a rounded grain of andalusite, experiment 7. (c) Dissolution of andalusite suggested bysubparallel alignment of irregular etched zones, experiment 10. (d) Growth of an anorthite lath and of euhedralcalcite, experiment 1. Scale bar is I pm long in a, b, c and d.

ANDALUSITE + CALCITE + QUARTZ = ANORTHITE + CO2 799

Occasionally, small euhedral laths (Fig. 4d) andprisms of andalusite are present. Although coarsequartz and calcite grains tend to be rounded, somedisplayweak "growth" steps. Fine euhedral calcitegrains are observed occasionally (Fig. ad). Dissolu-tion of anorthite is suggested by rounded grain-boundaries, by the presence of small etch-pits onan otherwise smooth surface (Fig. 5a), and by thescarcity of steps on grain surfaces. Additionalevidence of anorthite dissolution, such as numerousetch-pits, scalloping, pointed structures and chattermarks reported by other investigators (Berner &Holdren 1979, Holdren & Berner 1979, Allen &Fawcett 1982, Moody et al. 1985), were notobserved.

PLM observations indicate that calcite recrystal-lizes and becomes coarser-grained, even in experi-ments where it is not a stable phase. In experimentsperformed at temperatures above the equilibriumcurve, SEM observations show that fine-grainedcalcite tends to become very rounded. Althoughsome of the coarser calcite grains have stepssuggestive of growth, the steps tend to be anhedraland rounded (Fig. 5b). Calcite may have recrystal-lized during the increase in temperature at theinitiation of an experiment and subsequentlydissolved while in the stability field of anorthite. Itis unlikely that significant rounding occurredduring the quench because coarser grains of calcitethat remained in the stability field of the low-temperature assemblage tend to retain theireuhedral characler; even the fine grains of calcitewere found to be euhedral in some cases (Fig. 4d).

DISCUSSION

Equilibrium (1) also has been experimentallyinvestigated by Thompson (1976), Kerrick & Ghent(1979) and Jacobs & Kerrick (1981). Thompsonobtained very broad brackets at fluid pressures of2 kbar (400-475'C) and 0.5 kbar (375-455"C) bvmonitoring weight changes of quartz rods inexperiments lasting 2l days. The fluid compositionat the start of these experiments had an X(COJ of0.5, but the composition at the conclusion of theexperiments was not reported. Owing to the widthof the brackets, these data are consistent with allother experimental work on this equilibrium (Fig.1) and will not be discussed further here.

The bracket of Kerrick & Ghent was located bymonitoring the weight changes of quartz spheres,whereas the brackets of Jacobs & Kerrick werelocated by monitoring the mass of CO2 lost orgained. Natural phases were used as startingmaterials, and experiments were conducted forseven days in both of these studies. We also usedminerals in the starting malerial and employed thetechnique used by Kerrick & Ghent and Jacobs &Kerrick to measure fluid compositions. The brack-ets obtained by Kerrick & Ghent and Jacobs &Kerrick are plotted on Figures I and 6. Themidpoints of the brackets of Kerrick & Ghent andJacobs & Kerrick lie some 40'C higher than themidpoints of our data at a total pressure of 2 kbarand X(COr) = 0.06.

Berman et al, (1986) discussed the inconsistenciesthat may arise in experimental phase-equilibrium

Frc. 5. SEM photomicrographs showing dissolution features. (a) Dissolution of anorthite suggested by etch pits onan otherwise smooth grain, experiment l. (b) Dissolution and subsequent rounding of growth steps on calcite,exoeriment 3. Scale bar is I rrm in a and b.

800 THE CANADIAN MINERALOCIST

Q

(l)

t-iF

(€L{(l)q

E(l)

F

30 40

MoleFtc. 6. Comparison of experimental data and the positions of equilibrium (l) ar I and 2 kbar predicred with rhe

thermodynamic data of Berman (1988). Light and heavy curves were computed with ideal mixing and the equationof state of Kerrick & Jacobs (1981), respectively, Symbols mark the location of experimenral dara after adjusrmentaway from the equilibrium curve for quoted uncertainties (connecting lines) in temperature and fluid composition.In order to account for possible errors in calibration, a correction of 3oC was added arbitrarily to temperatureiluctuations shown in Table 1.

percent c0,

studies when reaction progress is assessed bymonitoring weight changes of a single crystal orthe mass of COr. Large uncertainties arise indetermining the point at which zero change inweight occurs when arbitrarily selected polynomialsare used to fit weight-change data collected on bothsides of an equilibrium. Berman et al. (1986)showed that alternative polynomials fit to theweight-change data for And + Cal + Qtz = An+ CO2 produced larger uncertainties in theequilibrium position than cited by Jacobs & Kerrick(1981). Furthermore, Monte Carlo techniquesemployed by Berman et al. (1986) suggest thar the2 kbar brackets of Jacobs & Kerrick at 370. and400oC may be more appropriately viewed ashigh-temperature [(low XCOj] "half-brackets"because the data do not extend significantly on thelow-temperature f(high XCO)J side of the equi-librium. This is especially true of the 400.C data,

for which 'the 2o confidence bands of all threepolynomial functions employed in the analysis ofBerman et ol. (1986) failed to define the low-temperature side of the equilibrium.

The results of experiments 14 and l8 of thepresent study agree well with the 370oC bracket ofJacobs & Kerrick (1981). Experiment 18 requires,however, the position of equilibrium (l) to be atsomewhat lower temperature than their 400'Cbracket (Fig. 6), supporting the above interpreta-tion of the 400'C data as a high-temperaturehalf-bracket. A much more pronounced inconsis-tency is apparent between the experimental datareported in this study and the 450oC bracket ofJacobs & Kerrick (1981). This discrepancy cannotbe attributed to the same source. as their dataextend further on the low-temperature than thehigh-temperature side of the ACO, line defining theequilibrium position. As the actual changes in

JacobsKer r i ck

Ker r i ck (1981)Ghent (1979)

Th is s tudykbar da ta

kbar da ta

2ul

l ig

And + Cal

ANDALUSITE + CALCITE + QUARTZ = ANORTHITE + CO2 801

weight observed by Jacobs & Kerrick (1981) areexceedingly small (0.02-0.05 mg), one possibleexplanation for the discrepancy is that CO2 wasconsumed by a small and undetected amount ofback-reaction during the quench. In comparison,the change in the CO2 content of the fluid in allbut two of our experiments was greater than 0.llmg (Table l). Because the longer run durationsproduced more definitive changes in the propor-tions of reactants and products, we believe that ournew experimental data constrain the position of theAnd + Cal + Qtz = An * CO2 equilibrium morereliably than the earlier data.

The thermodynamic data-set derived by Berman(1988) incorporates strong constraints on thedifference in free energy among the minerals inequilibrium (l), obtained through optimization ofcalorimetric data and phase-equilibrium data in-volving volatile-free as well as H2O- and COr-bear-ing fluids (data summarized in Figs. 4-13 ofBerman 1988). Berman (1988) found that thesecombined constraints would permit the position ofequilibrium (l) to be no higher than 415oC atX(CO) = 0.7 and 2 kbar, approximately 35oCbelow the bracket of Jacobs & Kerrick (1981). Ournew experimental data for equilibrium (l) confirmthese thermodynamic calculations. If experimentaluncertainties are considered, the position ofequilibrium (l) predicted with the data-base ofBerman (1988) is consistent with all our experimen-tal data with the exception of experiment 18, whichis about 5oC lower than calculated. The nominalconditions of the experiments suggest, however,that equilibrium (1) lies at least l0"C lower intemperature than predicted. The very small amountof impurities in anorthite and andalusite (see above)have been ignored in these calculations, as theyhave a negligible effect on the position of thecomputed equilibrium-curve.

The most rigorous treatment of this differencebetween the experimental and thermodynamic datarequires small adjustments in the thermodynamicproperties of all minerals in the CaO-Al2O3-SiOr-H2O-CO2 system that were used by Berman (1988)to help constrain the properties of the minerals inequilibrium (l). Such an analysis is beyond thescope of this paper, and best awaits resolution ofseveral other inconsistencies among experimentaldata, most notably in the positions of theequilibrium

Calcite + Quartz : Wollastonite + CO2 (3)

studied by Greenwood (1967), Ziegenbein &Johannes (1974), and Jacobs & Kerrick (1981). Thethermodynamic data of Berman (1988) are consis-tent only with Greenwood's data (Fig. 8a of

Berman 1988). Preliminary analysis indicates thatthe present experimental determination of equilibrium (l) at lower temperature than computedwith Berman's thermodynamic data is morecompatible with the data for equilibrium (3) ofZiegenbein & Johannes (1974) and Jacobs &Kerrick (1981).

The general agreement between the experimentaldata reported by Kerrick & Ghent (1979) andJacobs & Kerrick (1981) for equilibrium (l) ledBerman (1988) to attribute the discrepancy betweenthese data and the predictions based on ther-modynamic data to inaccuracies in the equation ofstate for fluids proposed by Kerrick & Jacobs(1981). However, our new experimental data forequilibrium (l) contradict the earlier experimentaldata and indicate that the position of thisequilibrium computed with the Kerrick & Jacobs(1981) equation of state (Fie. 6) is in excellentagreement with the experimental data in bothH2O-rich fluids and CO2-rich fluids. In contrast,the position of equilibrium (l) computed at 2 kbarassuming ideal mixing of HrO and CO2 (Fig. 6) isstrongly at odds with the combined experimentaldata-set. Thus, within the limits allowed by thewidths of the present brackets, the new experimen-lal data provide good evidence that the Kerrick &Jacobs (1981) model affords a reasonable ap-proximation of the mixing properties of H2O-CO,fluids in this P-T region.

ACKNowLEDcEMENTS

M.J. Holdaway is gratefully acknowledged forproviding the natural andalusite and anorthite.SEM observations were performed in the electronmicroscopy laboratory in the Department ofZoology at the University of Maine; SEM workcould not have been completed without the cheerfulhelp and advice of Kelley Edwards. Perceptivereviews by D.A. Hewitt and D.M. Jenkinsmarkedly improved this contribution and. aregratefully acknowledged.

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Received September 25, 1990, revised manuscriptaccepted September 8, 1991.