AmericanMineralogist, Volume 64, pages 294-303, 1979

Thestability of anthophyllitein thepresence of quartz

JosnpHV. CsnnNosry. JR.eNo Launrr KNeppAurror D epart ment.oJ' G eol ogical S ciences Uniuersityof Maine at Orono Orono,Maine04473

Abstract

The dehydrationreactions anthophyllite : enstatite+ quartz + fluid (l) and : anthophyllite+ quartz+ fluid (2) havebeen bracketed, with reversedexperiments and PHzO = Ptotal. Smoothcurves drawn through the bracketingdata for reactions(l) and (2) pass throughthe coordinates 2 kbar,770";1.5 kbar,755"; I kbar,730";0.5 kbar,680. and 3 kbar, 738";2kbar,7ll"; 1.5kbar,697o; I kbar,678';0.5kbar,655'C, respectively. Synthetic quartz, anthophyllite,enstatite, and talc were used as startingmaterials. Reversibility was establishedby determiningthe relativegrowth or diminution(as judged by examiningrelative intensitiesof X-ray reflections)of the high- with respectto the low-temperatureassemblages. Both curvesare consistentwith the solubilitydata of Hemleyet al. (1977b)at I kbar but are not entirelycompatible with the hydrothermaldata of Greenwood(1963). Our experimental dataindicate that the phaseboundaries for reactions(l) and (2) intersectthe phaseboundary for the reaction(3) talc : 3 enstatite+ quartz + Hro at low pressure(below 200 bars) and are,therefore, more consistentwith the anthophyllitephase diagram proposed by Hemleyel al. than with the diagramproposed by Greenwood. Gibbs energydifference functions calculated using bracketingdata for reactions(l), (2), and (4) talc i forsterite= 5 enstatite+ HrO permit simultaneousevaluation of the free energiesof formationof talc(-5513.69+4.69 kJ mol-r),enstatite (-1456.3g+ l.g3 kJ mol-r), and anthophyllite(- I 1,323.26+5.35kJ mol-r) from the elementsat 298K and I bar.

Introduction reported,and modifiedthe topology of the phase Pure Mg-anthophyllite was first synthesizedby diagramhe proposedin 1963.Greenwood's revised Bowenand Tuttle (1949, p. 450),who concludedthat topology(Fig. 1) containedboth possibleenantio- "anthophyllitehas no stablerange of existencein the morphicforms of the anthophyllitephase diagram, presenceof water vapor." Using severalcombina- althoughhe preferredthe high-pressureportion. : tions of naturaland syntheticstarting materials, Fyfe Chernosky(1976) reversed the reactions (3) T 3E : (1962) demonstratedthat anthophyllitehas a true + Q + H,O and (4) T + F 5E + H2O at water stability field in the presenceof excesswater. Green- pressuresbelow 4 kbar and found that:(l) the reac- : : wood (1963)reversed four reactionsinvolving Mg- tionsT 3E + Q + HrO andA 7E + Q + HrO : anthophylliteand proposeda phasediagram which intersectat the [F] invariant point near PHzO 5 containeda stabilityfield for anthophyllite.The sta- kbar rather than at 20 kbar, provided that Green- bility field was depictedas a relativelynarrow band wood's preferredslope for the anthophyllite-bearing 85'C wide which extendedfrom low pressureto reactionis takenat facevalue, and (2) the slopesfor : : PHrO : 20 kbar. Greenwood(1971) later recalcu- the reactionsT + F 5E + H2Oand (6) 9T + 4F latedthe slopeof the vapor conservativereaction (5) 5,A.+ 4HrO are inconsistentif Greenwood'spre- A : T -| 4E (seeTable I for symbolsand notations), ferred topologyis correct. showed that it was much steeperthan previously On the basis of -aqueous-solutionequi- libria performedat PH2O: I kbar, Hemley et al. I Presentaddress: Department of GeologicalSciences, Virginia (1977b)suggest that the low-pressureportion of the PolytechnicInstitute and State University,Blacksburg, Virginia phasediagram (Fig. l), which showsthe stability 24061. field of anthophylliteexpanding with increasingtem- Wo3-004X/79/0304-0294$02.00 2sq CHERNOSKY AND AUTIO: STABILITY OF ANTHOPHYLLITE 295 peratureand pressure,is correct. Hemley et al. lo- Table l. Symbolsand notations catedthe [Q] and [F] invariantpoints near 500 and 200 bars PHrO respectively. A anthophyllite, Msrsir0rr(0tl), Becauseanthophyllite is a potential indicator of F fors ter i te, I'ls2si04 ltgsi03 pressure E orthoenstatite, and temperaturein some metamorphic T talc, MSrSi40,O(0H), rocks,we decidedto reinvestigatethe phaserelations 0 quartz, Si02 aboutthe [F] invariantpoint. The reactions(l) A : G; standard Gibbs free energy of formation (298'15 K, 7E + Q + H,O and(2) 7T : 3,{ + 4Q + 4H,O were I bar) of a phase from the elements, in J rcl-r reversedbetween 500 and 3000 bars PHrO in orderto AG: Gibbs free energy of reaction at I bar and 298.15 K determinethe correcttopology for the anthophyllite c* Gibbs free energy of H20 at Te and Pe according to Burnhamet al.'s (1959) data, consistent with the phase diagram.Tight bracketsfor thesetwo reactions standard state of 298.15 K and I bar in P-T spacepermit refinement of the freeenergies of S; standard entropy of formation (298.15 K, I bar) of a talc, enstatite,and anthophyllitecalculated from ohase frm the elments, in J rcl-I deg-r phase-equilibriumdata by Zen and Chernosky Otf,, change of the entropy of formation of the sol id phases (1976).Data upon which this paper is basedhave for a given reaction beenpresented orally (Chernosky and Knapp, 1977). v vol ume AV, volume change of the solid Phases for a given reaction Te,Pe the temperature and pressure at which an univariant Experimentalmethods reaction is at equi I ibrium I therrcchemical calorie o 4.1840 joules Sl.artingmaterial Mixtureswith bulk compositionscorresponding to ousmonitoring of "line" pressures,In order to con- MgO'SiO,and 3MgO.4SiO,were made by drying, servevalve stemsand packings,pressures were not weighing,and mixingrequisite proportions of MgO monitored daily. Rather, pressureswere carefully (Fisher, lot 787699)and SiOzglass (Corning lump monitoredat the beginningof an experimentto guard cullet 7940, lot 62221).MgO and SiOz glasswere againstpressure leaks; once it wasdetermined that a firedat 1000'Cfor threehours to driveoff adsorbed vesselwas leak-free, it wasisolated for periodsof up water. Enstatiteand talc were synthesizeddirectly to a month betweenpressure checks. Small pressure from the mixes whereasanthophyllite was synthe- sizedusing the "talc" mix. Examinationof the syn- theticproducts with a petrographicmicroscope and by X-ray diffractionrevealed them to be entirelycrys- talline. Starting materialsused to reverseeach reaction wereprepared by mixing synthetictalc, enstatite, an- thophyllite,and quartz in the appropriatepropor- tionsand grindingby hand for one-halfhour to en- surehomogeneity. The high-temperatureassemblage constituted50 weightpercent (excluding HrO) of the startingmaterial for both reactions.The dried solid phaseswere combined with excessdistilled deionized HrO and sealedin l.25cm-longgold capsules. P rocedure All experimentswere performedin horizontally- mounted, cold-seal hydrothermal vessels(Tuttle, 1949). Pressureswere measuredwith factory-cali- brated,sixteen-inch Heise gauges assumed accurate to 10.1 percent of full scale(0-7000 and 0-4000 bars). Becausethe Heisegauges were usedas "pri- after (1971), generally Fig. l. Schematic P-T diagram Greenwood mary" standards,they were kept at room showing the high- and low-pressure intersections of the vapor- pressure,as suggestedby the manufacturer;smaller conservative reaction A = T + 4E with the [Q] and [F] invariant diameter,less accurate gauges were used for continu- points. Reactions are numbered as in the text. 296 CHERNOSKY AND AUTIO: STABILITY OF ANTHOPHYLLITE fluctuationsdue to temperaturedrift wereunavoid- experiment,assuming that the standardthermo- able, but experimentswhich experiencedpressure couplewas adequatelycalibrated. Corrections were dropsof greaterthan 50 bars werediscarded. Pres- usuallyon the order of 0-5oC. Experimentswere suresare believedaccurate to * I percentofthe stated checkedfor leaksboth beforeand afterhydrothermal value. treatmentby heatingthe chargecapsules and check- Becausetemperature drifted during the experi- ing for loss of HzO. Becausequartz was generally ments,daily temperaturereadings were averaged and leachedfrom experimentswhich leaked,such experi- the standarddeviation of the temperaturereadings mentswere discarded. cafculated;temperature errors are reportedas L2 The products of each experimentwere examined standarddeviations about the mean and represent with a petrographicmicroscope and by X-ray powder error due to temperaturedrift alone. Two other diffraction. Becausereaction rates at temperatures sourcesof error in temperaturemust be considered; nearthe equilibriumcurves were sluggish, complete namely,errors due to a temperaturegradient along a reactionwas generally not obtained.Judgment as to capsuleand thosedue to inaccuratethermocouples. which assemblageis stableat a givenpressure and Temperaturegradients in the pressurevessels used temperaturewas based on an examinationof all ma- wereall lessthan I oC overa workingdistance of 3.0 jor reflectionsfor the phasesof intereston a diffrac- cm (temperaturecalibration performed at room pres- tometer trace over the interval 5o to 40o N (CuKa sure). Becausebracketing experiments were per- radiation).A reactionwas considered reversed if a 25 formed in l.25cm-longsealed gold capsules,error percentchange in the intensitiesof X-ray reflections dueto temperaturegradients in the pressurevessels is relativeto thoseof the startingmaterial could be assumednegligible. The temperaturecalibration for observedafter the completionof an experiment.Mi- eachshielded thermocouple was checked after each croscopicobservation of the experimentalproducts experimentby heatingthe vesselto the temperature revealeddistinct textural features (described in detail of the experimentat a pressureof I bar and com- later) which indicatedreaction direction. Unfortu- paringthe measuredtemperature to that of a pre- nately,the sensitivity obtained using textural features viouslystandardized thermocouple placed inside the to judge reaction direction was not significantly bomb.This procedureinsures internal consistency in greater than the sensitivityafforded by the X-ray temperaturereadings among pressurevessels, cor- technique.Consequently, textural features were used rects for the temperaturedifference between the to confirm the X-ray resultsbut werenot usedas the charge capsule and external measuring thermo- solecriteria for judgingreaction direction. couple,and resultsin an accuratetemperature for an Unit-cell parametersfor syntheticphases used in the starting material were calculated by refining Table 2 Unit-cell parametersand volumes of syntheticenstatite, power patterns obtained using an Enraf-Nonius talc, and anthophyllite FR552 Guinier cameraand CuKa radiation;CaF, (BakerLot 91548,a : 5.4620+0.00054),standard- enstatlte talc anthophyllite izedagainst gem diamond(a : 3.567034,Robie el al., 1967),was usedas an internal standard.Least- 18.236 ( 10) s.29t(6) 18.s7 (4) squaresunit-cell refinementswere performed with a b 8.822(3) 9.169(7 ) 17.9L(4) computerprogram written by Applemanand Evans c 5.176(1) 18.982 (r6) s. 2s(1) (1973). 832.86 (43) 909.47 (L.28) 1748.0(3.7) 99"4', Results N 19 I4 16

s CaF CaF2 CaF, qnd phases z Synthesis characterizationof Enstatite(MgSiOs) was synthesizedhydrother- Fjgures in trEentheses represent the esti@ted mally at 815"Cand PHrO : I kbar in fiveto seven sxandard deviaxion in terms of J.east units cited for tlre value xo their ircdiate lefx; xhese uncer- daysfrom a mixtureof MgO andSiOz glass. Crystals taiaties wete calcuiated using the celT refinqent. werefine-grained (l0p), prismatic,and had parallel prqram and represent precision onlg. Ce77 parile- Xers are expressed in angstroms. extinction.The X-ray patternand unit-cellparame- ters(Table 2) comparefavorably to the powderpat- Abbreviaxions: N = nunbq of teflections used it utjt ce77 refinqenx; s = x-rag standard. tern of orthorhombicenstatite from the Bishopville meteorite(tsru powderpattern 7-216). CHERNOSKYAND AUTIO: STABILITY OF ANTHOPHYLL]TE

Talc [M93Si4O,'(OH)r]was synthesizedhydro- bracketingexperiment as it is in the startingmaterial. thermally at 680"C, PH2O = 2 kbar in five to ten Interactionof triple-chainimpurities with the fluid days.The syntheticproduct typically crystallized as phaseis probablyminimized because dissolution of aggregatesof very fine-grainedplates and contained anthophylliteduring a bracketingexperiment pre- lessthan 0.5 percentforsterite as an impurity,which sumably occurs along grain boundaries,and the could be due to leachingof silicafrom the starting triple-chainimpurities are located within anthophyl- materialor to the initial preparationof a starting lite grains. materialdeficient in SiOz.The powderpattern and Quartz (SiOr)was synthesizedtogether with an- unit-cellparameters of talc (Table2) comparefavor- thophyllitewith synthetictalc as a startingmaterial. ably with thoseof natural(esrru powder pattern l3- The positionsof the major X-ray reflectionsof the 558)and synthetic(Forbes, 1971) talc. syntheticquartz compare favorably to thoseof natu- AnthophylliteIMgrSirOrr(OH)r] was synthesized ral quartz from Lake Toxaway, North Carolina together with cristobalite,enstatite, talc, forsterite, (,rsrrrrpowder pattern 5-0a90);unfortunately, the and quartzby hydrothermallytreating synthetic talc number of reflectionsrequired for a unit-cellrefine- at 840oC,PH,O : 0.5 kbar for l2 hours.A mixture ment was not observedin the syntheticproduct, of anthophylliteand quartz togetherwith about 5 whichcontained less than ll percentquartz. percenttalc was obtained by hydrothermallytreating : the productsof the previousexperiment at 735oC, The reactionA 7E + Q + HrO PH,O : 3 kbar for 4l days.Anthophyllite synthe- Startingmaterial used for reversingreaction (l), A sizedin thismanner occurred as extremely fine grains : 7E + Q + HrO, was obtainedby mixing pure intimatelyintergrown with talc, and as somewhat syntheticenstatite together with anthophylliteand larger (l0p) prismaticcrystals having parallelex- quartz. The starting material containeda small tinction.The X-ray powderpattern and unit-cellpa- amount((5 percent)of talcwhich, however, reacted rameters(refined assuming the anthophyllitespace away in all but threeexperiments (Table 3, Experi- group Pnma) correspondfavorably to thoseof syn- ments 5, 21, and 24) located near the metastable thetic(Greenwood, 1963) and natural(Finger, 1970) extensionofthe phaseboundary for reaction(3), T : anthophyllite.Because several new triple-chain min- 7E + Q + HrO. Petrographicexamination of experi- eralscreated during the reactionof anthophylliteto mentalproducts which showeddefinite reaction re- form talc have recentlybeen reported (Veblen and vealedthat the dimensionsof crystalsbelonging to Burnham,1975,19761' Veblen et al., 1977),we com- paredthe powderpatterns of syntheticanthophyllite : to calculatedpowder patternsof , Table 3. Experimentsbracketing the reactionA 7E + Q + HrO clinojimthompsonite,and chesteriteprovided by Exper i ment T 'HzoP Duration D. R. Veblen.Reflections belonging to thesephases numDer ('c) ( hour s werenot observedin the syntheticanthophyllite we (kbars) usedas startingmaterial. D. R. Veblenkindly exam- 24 564(t.5) 0.5 91a A(+)E(-)q(-)r(+) tl inedthe startingmaterial and theproducts of 687Q\ 0.5 2607 A(-)E(+)a(+) s several il 758(3' 0.5 I 080 A(-)E(+)q(+) }/ reversalexperiments on both sidesof eachphase 22 699.5(r.5) r.0 8r5 A(+)E(-)a(-) M boundarywith a high-resolutiontransmission elec- 2l 7t9.5(3.5) r.0 A(+)E(-)q(-)r(+) il tron microscopeand observedisolated slabs of a r9 760(r) r.0 974 A(-)E(+)a(+) s triple-chainsilicate parallel to 010 of anthophyllite. t2 731(t., t.5 1273 A(+)E (-) A(-) !' r3 752(2) 1.5 t224 A(+)E(-)0(-) !'r The percentageof triple-chainsilicate varied (5-15 r4 767(4) r.5 r t05 A(-)E(+)a(+) H percentby volume)from onegrain to the next.The 6 133(2) 2.0 | 080 A(+)E(-)A(-) r.r 9 75r0 .5) 2.0 A(+)E(-)a(-) r1 triple-chain silicate apparently forms when an- 5 7658.5) 2.0 7t+\ A(+)E(-)a(-)r(+)r,,l thophylliteis initiallysynthesized outside its stability 25 775(2') 2.0 A(-)E(+)Q(+) H field from talc.X-ray reflectionsfrom the triple-chain 20 754.5Q.o',) 3.0 8r8 A(+)E (-)A(-) H silicatewere not observedbecause the isolatedslabs areonly oneunit cellthick. Growxh or dininution of a phase is jndicated bg a (+) or (-) restrEctivefg. ALf assedbTages incLuile vatrbt. Patenthesized We assumethat the presenceof triple-chainimpu- numbers represent two standatd Cleviations jn terns of Jeast units cited for the mean tenpqatutes xo their iMediate feft. Sgnbois rities does not significantlyaffect the experimental s. M, and w are quaTitative esti@tes of the extent of reaction and tepresent gr@ter than 80 peEcent' 80 to 50 [Etcent, and fess results,because the ratio of anthophylliteto triple- than 50 petcent teacxion' respectiveTg. chain silicateis the sameafter the completion of a 298 CHERNOSKY AND AUTIO: STABILITY OF ANTHOPHYLLITE the stableassemblage increased, whereas crystals be- pure naturalanthophyllite (2.77 percent MnO, 1.26 longing to the unstableassemblage had corroded percentCaO, and 0.59percent ALO3) from Falcon- grainboundaries. Several additional textural features ville,New York. The phaseboundary determined in wereobserved in experimentsconducted on thehigh- this study passesthrough an equilibriumpoint at temperatureside of the phaseboundary: (1) quartz 729oC, PH2O: I kbar calculatedby Hemleyet al. often recrystallizedto the point where undulatory (1977b),using thermochemical values derived from extinctionbecame evident, (2) finequartz grains (2p) their experimentaldata on severalreactions in the occurredas inclusionsoriented parallel to the c axis systemMgO-SiOz-HzO. of enstatite,and (3) very fine (0.5p)quartz grains = wereobserved to clusterabout the peripheryoflarger The reaction7T 34 + 4Q + 4HrO enstatitecrystals, suggesting that a singleanthophyl- The starting material used for reversingreaction lite grain had reactedto enstatite* quartz. (2), 7T : 3,{ + 4Q + 4HrO, consistedof an in- Criticalexperiments used to bracketthe position of timatelyintergrown mixture of fine-grainedtalc (50 the univariantcurve for reaction(l) are setdown in percent),anthophyllite (44.7 percent),and quartz Table3 and areplotted on Figure2, wherethey are (5.3 percent),all synthesizedsimultaneously from a comparedto resultsof otherworkers who haveused mix havingthe compositionof talc.The startingma- both syntheticand naturalanthophyllite as starting terialalso contained a smallamount ((0.5 percent) material.The smoothcurve through the experimental of a fine (30p),colorless, isotropic impurity having - bracketswas constructedusing a log /HrO l/T conchoidalfracture and ne - 1.58.The amountof diagram.Although our experimentaldata are consis- this impurity decreasedsubstantially during the tent with all but one (PH,O : 2.6 kbar. 775"C) bracketingexperiments. reversalobtained by Greenwood(1963), the slopeof Althoughtextural relations alone did not provide the curvedrawn throughour data is shallowerthan unambiguouscriteria for determiningreaction direc- the curve (Fig. 2, dashedline) suggestedby him. tion, severaltextural featuresconfirmed X-ray re- Greenwood also convertednatural anthophyllite sults.An experimentconducted within the stability containingll.12 percentFeO to enstatiteI quartz, field of the high-temperatureassemblage generally but did not attemptto convertenstatite * quartzto containedmany well-shapedanthophyllite needles Fe-bearinganthophyllite. Fyfe (1962)obtained three and recrystallizedquartz grainshaving undulatory reversalsat PHrO : 2 kbar (Fig. 2), using a fairly extinction.On the other hand, experimentscon- ductedwithin the stabilityfield of the low-temper- ature assemblagecontained very fine, barelydistin- I E THIS STUDY I guishablequartz grains and occasionaleuhedral talc a O GREEN\iooD(l%3) II platesrather than anthophylliteneedles. A FyFE(tes2) A I Criticalexperiments which bracket the positionof o ' o € HTMTEYErar (re77) i the univariantcurve for reaction(2) arc setdown in Table4 and plottedon Figure3. The smoothcurve throughthe experimentalbrackets was constructed usinga log/HrO - l/T diagram.Our experimental data are not entirely consistentwith the reversals obtainedby Greenwood(1963). Although our phase boundarypasses through Greenwood'sbracket at PH2O: 2 kbar, it hasa shallowerslope than Green- wood's proposedphase boundary (Fig. 3, dashed line).Our curvepasses through an equilibriumpoint at 675ot7oC, PH2O: I kbar obtainedby Hemleyel al. (1977b),who monitored phasechanges of the rErp€RAIUR[ (o c) solidsand compositionsof the aqueoussolutions in Fig. = 2. Dehydration curve for the reactionA 7E + Q + H,O. equilibriumwith thesesolids. They useda varietyof Solid symbols representgrowth of anthophyllite; open symbols represent growth of the high-temperature assemblage. Size of starting materials, including natural anthophyllite rectanglesrepresents uncertainty in measurement of pressureand containing5.9 percent FeO or thesame anthophyllite temperature.Dashed curve is after Greenwood (1963). exchangedwith lM MgCl, at 720"and 2 kbar. CHERNOSKY AND AUTIO: STABILITY OF ANTHOPHYLLITE 299

The rmo ch e mi ca I cal cula t ions Table4. Experimentsbracketing the reaction7T : 3A + 4Q + 4H20 Zen and, Chernosky (1976) calculatedthe free energiesof talc, enstatite,and anthophylliteusing Expe r i ment T P, n Duration Cments phaseequilibrium data of Greenwood(1963) and numDer ("c) z (nours, (tbars) Chernosky(1976). Unfortunately, the uncertainties in the calculatedfree energieswere rather large: I5 613(3) 0.5 106r r (+)A(-)a(- ) t'l l4 630(3) 0.5 I051 r (+)A(-) a(- ) t'l + 10.5kJ mol-r for talc,* 17kJ mol-r for anthophyl- l0 6478.5) 0.5 25\\ T(+) A(-) a(-) lite, and +4 kJ mol-1 for enstatite.The bracketing l2 677(tl 0.5 to52 r (-)A(+) Q(+) ll data obtainedduring this study haveenabled us to 8 564(l) |.o 744 r (+)A(-) 0(- ) t1 687(3) 1.0 872 r (-)A(+)q(+) r refinethe freeenergies ofthese phases and reducethe 7t4(3) l.o 744 r (-)A(+)q(+) |t uncertaintiesof the calculatedvalues. l9 693(31 1.5 t826 r(+)A(-)q(-) !, The Gibbs free energyof formation of a phase 701(3) t.5 1399 r (- )A(+) 0(+) l.t from the elementsat 298K. I bar can be calculated 20 706(lr) z.v zlov r (+)A(-)a(-) t7 7t9Q) 2.O l0l4 r (-)A(+) Q(+) from theP-I coordinatesofa phaseboundary using 7 592(t) 3.0 799 r(+)A(-)q(-) the relation(Fisher and Zen, l97l\: r3 7zo(\l 3.0 |651 r (+)A(- )q(- ) 2l 727$) 3.0 2688 T(+)A(-)0(-) = : - t6 r (-)A(+) t, AG(Te,Pe) 0 AGl,"(298,1) I#sAS9;jT 742(\l 3.0 993 Q(+)

+ /l"A V"dP+ G*(HrO)(Te,Pe) Growth ot itininution of a phase is indicated bg (+) or (-) tespectiveLy. e.ll. asssrD.lages incftfre vatrw. Patenthesized The integrals Dunbers repregent ttu standtd deviations jr teare of Teast @its .fASf,"d1'and !A,V"dP have been ap- cited fot the rean t4peratures to tteit imediaxe.left. ,9gnbois proximatedby the quantitiesA.S|,,AZ and AV"L,P, S, t, and W are qbl.itative esti@tes of tIE extenX of reaccao0 and reptesent giteatet xhan 80 Eetcent, 80 to 50 petcent, ard -less respectively.The uncertaintyintroduced by making tIEn 50 lErcent teaction, reapativeLg. thesesubstitutions is small(Zen, 1969). We have chosento calculatethe free energiesof has been lucidly explained talc, enstatite,and anthophyllite by Bird and Anderson by simultaneously (1e73). evaluatingthe Gibbs energy differencefunctions In order to evaluate (Thompson,1973) cKA) - 7Go^E),7GiG)-3cKA), the differencefunctions, one - only needsthe freeenergies of quartzand forsteriteat and Gi(T) 5G?(E),for the reactionsA = 7E + Q + 298K, I bar,the molarvolumes and entropiesof the H2O,7T: 3,{ + 4Q + 4HrO,and T + F : 5E + HrO, respectively(Table 5). The uncertaintyattached to the differencefunctions calculated for each indi- vidual (Table experiment 5) wasevaluated by com- I t= IHIS SIUDY bining,in thecommonly accepted manner, the uncer- A FYFC0952) tainties associatedwith the free energiesof either a O GRECN\r/ooo (1963) quartzor forsterite,the entropiesand molarvolumes -+ HEMTEYrr Ar.(l9zz) of the solidphases, and the valuesfor G*(HrO).The uncertaintyattached to the averagevalue for each differencefunction (Table 5) wasobtained by adding the uncertaintiesin theprimary thermochemical data (Table6) to the uncertaintiesreflected in the finite width of eachexperimental bracket (Figs. 2 and 3). The "best" value for each differencefunction (G,) and its associateduncertainty (f,) were calculated usingthe relations:

Gt:ZT-rGtwi/Zil and TEMPERATURTec) Fig. 3. Dehydration curve for the reaction 7T : 3A + 4Q + (l/21=rw1)t/2 Si: 4HrO. Solid symbols represent growth of talc; open symbols represent growth of the high-temperature assemblage. Size of where g; is the uncertainty associatedwith Gi and,w1 rectangles representsuncertainty in measurementofpressure and = l/C1..This procedure was used by Zen (1972) and temperature Dashed curve is after Greenwood (1963). 300 CHERNOSKY AND AUTIO: STABILITY OF ANTHOPHYLLITE

Table 5. Gibbs energydifference functions for three equilibria in LGi - (Z-298)ASP,"+ (p- l)AV, + G*(H,O)?0 the system MgO-SiO,-HrO Each linear inequality of the set is obtained from an (r) a=78+Q+H2O experimentalreversal of the reactionA : 7E + Q +

c;(A) - 7c;(E) = c;(a) + c*(H2o) + ^vs^p - ^ssar H,O (Table 3). The rangeof permissibleASf,, values which satisfythe inequalitiesis 177.7to 201.4J mol-l P,kbar Lower temD. revelsaf Upper t@p. reversal deg-l which corresponds to Si(A) values ranging 0.5 664"c -Lrz7.2i!0.98 6870C -1729.74!0.97 -2398 -2422 1.0 720 -1128.0110.96 760 -1130.7510.95 from to J mol-1 deg-', respectively.We 1,5 752 -1128.3610.95 167 -rr29.3110.95 have adopted Mel'nik and Onoprienko's entropy 2.O 765 -1128.0910.95 775 -Lr28.63!0,95 value (-2411 J mol-1 deg-') for anthophyllite be- Average 1 1 sEandard error: !728,59!O.79 kJ ml-l cause it lies within the middle of this permissible = (2) 7T 3A + 4Q + 4H2o range of values, and have assignedto it an uncer- 7G;(r) - 3c;(A) = 4c;(a) + 4c*(n2o) + avs^p- assAr tainty of il2 J mol-r deg-r. - P,kbar Lower teop. reversal Upper teDp, reversal The calculatedvalue for Gi(T) 5GKE) (Table 5)

0.5 647 -462r.75!L.96 677 -4636,84rr.95 is about I kJ smallerthan the valueobtainedby Zen 1.0 664 -4616.88!1. 94 687 -4627.5O!r.93 and Chernosky (1976), who replaced the integral 692 -4623.O7!r.93 701 -4626,99!r.92 2.O 681 -4624.71.!7.92 7r9 -4630.3011.91 ILS?.dT by lASy,"AZ rather than by AS|,"A7";the 3.0 727 -4627.45!r.90 742 -4633.5711.90 mean value,ASr,", is obtainedfrom entropy of forma- Average 1 1 standard error: -\626.07!2.3\ kJ nol-I tion valuesfor successiveeven-hundred-degree inter- (3) T+F=5E+H2O vals.We did not usethe approximation made by Zen c;(r) - 5c;(E) = -ci(F) + c*(H2o)+ AvE^p- Ass^r and Chernosky becauseSi(A) is not known as a function of temperature;in addition, our procedure P,kbar Lower temp. reversal Upper temp. reversal requires that all three difference functions be calcu- 0.5 600 L769.92!r.)-0 62r 1767.78!L.O9 1.0 637 1768.9011.09 657 1767.05rr.08 lated in the same manner. Simultaneousevaluation 2.O 640 1770.38i1.08 663 1768.45!r.08 3.0 662 L769.30!I.O7 692 7766.87!r.O7 of the three difference functions (Table 5) yields 4.0 686 1767.79!L.07 706 1766,24!L.06 the following: Gi(A) : -11323.26+5.35, c?(E) : Average J I gtandard error tl68.2l!0.92 kJ rc1-r -1456.38+1.93,and GKT): -5513.69+4.69kJ mol-', where the errors are *2 standard deviations. Btacketing data fot teaction (3) is tt@ Chsnoskg 0.976). The calculated free energiesare in excellent agree- ment with those obtained by Zen and Chernosky solid phases(Table 6), and valuesfor Gx(HrO),the standardGibbs free energyof water, tabulatedby Table 6. Thermodynamic parametersof phases Fisherand Zen ( 197I ). The heatcapacities of quartz, G;(2e8,1) s;(2e8,1) v(2e8,1) forsterite,and talc havebeen measuredand the en- - _ -l- -l-deg -l- 'gf ' tropy valuesderived from thesemeasurements are kJ mol J mol J bar probably quality. of high We assumethat the entropy Anthophyllite -71323.26 -24rr 26.446 (6) of clinoenstatiteis a good approximationfor ortho- Msrsiror,(oH), rs.3s (3) !r2 (3) enstatite(Zen and Chernosky, 1976, p. ll58). Enstatite -1456.38 -29L.14 3.r476 A major sourceof uncertaintyis the entropyof MCSio3 11.93 (3) 10,461 (2) i0.0050 (5) anthophyllite;the valueused by Zen (1971)and by Forsterite -205r.70 -398.99 4.3786 MC2SiO4 11. 9* (4) j0. 88 (2) 10.0029 (s) Zen andChernosky (1976) was calculated by Mel'nik and (1969) Quartz -856.239 -182.49 2.2689 Onoprienko from Greenwood's(1963) sio2 !L.72 (1) r0.08 (2) 10.0003 (5) phaseequilibrium data for the reactionA : 7E + Q TaIc -5513.69 -r273.08 13.6252 + HrO. Althoughour bracketingdata for this reac- Mg3si4oto (oH) !4.69 (3) 10.71 (2) 10.0259 (s) 2 tion are consistentwith all but one of Greenwood's reversals(Fig. 3), they more closelydefine the posi- Nunber in parentheses reters to source of daXa: (7) Robje tion of the equilibriumcurve and henceprovide a and waidbaun (1968); (2) zen (1972); (3) xhis studgi (4) Eenjngwag and Robie (vnpub. daxa); (5) Robie eX af. (7967); (6) usefulcheck on the entropyof anthophyllite. zen (f97f).

We haveused Gordon's (1973) method for obtain- *Vafue cotrected in accotdance with the revised vafue for ing the rangeof permissibleAGf and ASf," values the heat of sofDtjon of qoartz. which togethersatisfy a set of linearinequalities of +Vafue reters to cfinoenstaxite. the form: CHERNOSKY AND AUTIO: STABILITY OF ANTHOPHYLLITE

(1976),but the associateduncertainties are consid- erablysmaller. Free-energyvalues for anthophyllite(- I1344.ll.+ 7.1I ), ensratite(- 1460.63+2,51),and talc (-5524.60 +3.77 kJ mol-') calculatedby Hemleyet al. (1977a) o and Hemfeyet al. (1977b)differ from thoseobtained o in this paper,despite excellent agreement among the o phaseequilibrium data. The discrepancy in thecalcu- lated free energiesof enstatiteand anthophylliteis directly relatedto the freeenergy value used for talc. Hemleyet al. (1977a)calculated Gi(T) from experi- mentaldata for the equilibrium l/T(K)xlo 3 - Mg3Sino'o(oH)' t^i*1,,.,(oH[ Fig 4 Log JH"O l/T plor for reactions(l), (2), and (3). Experimental data for reactions(l) and (2) are taken from this I + 2H4sio4study; data for reaction(3) are from Chernosky(1976) Lined area shows possiblelocation of IF] invariant point Limiting slopesof at 200oC, I00 bars; in order to perform the calcu- reactions have been plotted as they would have appearedif the lation, the free energiesof HnSiOoand chrysotileas experimentshad all beenperformed at a constanttotal pressureof well as entropies and molar volumes of talc and I bar. chrysotileand heat capacitiesof chrysotile,talc, H2, Oz, and Si are required. Hemley et al. (1977b) used of the phase boundary for the effect of pressureon values for @(T) and @(F) reported by Robie and volume change of the solid phases (Eugster and Waldbaum (1968) to calculate Gl(A) and cKE). Wones, 1962,p.9l). The correctionamounts to re- However, the value for Gl(F) containedin Robie and calculating data for Waldbaum (1968) has been revisedby Hemingway experiments at variable total pressureso that they appear as if they were and Robie (unpublisheddata) due to a revisionin the obtained at the same constant total pressure,using the ex- enthalpy of solution of a-quartz (Hemingway and pression Robie, 1977).Inasmuch as we used 28 experimen- talfy-determinedP-T coordinates and more recent Alogffi* : - A Vs (P - PexP)/2.303RT ancillary thermochemicalvalues, we believeour cal- where P is the pressureto which the experimentsare culated free energies for talc, enstatite, and an- thophyllite are more accurate. being adjusted, Pexp is the experimentally-deter- mined pressure, and n is the number of moles of Discussion water participating in the reaction. Phaseboundaries for reactions(l), (2), (3) Cursory inspection of the phase boundaries for and have beenplotted on a logfil"O - 1/T diagram(Fig. reactions(l), A : 7E + Q + H,O, and (2), 7T : 3A 4); all experimentshave been recalculatedto a total + 4Q + 4H2O,(Figs. 2 and 3) suggeststhat they will pressure of I bar. Individual reversalshave been intersect and generatethe [F] invariant point at a omitted from Figure 4 for the sake of clarity; how- water pressurebelow 200 bars. Additional constraint ever,the Iimiting slopesfor each reactionare shown. on the locationof the [F] invariantpoint is obtained In Figure 4, we by including phase-equilibrium data (Chernosky, constructing have assumedthat there is no solid solution between anthophyllite and the 1976)for reaction(3), T : 3E + Q + H,O. Provided triple-chain silicate and that anthophyllite does not that the activitiesof all the phasesparticipating in the changecomposition during the courseof the experi- reaction are unity and that steamis the only volatile ments. Figure 4 indicatesthat all three reactionscan speciesinvolved in the reaction,van't Hoffs equation : reducesto: intersectbelow PH,O 200 bars, and suggeststhat the phase diagram proposed by Hemley et al. (0 logf{j"6)0 | /T)"" : - (LH)r"/ 2.303R (1977b), who placed the [F] invariant point at 200 bars, is consistent with the data presentedin this This relation indicatesthat a dehydration boundary paper. However, becausethe reactionsintersect at a - will plot as a straight line on a log /HrO l/T low angle, we cannot locate IF] preciselynor can we diagram if A^H at constantP" is constant(Orville and rule out the possibility of a high-pressureinter- Greenwood, 1965).It is necessaryto correctthe slope section.In fact, Evans(1977) has suggestedthat both CHERNOSKY AND AUTIO: STABILITY OF ANTHOPHYLLITE the high- and low-pressureportions of the phasedia- Using the approximationsmentioned above, gram as depictedin Figure I may exist. Gibbs energydifference functions, l0 GKATJT'') Thus far, we have consideredthe triple-chain(TC) 73 GKE) and 73 GKT) 30 GP(A,JT''), silicatean impurity and have assumedthat it doesnot were calculated.These values were combined with affectthe location of the phaseboundaries. The chief thosefor GKT) - 5 GKE)(Table 5) andsolved simul- reasonfor assumingno solid solution betweenthe TC taneouslyto yield: GKT) -5514.00+12.78, silicate and anthophyllite is that the structures of cKE) -1456.44+2.83, and GKA,JT'') these two phasesare different. We know, however, - I1,785.67+15.66 kJ mol-'. that the TC silicateis not inert, sincethe ratio of TC Although the calculatedfree energy of A*ITto is silicateto anthophyllite remainsnearly constantdur- tentative,it suggeststhat the additionof the com- ing the reversalexperiments, even though anthophyl- ponent Mg,oSirrO3r(OH)nstabilizes pure Mg-an- lite eithergrows or decomposes.It is difficult to quan- thophyllite.The amount by which pure Mg-an- titatively evaluate the effect of the TC silicate because thophylliteis stabilizedwill undoubtedlychange as its thermochemicalparameters and its relationshipto moreaccurate values for the molar volumesand en- the host anthophylliteare unknown. tropiesof theA-JT solidsolutions become available; One way to obtain quantitativeinformation on the however,it is unlikelythat thedirection of change,at effectof the TC silicatein anthophylliteis to consider leastfor Mg-richcompositions, will be affected,i.e., that there is solid solution between the two phases. our calculatedfree energy for pureMg-anthophyllite Strictly speaking this approximation is incorrect. is probablya "leaststable" value. However, it is reasonablebecause (1) the structures Becauseour experimentalwork wasconducted us- of the two phasesare very similar, and (2) the ran- ing Fe-freephases as startingmaterials with water domly-distributed domains of the TC silicatein an- pressureessentially equal to total pressure,it neces- thophyllite are on the order of one unit cell thick. sarilyhas limited direct application to naturalrocks. Although Veblen observedthat the amount of TC The effectsof decreasingPH2O relative to Pfluid or silicatevaried (5 to l5 percent)from one anthophyl- to Ptotal are well known (Thompson,1955; Green- lite grain to the next, the averagewas about l0 per- wood, l96l) and will not be discussed.The effectof cent. Assuming the presence of l0 percent jim- substitutingFe2+ for Mg on the phaseboundaries thompsonite,Mg,oSi,rOrr(OH)o, in anthophyllite,the consideredin thispaper is a topicofcurrent debate. formula for the anthophyllite-jimthompsonitesolid Calculationsby Evansand Trommsdorff(1974) solution (ArJT,o) becomes Mgr.rSi..oO2r(OH)rr. assumingX'"(E) : 0.900,X'r(T) : 0.973+0.002, When recastto preservestoichiometry, reactions (l) andXyr(A) : 0.88+0.01suggest that the equilibrium and (2) become: A : T * 4E will occurat a lowerpressure or a higher temperaturein the -freesystem by an amount l0A,JT1o:73E + llQ + llHrO (la) givenby AT = RryAS lnK o 200+95'C.Evans and (1977)calculated that the temperature shift due to the substitutionof l0 mole percentFe for Mg in the 73T :30A,JT,. + 40Q + 40HrO (2a) otheranthophyllite-bearing reactions should be small In order to evaluatethe free energyof AroJTro,we (-+ l0"C), becausethe volumeand entropy changes need its molar volume and entropy. Although these aremuch greater for dehydrationreactions than for quantitiesare not known, the molar volume of the vapor-conservativereactions. However, Sanford "solid solution" shouldbe closeto the molar volume (1977)using differentvalues for the enthalpiesof of anthophyllite. The error introduced by using the formationof the phasesinvolved, calculated that the molar volume of the syntheticanthophyllite (Table 2) substitutionof Fe'+for Mg by an amountcommonly used in the starting material is negligible.We have observedin naturaloccurrences is sufficientto lower also assumedthat the entropy contribution of the TC the phaseboundary for thereaction 9T + 4F : 5A + "impurity" is within the limits (-2411*12 J mol-l 4HrO by l20o-l50oC,assuming PHrO : Ptotal. deg-') calculatedusing experimentaldata for the re- Sanford'sresult is surprisingin light of experimen- action A : 7F. + Q + H,O. Although this assump- tal data for the reaction7T = 3,A + 4Q + 4H2O, tion is highly suspectbecause configuration and mix- obtainedby Hemleyet al. (1977b)using as starting ing contributions to the entropy have been ignored, materialsboth natural anthophyllitecontaining 5.9 the error introduced is probably far smaller than the percentFeO and the sameanthophyllite in whichthe error arising from the uncertainties in the free iron had been exchangedfor Mg. Hemley et al. energiesof the other phasesinvolved in the reactions. (1977b,p.362)stated that "No systematicdifferences CHERNOSKY AND AUT]O.' STABILITY OF ANTHOPHYLLITE 103

were noted in the use of natural us. synthetic or revisedvalues for AH!,rrr and AG!,z". of some aluminosilicate exchangedus. unexchangedmaterials." Clearly, addi- .J Res U S. Geol.Suru.,5,413-429 tional experimentalwork on the stability of Fe-bear- Hemley, J. J., J. W. Montoya, C. L. Christ and P. B. Hostetler (1977a) Mineral equilibria in the MgO-SiO,-H,O system: l. ing anthophyllitewill be extremely usefulin resolving Talc-chrysotile-forsterite- stability relations.A m. J. Sci., this probleni. 277. 35t. D. R. Shaw and R. W Luce (1977b) Mineral Acknowledgments eqlilibria in the MgO-SiO,-H,O system: I I Talc-- Thisresearch was supported by NSF grant EAR74- 13393 A0l to forsterite-anthophyllite-enstatitestability relations and some J. V. Chernosky.We thank D. R. Veblen for examiningthe prod- geologic implications in the system Am J. Sci , 277, 353-383 uctsof severalexperiments with high-resolutiontransmission elec- Mel'nik, Yu P and V L Onoprienko (1969)Termodinamiches- tron microscopy Discussionsof anthophyllitephase relations with kiye svoystva antofillita. In Konstitutsiya i SuoystuaMineralou, B. W Evans,J. J. Hemley, H. J. Greenwood,and E-an Zenhave Vol. 3 (Thermodynamicproperties of anthophyllite,In Cons titu- been stimulating Perceptivereviews by H. J. Greenwood, T M t.ionand Properties of Minerals, Vol 3), p. 46-55 Akad. Nauk Gordon, T J. B. Holland, and E-an Zen are greatly appreciated. Ukr. SSR, Kiev Orville, P. M and H. J. Greenwood (1965)Determination of AH References of reaction from experimental pressure-temperaturecurves. Am. J. Sci. 263.678-683. Appleman, D. E. and H. T. Evans,Jr. (1973) Job 9214 Indexing Robie, R. A., P. M. Bethke and K. M. Beardsley(1967) Selected and least squares refinement of powder diffraction d,ata Natl. X-ray crystallographicdata, molar volumes, and densitiesof Tech. Inf. Seru, U.S. Depr Commerce, Springfield, Virginia, minerafs and related substances.U.S Geol Suru. Bull 1248 DocumentPB-216 188. - and D R Waldbaum (1968)Thermodynamic properties of Bird, G. W. and G. M. Anderson (1973) The free energy of mineralsand relatedsubstances at 298.15'K (25.0'C) and one formation of magnesiancordierite and phlogopite Am. J. Sci , atmosphere(1.013 bars) pressureand at higher temperatures. 273, 84-91 U.S. Geol. Suru. Bull. 1259. Bowen, N. L and O. F. Tuttle (1949)Thesystem MgO-SiOr-HrO. Sanford,R F. (1977)The coexistenceofantigorite and anthophyl- Geol Soc. Am Bull.,60, 439-460. lite in uftramafic rocks (abstr.). Geol. Soc. Am. Abstracts with Burnham, C. W., J R. Hollaway and N. F Davis (1969) Ther- Programs, 9, I I 54-l 155. modynamic properties ofwater to 1000"C and 10,000bars. Geol. Thompson, A. B. (1973)Analcime: freeenergy from hydrothermal Soc. Am Spec. Pap 132. data Implications for phase equilibria and thermodynamic Chernosky,J. V., Jr. (1976)The stabilityfield of anthophyllite-A quantities for phases in NaAlOz-SiOz-HzO. Am. Mineral., 58, reevaluationbased on new experimentaldata. Am Mineral.,6l, 277-286 ll45-1155 Thompson, J B, Jr (1955) The thermodynamic basis for the - and L. A. Knapp (1977)The stabilityof anrhophylliteplus mineral faciesconcept. Am. J. Sci., 253, 65-103 quartz. Geot. Soc Am. Abstructs with Programs,9,927. Tuttle, O. F. ( 1949)Two pressurevessels for silicate-waterstudies. Eugster,H. P and D. R. Wones (1962) Stability relations the of Geol. Soc. Am. Bull.. 60. 1727-1729. ferruginousbiotite, annite.J Petrol.,3,82-125. Veblen, D. R. and C. W. Burnham (1975) Triple-chain biopyri- Evans,B W (1977) Metamorphism of Alpine peridotite and ser- boles:newly discoveredintermediate products of the retrograde pentinite. Ann. Reu. Eart.h Planet. Sci., 5, 397-447 anthophyllite-talctransformation, Chester, Vt. (abstr.). Trans. - and V. Trommsdorff(1974)Stability of enstatite* talc,and Am. Geophys. Union, 56, l0'76 CO2-metasomatismof metaperidotite,Val d'Efra, Lepontine - and - ( 1976)Biopyriboles from Chester,Vermont: the Alps. Am. J. Sci 274, 2'74-296. , first mixed-chain silicates(abstr.). Geol. Soc. Am Abstructs wit.h Finger, L. W. (1970) Refinementof the of an- Programs,S, ||53. thophyflite Carnegie Inst. Ll/ash. Year Book,68, 283-288. -, P. R Buseck and C. W. Burnham (1977) Asbestiform Fisher, J. R and E-an Zen (1971) Thermodynamic calculations chain silicates:new mineralsand structural groups.Science, 198, from hydrothermal phase equilibrium data and the free energy 359-365 ol H,O Am. J. Sci . 270. 297-314 Zen, E-an (1969) Free energy of formation of pyrophyllite from Forbes, W. C. (1971) Iron content of talc in the system hydrothermal data: values,discrepancies and implications.,42. MgrSinO,o(OH),-Fe.SioO,o(OH')2.J Geol., 79, 63-74 Mineral , 54, 1592-1606. Fyfe, W. S. (1962)On the relativestabilities of talc, anthophyllite - (1971) Comments on the thermodynamic constantsand and enstatite.Am. J 5ci..270. l5l-154. hydrothermal stability relations of anthophyllite Am. J. Sci, Gordon, T M. (1973)Determination of internallyconsistent ther- 270.t36-150. modynamic data from phaseequilibrium experiments.J. Geol , - (1972) Gibbs free energy,enthalpy and entropy of ten rock- 81, t99-208 forming minerals:calculations, discrepancies, implications ln Greenwood,H J. (1961)The systemNaAlSi,Ou-HrO-argon: total Mineral.. 57. 524-553 pressureand water pressurein metamorphism. J Geophys.Res , - and J V Chernosky, Jr. (1976) Correlated free energy 66, 3923-3946 valuesof anthophyllite, brucite, clinochrysotile, enstatite, forste- - (1963)The synthesisand stability field ofanthophyllite. "/ rite, quartz and talc. Am. Mineral , 61, 1156-1166. Petrol..4.3l7-351. - (1971) Anthophyllite. Corrections and comments on its stability Am. J. Sci , 270, l5l-t54. Hemingway,B. S. and R. A Robie (1977)Enthalpies of formation Manuscript receiued,May 10, 1978: of low albite (NaAlSi,O,), gibbsite (AI(OH)3), and NaAlO,; acceptedfor publicarion, July 19, 1978