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The Stability of Anthophyllite in the Presence of Quartz

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The Stability of Anthophyllite in the Presence of Quartz 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 talc : 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 mineral-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
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