Enstatite + CO, and Magnesite: Periclase+ Co2oand Enthalpiesof Formation of Enstatite and Magnesite Anonr,Q,M

American Mineralogist, Volume 80, pages 1252-1260, 1995

Experimental determination of the reactionsmagnesite * quartz : enstatite + CO, and magnesite: periclase+ CO2oand enthalpiesof formation of enstatite and magnesite ANonr,q,M. Kozror.,* Rornnt C. NnwroN Department of the GeophysicalSciences, University of Chicago,Chicago, Illinois 60637, U.S.A.

Arsrnlcr The equilibriumP-Z curveof the reaction MgCO. + SiOr: MgSiOr + CO, (l) magnesite qmrtz enstatite was determined by reversed reactions in well-calibrated, solid-pressuremedium (NaCl) piston-cylinder apparatus.External hematite + magnetite + HrO /o, buffers were used in all experiments. A reversedpoint at 715 "C ofthe reaction MgCOr: MeO + CO2 Q) magnesite periclaa was obtained in an internally heated Ar pressurevessel, using a coarse-glainedsynthetic periclase to inhibit quenching back reaction, along with synthetic magnesite and silver oxalate.At7l5 "CourpressureforReactionI is 10.4+ 0.2kbar,andforReaction2it is 590 + 30 bars. From these two reactions we derive LH'l.2eg: 33.41 + 0.83 kJ for enstatite from the oxides. Using an extrapolatedheat-capacity equation for magnesite,we obtain LH!,,2'B: -116.68 kJ (from the oxides)and -llll.68 kJ from the elementsfor magnesite,somewhat lessnegative than the values of either the Holland and Powell (1990) or the Berman (1988) data sets. A calculated 2 kbar temperature for the magnesite + quartz reactionis 492 oC,about 25 'C lower than the data set predictions but in agreement with a prediction based on natural occurrencesof magnesite, quartz, and enstatite in metacarbonatesfrom the Alps by Trommsdorffand Connolly (1990).

INrnonucrroN land and Powell, 1990); uncertainties in its thermody- properties propagate Enstatite,MgSiOr, is one of the most important of Earth namic throughout the data sets,with materials. It is the dominant member of orthopyroxene far-reaching effectson all the tabulated data. solid solutions, which is the secondmost abundant min- Despite the chemical simplicity of enstatite, its ther- properties eral speciesin the upper mantle and a definitive phaseof modynamic have eluded exact definition, with the major peridotite assemblages.The presenceof ortho- adverseconsequences for calculations of mineral equilib- pyroxene in quartzofeldspathic and mafic crustal rocks ria. The relative insolubility of MgO in aqueousacids and virtually defines the granulite facies of metamorphism. salt melts has hindered measurementof the enthalpy of (Charlu Enstatite is a component of the COr-metasomatic ultra- formation et al., 1975; Torgeson and Sahama, mafic rocks that have figured importantly in defrnition of 1948)and, hence,of the free energyof formation. Newton (1987) the P-7"history of alpine metamorphism (e.9.,Evans and summarized the large spread of measuredvalues properties Trommsdorfl 1974). Orthopyroxene is a characteristic of these and concluded that measured ther- pre- phasein felsic to mafic volcanic rocks and the large lay- mochemical data for enstatite were not sufficiently practical ered intrusions. Much of its importance to petrogenesis cise for utility. lies in its major role as a participant in mineral reactions Many workers (e.9.,Helgeson et al., 1978) have real- that are the basisfor geothermometry,geobarometry, and ized that if suitable high-quality experimental reactions properties oxygen fugacity analysis.As the simplest and most easily are available, thermodynamic of minerals can characterized magnesiansilicate, enstatite anchors the self- be derived from these data more accurately than from physicochemical consistentthermodynamic data sets(Berman, 1988; Hol- other measurements.For enstatite, the experimental databaseis marginal for this purpose. The only stable formation reaction of enstatite that can be * Presentaddress: Department of Geology,University of Day- investigated in accessiblepressure-temperature space is ton, Dayton, Ohio 45469, U.S.A. the decarbonation reaction of magnesitein the presence 0003-004x/95/ | | r2-r252s02.00 r252 KOZIOL AND NEWTON:EXPERIMENTAL STUDY OF MAGNESITE t253

of quartz: MgO + SiO,: MgSiOr. (3) periclas qurtz MgCO, + SiOr: MgSiO. + COr. (l) enstatite magnesite quflz eDstatitc BecauseReaction 3 does not involve magnesite, the unknown high-temperature thermal properties of mag- This reaction has not previously been reversed because nesite are not an issue in deriving the free energy offor- of very slow reactions in silicate systemsin which CO, is mation of enstatite.The heat-capacityfunctions of ensta- the only volatile substance.An equilibrium temperature tite, periclase, and quartz are very well-measured of 521 "C at2kbar was estimatedby Johannes(1969) by quantrtles. projection of other experimental reactionsin mixed HrO- CO, systemsto pure COr. Using this point, the formation Exprnrvrnxur, METHoDS propertiesof enstatitemay be derived in conjunction with the experimentally determined reaction Piston-cylinder experiments Experiments on reaction magnesite quartz, MgCOr: MgO + CO,. (2) the of and Reaction l, were made in a 0.75 in. diameter piston- magnesite prcriclare cylinder apparatus with NaCl pressure medium and a The two determinations of the univariant P-?" curve of graphite heater element. Calibrated matched pairs of Reaction 2 are somewhat discrepant.The broad reversed W-25o/oRe vs. W-30/oRe thermocouples measured the brackets of Harker and Tuttle (1955) place the reaction temperaturesand served as automatic temperature con- 'C curve approximately 15 higher than the two tight re- trol sensors.The manufacturer's statedprecision is better versedbrackets of Johannesand Metz (1968). Additional than + I "C, and the temperature-controllervariation was uncertainty arises from the poorly known heat capacity less than +2 "C. However, the need for a double-capsule of magnesite above 400 {: the data sets use extrapola- high-f", buffer to insure purity of the CO, gas phase re- tions of lower temperature data, and none of these ex- sulted in a separation of as much as I mm between the trapolations consider the possible onset ofcarbonate ro- thermocouple tip and the sample charge. The samples tational disorder at higher temperatures.All these were positioned in a level interval of the temperature uncertainties compound the difficulty in extracting the gradient, on the basis ofprevious gradient measurements, enthalpy and free energy of formation of enstatite from but the temperature uncertainty required independent phaseequilibria. fixed-point calibrations as describedbelow. Experiments Both the Berman(1988) and Holland and Powell(1990) were conducted under piston-out conditions, and maxi- data setsyield a 2 kbar equilibrium point near 520.C for mum nominal pressure variation during an experiment Reaction l, in apparentagreement with Johannes's(1969) was +70 bars. There is almost no frictional correction projection. However, Trommsdorff and Connolly (l 990) for NaCl under these conditions; the pressuresare be- have challengedthe data set predictions. Their revision lieved accurateto 4200 bars at l0 kbar. is based on analysis ofnatural paragenesesin carbonate Samples consisted of homogenized powder mixes of rocks from the Alps. According to their assessment,Re- synthetic enstatite, synthetic magnesite,and natural quartz action I should lie at significantly lower temperatures than sealed in I mm diameter platinum tube segmentswith the reaction of dolomite + quafiz to diopside and COr. weighed amounts of silver oxalate (AgrCrOo)as the CO, The latter reaction was experimentally determined by source. Magnesite was synthesizedfrom Baker hydrous Slaughteret al. (1975) to be closeto 520.C at 2 kbar. magnesium carbonate reagentby recrystallizing under CO, Trommsdorff and Connolly's revision locates a 2 kbar pressureat 400 'C and about I kbar in a Morey vessel equilibrium point of Reaction I near 490 .C. Their con- for 2 d. The result was well-crystallized magnesitewith sequent revision of PTX topology in the system CaO- averagegrain size of about l5 pm. The unit-cell constants MgO-SiOr-HrO-CO, is in better agreementwith the ob- were determined by powder X-ray difractometry, scan- served metamorphic zonation in metacarbonatesin the ning at Vq"/minwith an internal quartz standard. Five central Alps than the topologies predicted by the widely major peaks in the range 30-55' 20 (CuKa, radiation) used data sets.They suggestthat the discrepancywith the were usedto calculatethe cell constantsby a least-squares Berman (1988) data set can be rectified by assuminga method:ao: 4.636+ 0.001and bo: 15.016+ 0.001 free energy of formation of magnesite2 kJ more positive A. Enstatite was synthesizedfrom this magnesitemixed than that used by Berman (1988). This assumptionis intimately with finely ground natural quartz in sealed equivalent to relocating Reaction 2 to considerablyhigh- platinum containersat 1000'C and 15 kbar for 6 h. The er pressuresand lower temperaturesthan those predicted product was nicely crystallized enstatite averaging30 pm by Berman. The discrepancyin Reaction I could, how- in length with the following unit-cell constants,based on ever, be equally well accountedfor by an adjustment of seven X-ray diffraction peaks in the range 26-40'20: ao the free energy of formation of enstatite. :18.226 + 0.003,r0:8.811 + 0.002,andco:5.178 The present work seeksto resolve these uncertainties + 0.001 A. Another sample of synthetic enstatite used in using improved experimental determinations of Reac- some experiments was prepared by Eckert et al. (1992) tions I and 2. A linear combination of these reactions from reagentMgO and SiO, by dry synthesisat 1400 "C yields the formation reaction of enstatite and L7-20 kbar for 24 h. r254 KOZIOL AND NEWTON:EXPERIMENTAL STUDY OF MAGNESITE

TABLE1. CO, yield tests of Ag,C,Oo on the basis of weight (g) of capsule and contents Expt. Beforedecarbonalion expt. Pt capsule o.'18777 0.18358 0.17454 0.181 94 0.30180 + AgrCrO4 0.19415 0.19039 0.18160 0.18966 0.31136 Sealed 0.19410 0.19034 0.181 56 0.18963 0.31130 Aftel decarbonationexpt. Pt capsule 0.19408 0.19033 0.18157 0.18961 0.31130 Punctured 0.19228 0.18838 0.17956 0.18743 0.30859 Dried 0.19228 0.18839 0.17956 o.18744 0.308s5 ActualCO, released(mg) 1.80 1.95 2.01 2.18 2.71 ExpectedCO, (mg) 1.84 1.97 2.04 2.23 2.76

All experiments were conducted with a hematite * which is negligible in terms of displacementof decarbon- magnetite buffer consisting of 35 mg of reagenthematite ation equilibria. It is possible that graphite is metastably and l0 mg of HrO sealedwith the platinum sample cap- oversteppedunder our experimental conditions, and that sule in a 3 mm diametergold tube segmentof 0.15 mm CO and HrO contents of the vapor phase may not be wall thickness. Experiments were considered well buf- negligible. However, Rosenbaum and Slagel(1995) fered if there was no weight loss of the gold capsule at showed by mass spectrometrythat quenchedcharges ini- the end of the experiment, and if the presenceof both tially consisting of silver oxalate in platinum capsules, magnetite and hematite was verified optically after open- buffered externally by hematite + magnetite * HrO, con- ing this outer capsule. tained no detectableCO after being held at 800 "C and 8 kbar for 24 h. Failure to detect significant weight loss in Characterization of vapor phase drying of our punctured capsulesconfirms the virtual ab- Purity of the CO, gas phase is a critical issue because senceof HrO in the gas phase of the platinum capsules. admixture of other volatile componentswould deflectthe We conclude that the gas phaseinside the platinum cap- decarbonation equilibrium point to lower temperatures suleswas effectively pure COr. and higher pressures.To check the purity of the CO, yielded by our silver oxalate (ICN Pharmaceuticals),we Determination of reaction direction performed five puncture weight-loss tests under liquid Each experimental charge contained approximately 4 nitrogen on quenched platinum capsulesthat had been mg of AgrC.Oo to provide approximately I mg of CO, sealedwith several milligrams of the silver oxalate and fluid at experimental conditions. The exact amount of heatedto 400'C for l-2 d in a Morey vesselunder an AgrCrOoas weighed on a Mettler AE240 microbalance external CO, pressure of several hundred bars. The re- was noted for each experiment. Reaction direction was sults of the tests are given in Table l. Within weighing monitored by careful measurementof the amount of va- error, the CO, yields were as expectedfrom the formula por present at the end ofan experiment. AgrCrOo, except for a possible, marginally detectable IJpon removing and cleaning of the platinum sample (about l0lo) mass deficiency. Subsequentdrying showed capsule, our procedure was as follows. The capsule was that this deficiency did not result from H,O in the silver initially weighed. Any leaks during the experiment were oxalate. Nitrogen also is ruled out: If nitrogen were pres- discovered at this time. The capsule was partially im- ent in the form of ammonium oxalate, silver nitrate, or mersed in liquid Nr, punctured, then removed from the ammonium nitrate in the silver oxalate,the puncture mass liquid Nr. In warming from liquid N' temperature, the loss would have been greater than expected.We suggest CO, boiled away from the punctured capsulewhile HrO that the slight CO, deficiencyresulted from photodecom- (if present)was still frozen. The capsulewas immediately position ofthe silver oxalate. reweighed,then heated at 120'C for at least 20 min and H, infiltration into the platinum sample capsulesoc- weighed again. There was no further weight loss, indicat- curred during the experiments, reacting to HrO and car- ing no detectableHrO in the samples. bon monoxide or graphite, until the H, pressure,calcu- Direction of Reaction I could be monitored by the lated to be 0.25 bar at the hematite + magnetite buffer weight loss upon puncturing of the sample capsule and at 1000 K and l0 kbar, was equilibrated in the charge. by comparing powder X-ray diffractometer scansof the The absenceof graphite in the samples places an upper same chargesbefore and after the experiments. Release fugacity limit of 48 bars of HrO inside the COr-filled upon puncture of an amount of CO, that was less than platinum capsules,on the basis ofcalculations using data the amount initially presentas AgrCrOoindicated groWh for hematite, magrretite,COr, and HrO from Robie et al. of magnesite and quartz and consumption of enstatite (1978),Shmulovich and Shmonov (1978),and Burnham and COr. Releaseof an amount of CO, that was greater et al. (1969).Using the Lewis and Randall rule, this fu- than the amount initially presentindicated consumption gacity is equivalent to an HrO mole fraction of 0.003, of magnesiteand quartz and growth of enstatiteand COr. KOZIOL AND NEWTON: EXPERIMENTAL STUDY OF MAGNESITE 1255

In every casethis was supported by the relative growth or reduction of X-ray peak heighls in the X-ray difrac- -En- tron scans. Mo Qz-C02

Calibration of pressureand temperaturereadings A test of the pressureand temperature calibration of A Zo -An- Ky-Qz - H20 our experimental assemblieswas provided by A determin- I ing the reaction of anorthite + H2O to zoisite + kyanite A + quartz. This reaction is readily reversibleat conditions = t0. A almost the same as @ those of our most definitive reversals :< of Reaction I (near l0 kbar and 700 "C) in experiments lrJ of the same duration (a few to several days). The anor- G thite-HrO reaction was determined in internally heated a a gas-pressureapparatus by Jenkinset al. (1985)and Chat- He. te{ee et al. (1984) and is one ofthe reactionsbest con- o- strained by the standard data setsin that thermodynamic data are available for all the phases. Synthetic anorthite was preparedhydrothermally from reagent silica glass, 7-AlrO3, and calcite mixtures at 8 kbar and 730'C for 48 h. The high-pressurezoisite-bearing assemblagewas prepared from the same mixture, with 30 mg of HrO added, at 15 kbar and 700 "C for 48 h. The mix was seededwith a small amount of kyanite, and kyanite was the only AlrSiO, polymorph produced along 700 7t0 with well-crystallized zoisite and quartz. Subequal por- TEMPERATUREDEG C tions of the 8 and l5 kbar sampleswere thoroughly mixed Fig. 1. Reversalexperiments of the calibrationreaction 2 under acetoneto make a reversal mix. X-ray diffraction zoisite(Zo) + kyanite (Ky) + quartz (Qz) : a anorthite (An) + peak heightsof the starting mix were comparedwith those HrO at 700lC (squares:solid : zoisite-kyanite-quartzstable, of the quenched samplesto assessthe reaction progress. open: anorthite-HrOstable) and the reactionmagnesite (Ma) The calibration samples were sealedin I mm platinum + quartz: enstatite(En) + co, at 715 "c (triangles:solid : tubes with 200loexcess HrO, and thesein tum were placed magnesite-quartzstable, open : enstatite-Co,stable). The cal- in 3 mm gold tubes with l0 mg of HrO and 30 mg of ibration experimentsagree closely vrith the predictionsof the calcite (rather than hematite). The geometryof thesedou- Holland and Powell(1990) (HP 90, dashedline) and Berman (1988)(B ble capsulesexactly emulated the geometry of the buf- 88, dotted line) data sets,but the magnesite-quartz reactionwas reversed about I kbarhigher than the dataset pre- fered experiments on Reaction l. Calcite was used in- dictions.This comparisonshows that our resultson the latter stead of hematite possibility to avoid the of difrrsion of reactionare not the productoftemperature or pressuremiscal- Fe through the platinum capsule,which might influence ibration,but ratherthat thereis someinternal inconsistency in zoisite stability. These experiments were performed in both datasets. the same smooth carbide pressurechamber as the Reac- tion I experiments and using the same apparatus and thermocouple. Results of these calibration experiments Muscle ShoalsElectrochemical Company and had a max- are shown in Figure 1. The Holland and Powell (1990) imum impurity of 0.1 wto/oCaO. Use of coarselycrystal- data setpredicts a pressureof9.7 kbar at 700'C, in agree- line periclase eliminated the problem of back reaction ment with the present experiments, and Berman (1988) during quenching to magnesite,a problem encountered gives 9.6 kbar. The present Reaction I experimentsnear by all previous investigators of this equilibrium. Hema- l0 kbar and 700 "C are therefore not subject to substan- tite + magnetite buffered double capsulesof the same tial uncertainty of apparatuscalibration. geometry as that used for Reaction I were positioned in the gas-pressurevessel so that the two thermocouples were Internally heated gas-vessel experiments at the extreme ends of the platinum capsule (about 0.8 A tight reversal on Reaction 2, the magnesitedecom- cm apart). Sintered hematite powder as an H getter was position equilibrium, was determined at 715 'C in an packed around the gold capsule inside a thick copper internally heatedgas-pressure vessel with argon medium. holder, which greatly reduced the temperature gradient The starting material consistedof a homogeneousmix of in the sample vicinity. Temperature readings of the two nearly equal massesof ground synthetic magnesite and sheathed chromel-alumel thermocouples were always 'C periclase.The sample powders were heatedat 350 "C im- within 5 of each other and usually within 2 "C. Pres- mediately before sealing with silver oxalate in platinum sureswere read from a Heise bourdon-tube gaugewith a tube segments.The original periclasewas in the form of precision of +4 bars. Constancy of pressurewas moni- large limpid crystals made by an arc-fusion process by tored by the output of a manganin-resistancepressure t256 KOZIOL AND NEWTON:EXPERIMENTAL STUDY OF MAGNESITE

TABLE2. Reversedexperiments on the reactionmagnesite + t8 guartz: enstatite+ CO,(Reaction 1)

Expected Actual T P f Corloss CO,loss Results oa Expt. ("C) (kbar) (h) (mS) (mS) by XRD 5.105 638 7.5 353 1.12 0.29 magn+ qtz 5.56 640 7.9 166 1.02 0.68 magn+ qtz t4 a 5.86 660 8.1 211 1.07 1.11 no reaction a 5.104 663 7.5 291 1.24 1.51 en + CO, 5.95 670 8.0 238 1.21 1.36 en + CO2 E Mognesite* Ouortz 5.54 680 8.0 136 0.85 1.31 en+ CO, 5.22 700 8.5 211 0.82 1.49 en + CO, @ 5.21 700 9.0 141 1.24 1.23 no reaction :< a, 5.32 700 9.2 138 0.91 1.26 en + CO, ME-1 700 9.2 214 1.65 1.41 no reaction rrr l0 $ 5.24 700 9.4 119 0.88 0.79 no reaction E 5.33 700 9.9 120 0.90 0.86 no reaction a ME-2 715 9.2 190 3.71 4.01 en + CO,

Tlele 3. Data for experimentME-8 TABLE4. Reversed experiments on the reaction magnesite : periclase + CO, (Reaction 2) Weight (g) Betoreexpt. Expected Actual T P f CO,loss CO, loss Results Pt capsule 0.14805 Expt. fc) (bar) (h) (mg) (mS) by XRD + Ag oxalate 0.15311 + Startingmix 0.15585 MP-16 717 560 188 1.03 2.06 periclase Welded 0.15576 MP-15 715 579 101 0.81 0.84 no reaction '114 Afler expt." MP-17 715 602 1.28 1.38 no reaction MP-14 715 621 102 1.26 0.63 magnesite Pt capsule 0.15580 MP-12 7't5 900 138 1.18 0.66 magnesite Afterpuncturet- 0.15405 Driedat 120rc, t h 0.15405 /Vote.'All experiments were performed in aninternally heated gas-vessel Driedat 350rc, 0.5 h 0.15404 apparatus(see text). Uncertainty in temperaturemeasurement is +2 "C. Uncertaintyin pressuremeasurements is +4 bars. Nofej ME-8 experimental conditions were 715 9, 1O.2kbat, tor 271 h. The starting mix was enstatite, magnesite, and quartz, about equal masses of each, finely powdered. ExpectedCO, loss was 1.47 mg. Actual CO, loss was 1.75 mg. - X-ray diffraction analysis showed nearly complete reaction to ensta- The discrepancyof our results on Reaction I with the tite, with no detectable magnesiteand a small amount of quartz present. -" data setsindicates the need to revise the thermodynamic Capsule was punctured as it was partially submergedin liquid Nr. data for either enstatite or magnesitebecause the data of quarlz and CO, are very well known in the temperature and pressurerange around 700 "C and l0 kbar. By using ple of experimentno. 5.97,Table 2, with an internal quartz our data for Reactions I and2 simultaneously,magnesite standard. No angular departuresfrom the peak positions may be eliminated and the Gibbs free energy of the re- of the starting material were detected in this 870 "C sultant Reaction 3, the formation reaction of enstatite, expenment. may be obtained directly. The only quantities that need Table 4 lists our experimentson Reaction 2, and these be considered, apart from the equilibrium pressuresof and all other reversedexperiments on magnesitedecom- Reactions I and 2, are the heat-capacityfunctions ofen- position up to 1000 bars are shown in Figure 3. Our ex- statite and its component oxides, the molar volumes, the periments are consistent with the two carefully deter- compressibilities and thermal expansions of the solid mined bracketsof Johannesand Metz (1968)but slightly phases,and the .f.o, at 10.4 and 0.6 kbar. The former violate one of the reversed experiments of Harker and fugacity requires a slight and straightforward extrapola- Tuttle (1955). The discrepancy with the latter authors tion from the tablesof Shmulovichand Shmonov(1978) could result from fast back reaction in slow-quenching cold-sealapparatus, which could give a falsely high-tem- /l peratureimpression of magnesitestability, as emphasized r000 Ia / by Johannesand Metz (1968). Our data constrain the equilibrium within the pressureinterval of 560-620 bars 'C. at 7l5 This is consistent with the prediction of the Holland-Powell data set but is considerably higher than the 430 bars predictedby Berman (1988). E Mognesite (D

DrscussroN L! Our results support the prediction of Trommsdorffand G.= o', Connolly (1990), made on the basis of analysisof natural a ./' LJ F parageneses, E that the reaction ofmagnesite and quartz to o- Periclose+ COz produce enstatite and CO, indeed lies higher in pressure and lower in temperature than predicted by the data set of Berman(1988). pressure The discrepancyin at 700.C )'o t is about I kbar, and there is a slightly larger discrepancy with the prediction of Holland and Powell (1990).If this 400l_ discrepancypersists down to 2kbar, as is likely, the equi- 650 700 750 800 librium temperature of Reaction I would be about 495 TEMPERATUREDEG C {, close to the value shown by Trommsdorff and Con- nolly (1990). Their revision of the topology of mineral Fig. 3. Experimental reversals of the equilibrium magnesite : periclase+ CO, in the range400-1000 bars.Circles : present equilibria in the systemCaO-MgO-SiOr-COr-HrO is sup- experiments. Half-solid circles indicate no observablereaction. ported. The value of field testing thermodynamically cal- Squares: Harker and Tuttle (1955). Triangles : Johannesand culated mineral equilibria receivesan additional recom- Mefz (1968). Curve A is calculated from the data of Table 5. mendation, in support of those who have advocatedthis Curve B is the prediction of Berman (1988). The predicted curve method (Bergand Dokka, 1983;Johnson, 1963: Pattison, of Holland and Powell (1990) is similar to the present experi- I 992; Schuiling,I 957). mental curve. 1258 KOZIOL AND NEWTON:EXPERIMENTAL STUDY OF MAGNESITE

Taele5, Thermodynamicdata and sourcesrelevant to derivationsin the text

Sr.u a bt10. cx101 dx10s vN aV x 105 bY x 106 Ref.' Enstatite 66.27 350.7 -147.3 167.9 5.826 -4296.0 3.131 9.0 2.3 1,2 Magnesite 65.09 81.1 19 52.254 - 183.20 0 0 2.803 10.6 3.0 2,3* d quanz 41.46 81.145 18.283 - 18.099 0.5406 -698.46 2.269 8.0 s.9 2,4 P quarlz 57.959 9.330 183.471 0 0 2.367 0 2.6 2,4 Periclase 26.94 65.211 - 1.2699 -46.185 n -387.24 1.125 4.6 0.7 2,3 CO, 213.79 87.82 -2.644 70.641 0 -998.86 3 Corundum 50.92 157.36 0.719 -189.69 n -998.04 2.558 6.4 0.9 3 Pyrope 266.27 544.95 20.68 -833.12 0 -2283.0 11.318 29.8 6.3 2, s Nofej Heatcapacity:cp: a+ bT+ cT-2 + dT + er-h. specificvolume:y(l:P): v*'r (r- 298xay) - Plpvl. Unitsforentropy(S), heatcapacity coefficients(a-e), volume (y), expansivity(oV), and compressibility(By) are in joules, kelvin, and bars. *References:1:Krupkaetal.(1979),2:HollandandPowell(1990),3:Robieetal.(1978),4:Hemingway(1987),5:Haselton(1979).Ref. 4 givesAH(4-quartz toB-quartz):625 J al844K. Ref.5 givesH","- Hn":295330 + 1180J and74290J 300J forpyropeand enstatite, respectively.Cohen and Klement (1967) give a - B quaftz equilibriumat 5990 bar and 988 K. Shmulovichand Shmonov (1978) give interpolatedand extrapolated fco.at 988 K: 682 J at 600 bar; 349261 J at 10400 bar. '- Magnesite heat-capacitycoefficients are the extrapolation of Ref. 3.

to l0 kbar. Quantities necessaryfor the calculations are to minimize solvent interactions with the basic (MgO) contained in Table 5. The resulting Gibbs free energyand and acid (SiOr) oxides that arise when they are dissolved enthalpy of formation of enstatite from the oxides are, separately,as was formerly the procedure in high-tem- respectively,-32.78 ! 0.73kJ and -33.41 + 0.83kJ at perature solution calorimetry. Chai and Navrotsky (1993) 298 K. These quantities are compared in Table 6 with also obtained a value for 4118close to that ofBrousse et others resulting from solution calorimetry and those tab- al. (1984). Their value of -33.9 + 2.8 kJ/mol was cal- ulated in several data set compilations. culated from their calorimetry experimentson magnesite There is good agreement between our direct experi- and quartz and the work oflto et al. (1990).The present mental determination of Atf of enstatite and the several phaseequilibrium value ofAllP (enstatite)is more precise experimentally derived data set predictions, except for by a factor oftwo than the Brousseet al. (1984)and the the value of Holland and Powell (1990), which is I kJ Chai and Navrotsky (1993)calorimetric values. less negative. Our value of the enthalpy of formation of An enthalpy of formation of magnesitefrom the oxides enstatite is close to that of the most recent solution ca- at 298 K can be derived from the present determination lorimetric determination, that of Brousseet al. (1984). of Reaction 2. The data of Table 5 are used in these These workers made use of a eutectic (Li,Na)rBrOo sol- calculations.These calculations use the Robie et al. (1978) vent operating at 800 "C, which is more reactive to MgO extrapolation of the heat-capacityfunctions of magnesite. than the calorimetric solventsformerly used.The Brousse Our calculationsgive -116.68 ! 0.47kJ, or -llll.68 et al. (1984) study simultaneouslydissolved mixtures of kJ from the elements,compared with - lll2.48 + 0.81 MgO and SiO, on the composition MgSiO. to maintain kJ of Holland and Powell (1990)and -1113.64 kJ of the stoichiometry of Reaction 3. This procedure appears Berman(1988). A recentrevision by Chernoskyand Ber- man (1989)based on experimentalphase-equilibrium relations of magnesite, spinel, and clinochlore yields - TreLe6, Determinationsof enthalpyof formationfrom the ox- AHc..2s|(magnesite) of I I 14.51 kJ, in worse agreement ides( AHf"ru) of enstatite, MgSi03 in kilojoules per mole with our phase-equilibrium value. It appears that the source of the discrepancy in Reaction I of the Berman AH?.""" (1988) primarily in A1{ magnesite,which (kJ/mol) Reference Solvent,f(K) data set lies of is about 2 kJ too negative. The source of error in the Calodmetric determination Holland and Powell (1990) data set exists in the A-F13of -35.73+ 0.63 Torgeson and Sahama (1948) HF, 346 -35.73+ 0.83 Shearerand Kleppa(1973) Pb,8,O5,970 both magnesiteand enstatite.Because of the lack of mea- -36.90+ 0.71 Charluet al. (1975) PbrB,O5,970 sured heat capacities of magnesite in this temperature -34.18 + 1.92 Kiselevaet al. (1979) Pb,BrOs,1170 range, it cannot be stated at present how much the un- -35.98+ 0.88 Chatillon-Colinetet al. (1983) (Na,Li)BO,,1170 -33.93+ 1.76 Brousseet al. (1984) (Na,Li)BO,,1073 certainty in high-temperature entropy of disordering of -33.9 + 2.8 Chai and Navrotsky(1993)' magnesite contributes to the discrepancy. The stability Phase-equiliblium determination curve of magnesitecalculated from the data of Table 5 is -33.46 Day et al. (1985) shown in Figure 3. The calculatedcurve (curve A) barely -33.65 Bermanet al. (1986) 1000 bar bracket of and Metz -34.15* Saxena and Chatterjee(1986) satisfies the Johannes -33.35 Berman(1988) (1968); the marginal agreementmay be the result of the -32.48+ 1.O4 Hollandand Powell(1990) onset of significant temperature-dependentdisorder of -33.41+ 0.83 present study magnesite,not consideredin the heat-capacityexpression 'Calculated as describedin the text. of Table 5. This disorder might stabilize magnesite to '" Uses C, data Table from of 5 to correct 970 K. higher temperaturesthan predicted by Table 5. KOZIOL AND NEWTON: EXPERIMENTAL STUDY OF MAGNESITE t259

The presentimproved enthalpy of formation value for equilibrium: Clinochlore + 2 magrresite: 3 forsterite + spinel + 2 enstatite may be used in conjunction with the heat of CO, + 4 HrO and revised thermodynamic properties for magnesite. 289. 249-266. solution work Eckert (1992) yield American Journal of Science. of et al. to a calori- Cohen, L.H., and Klement, W.J. (1967) High-low quartz inversion: De- metric Aff for pyrope garnet, MgrAlrSirO,r. They mea- termination to 35 kilobars. Journal ofGeophysical Research,72,4245- sured the solution enthalpy of synthetic pyrope and of a 4251. chemically equivalent homogenousmixture of synthetic Day, H.W., Chernosky,J.V., and Kumin, H.J. (1985) Equililbria in the enstatite and corundum in molten PbrBrO5 at 973 K. system MgO-SiOr-HrO: A thermodynamic analysis. American Min- eralogist,70, 237-248. Their value of A,F10at that temperature for the reaction Eckert, J.O., Newton, R.C., and Kleppa, O.J. (1992) Enthalpy changeof of pyrope to 3 enstatite* corundum is -27.73 + 4.33 the reaction 3/2 enstatite + corundum: pyrope. Geochimica et Cos- kJ. The heat contents of synthetic pyrope and enstatite at mochimica Acta. 56. 3971-3978. 973 K were determinedprecisely by Haselton(1979) us- Evans, B.W., and Trommsdortr,v. (1974) Stability of enstatite + talc, of metaperidotite,Val d'Efra, kpontine Alps. ing drop calorimetry, was and COr-metasomatism as that of corundum much ear- American Journal ofScience. 283A. 274-296. lier by numerous workers, summarized in Robie et al. Harker, R.I., and Tuttle, O.F. (1955) Studies in the system CaO-MgO- (1978). Relevant data are given in Table 5. Thesedata CO,: I. The thermal dissociation of calcite, dolomite and magnesite. unambiguously yield a calorimetric AHl.rn"of pyrope of American Journal of Science,253, 209-224. pyrope-grossulargarnets -70.39 + 4.71kJ from the oxides,or -6282.42 kJ from Haselton, H.T. ( I 979) Calorimetry of synthetic and calculatedstability relations. Ph.D. thesis,University ofChicago, the elements. This value is in very good agreementwith Chicago,Illinois. the value of -6283.32 + 3.23 kJ of Holland and Powell Helgeson,H.C., Delany, J.M., Nesbitt, H.W., and Bird, D.K. (1978) Sum- ( I 990) derived from experimentalphase-equilibrium data; mary and critique of the thermodynamic properties of rock-forming the discrepancybetween solution-calorimetry and phase- minerals. American Journal of Sciencn,2784, 1-204. (1987) from to 1000 K and pyrope Hemingway, B.S. Quartz: Heat capacities 340 equilibrium determinations of A,FIpof cited by revised values for the thermodynamic properties.American Mineral- Holland and Powell ( I 985) is resolvedby the presentwork. og;st,72, 273-279. The Berman (1988) value of -6286.55 kJ is marginally Holland, T.J.B., and Powell, R. (1985) An internally consistentthermo- in agreementwith our own value. dynamic dataset with uncertaintiesand correlations: II. Data and results. Journal of Metamorphic Petrology, 3, 343-353. -(1990) An enlargedand updated internally consistentthermody- AcxNowr,nncMENTs namic dataset with uncertainties and correlations: The system KrO- NarO-CaO-MgO-MnO-FeGFepr-AlrOr-TiOr-SiOr-C-HrOr. Joumal of This researchwas supported by National ScienceFoundation grant EAR- Metamorphic Petrology, 8, 89-124. 9310264 to R.C.N. Careful reviews by Alan Woodland and an anony- Ito, E., Akaogi, M., Topor, L., and Nawotsky, A. (1990) Negarive pres- mous reviewer led to significant improvement of this paper. sure-temperature slopes for reaction forming MgSiO, perovskite from calorimetry. Science,249, 1275- 1278. RnrnnrNcns cITED Jenkins, D.M., Newton, R.C., and Goldsmith, J.R. (1985) Relatiye stability of Fe-freezoisite and clinozoisite. Journal of Geology, 93, 663- Berg, J.H., and Dokka, J.A. (1983) Geothermometry in the Kiglapait otz- contactaureole, Labrador. American Journal of Science,283, 414-434. Johannes,W. (1969) An experimental investigation of the systemMgO- Berman, R.G. (1988) Internally-consistentthermodynamic data for min- SiOr-HrO-COr.American Journal of Science,267, 1083-l104. erals in the systemNarO-KrO-CaO-MgO-FeO-FerOr-AlrOr-SiOr-TiOr- Johannes,W., and Metz, P. (1968) Experimentelle Bestimmungen von H,O-COr. Journal of Petrology, 29, 445-522. Gleichgewichtsbeziehungenim System MgO-COrHrO. Neues Jahr- Berman, R G., Eng,i,M., Greenwood, H.J., and Brovsn,T.H. (1986) Der- buch fiir Mineralogie Monatshefte, l5-26. ivation of internally-consistentthermodynamic data by the technique Johnson, M.R.W. (1963) Some time relations of movement and meta- of mathematicalprogramming: A review with application to the system morphism in the Scottish Highlands. Geologieen Mijnbouw, 42, l2l- MgO-SiOr-HrO. Journal of Petrology,27, 133l-1364. t42. Brousse,C., Newton, R.C., and Kleppa, O.J. (1984) Enrhalpy of forma- Kiseleva,I.A., Ogorodova,L.P., Topor, N.D., and Chigareva,O.G. (1979) tion of forsterite, enstatite,akermanite, monticellite and merwinite at A thermochemical study of the CaO-MgO-SiO, system Geochemistry 1073 K determined by alkali borate solution calorimetry. Geochimica International. 16. 122-134. et Cosmochimica Acta, 39, 1487-1497. Krupka, K.M., Robie, R.A., and Hemingway, B.S. (1979) High-temper- Burnham, C.W., Holloway, J.R., and Davis, N.F. (1969)Thermodynamic ature heat capacities ofcorundum, periclase, anorthite, CaAlrSirO, glass, properties ofwater to 1,000'C and 10,000bars. Geological Society of muscowite, pyophyllite, KAlSi3Os dass, grossular, and NaAlSirO, glass. America SpecialPaper, 132, 1-96. American Mineralogist, 64, 86-101. Chai, L., and Navrotsky, A. (1993) Thermochemistry of carbonate-py- Newton, R.C. (1987) Thermodynamic analysisof phaseequilibria in sim- roxene equilibria. 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phases:The system CaO-MgO-AlrOj-SiOr. Journal of Petrology, 27, thermodynamic study of equilibria in the systemCaO-MgO-SiOr-H'O- 827-842. COr. AmericanJournal ofScience,275,143-162 Schuiling, R.D. (1957) A geo-experimentalphase diagram of AlrSiOj. Torgeson, D.R., and Sahama,T.G. (1948) A hydrofluoric acid solution Proceedingsof the Netherlands Academy of Sciences,B series, 60, 220- calorimeter and the determination of the heats of formation of MgrSiOn, 226. MgSiOr, and CaSiOr. Joumal of the American Chemical Society, 70, Shearer,JA., and Kleppa, O.J. (1973) The enthalpies offormation of 2156-2160. MgAl,Oo, MgSiOr, MgrSiOn, and AlrSiO, by oxidemett calorimetry. Trommsdofl V., and Connolly, J.A.D. (1990) Constraints on phasedi- Journal of Inorganic Nuclear Chemistry, 35, 1073-1078. agram topology for the system CaO-MgO-SiOr-COr-HrO. Conribu- Shmulovich, K.I., and Shmonov, V.M. (1978) Tables of thermodynamic tions to Mineralogy and Petrology, LO4,l-7. propertiesofgases and liquids (carbondioride), I 65 p. USSR Academy of Sciences,Moscow Bureau of Standards, Moscow. MeNuscnrrr RECETvEDDecervrnrn 8, 1994 Slaughter, J., Kerrick, D.M., and Wall, V.J. (1975) Experimental and Mer.ruscmp'rAccEprEDJvr-v24, 1995