AmericanMineralogist, Volume 67, pages653-675, 1982

Plagioclasestability at elevatedtemperatures and water pressures

JUI-taN R. Gor-osurrn Department of the Geophysical Sciences The University of Chicago Chicago, Illinois 60637

Abstract

The breakdownof plagioclaseto zoisite,kyanite, quartz, and a more sodicplagioclase in the systemNaAlSilOrCaAlzSizOrHzO has been investigatedwith reversedreactions at -Ana6 8, 9, and l0 kbar. At these pressuresall plagioclasecompositions from to Anlss break down alongthe sameP-T curye as pure anorthire,which can be describedby the equationP : -4590 + 20.4T(P: bars, f : "C). The isobariccurves in the T-X sectionare -Anas thus representedby a horizontalline (constanttemperature) between and An16s.At compositionsmore sodic than -Anas the curvesdip rather sharplyto lower temperatures' In the regionsbeneath the curves, the compositionof sodic plagioclasealong the down- dippinglimb is in equilibrium with Zo + Ky + Qz. The remarkablebehavior in the Ana6 Anlss region suggeststhat under the hydrothermalconditions of this study, regions of orderedanorthite develop in the plagioclaseand subsequentlyreact with water as would independentcrystals of An. Partial melting of pure An in the presenceof water beginsat 10.2 kbar and 725"C. At 10.8 kbar a region of partial melt extendsat nearly the same temperaturefrom An3sto An16s. The subsolidusexperimental data comparefavorably with observationson metamorphic rocks. Plagioclasecompositions are significantlyaffected by Pn,o as well as temperature and it is probablethat the configurationof the plagioclasebreakdown curve at moderateto high pressureseffectively cuts off the oligoclaselimb of the peristerite gap. Except for very high gradeor low pressuremetamorphic conditions plagioclases in the rangeAnas to Anvss are unstablewith respectto zoisiteor epidote.

Introduction experimentalwork could be done' Reactions in- volving zoisite, for example, were not feasible in Knowledgeof the subsolidusrelations in systems the older style rod bombswith a pressureceiling of containing the feldspar componentsand water is several kilobars. Plagioclasereactions involving essentialto the understandingof crustal metamor- diffusion exchangeare almost impossibly sluggish phic rocks. Surprisinglylittle experimentalwork on (Goldsmith, 1952), and even those aided by the breakdownof the intermediateplagioclase feldspars fluxing action of water tend to produce metastable at elevated temperaturesand pressureshas been productsand cannot be readily equilibratedat low done,and most of our interpretationsof the subsoli- or moderatewater pressures(Johannes, 1978)' A dusbehavior of plagioclasecomes from observation significantincrease in rate takes place in a number of progressivelymetamorphosed rocks. Goldsmith of differentreactions at pressureswell above2 kbar (1982)has reviewed the literature on the role of (Goldsmithand Newton, 1974 Clayton et al-, 1975) mineralogical reactions as well as the effect of but even so, in the current study at Ps"6 of 8 kbar structural state in affecting the composition of the andgreater, many of the mns were of two weeks' to plagioclases.The current investigationwas under- a month's duration. taken as a first step in the attempt to quantitatively evaluatethe stability limits of plagioclase,particu- Previousexperimental data and calculatedresults larly with respectto the breakdown of plagioclase on reactionsof interest to zoisite, kyanite, quartz, and a more sodic plagio- clase. Before the day of readily available piston- Boettcher and Wyllie (1967, 1969)investigated cylinderand high-pressuregas-system devices little the systemalbite-water; the join is effectivelybina- 0003-004x/82/070E-0653$02. 00 653 654 GOLD SMITH : PLAGIOC LAS E STABT LITY

r6 vapor. Incongruent melting of anorthite and water beginsat 725'C and 10.2kbar, and is unusualin that the subsolidusassemblage anorthite + vapor lies on the high-temperatureside of the boundary. t4 Storre and Nitsche (1974),noting reports of an increasingnumber of occurrencesof margarite in metamorphic rocks, experimentally investigated o o the -_ 12 following reactionsinvolving margarite:

O

tJ- rv 4 margarite I 3 quartz? 2 zoisite + 5 kyanite + 3H2O (2) Previouslydetermined reactions involving phases of interestare: margarite ? anorthite * corundum + H2O (3) (Chatterjee,197 l; Y elde, 197l) 6L_ 2 zoisite * kyanite € 4 anorthite g00" 500" 600" 7000 goo" * (4) oC corundum + H2O Temperoture, (Newton, 1966) Fig. l. Some of the P-I relations in the system anorthite- 2 zoisite + kyanite + qvartz water (CaAl2Si2OrH2O). = : An anorthite, Ky l5yili{s, e2 : =:4 anorthite * H2O (5) quartz, Zo = zoisite, L : liquid, = (phases V vapor intersected (Newton, on the join An-H2O). Curve (l) is for the reaction 4An + V = 1966) 2Zo + Ky + + V. (2) Qz Curve is for the reaction Zo + Ky + ez From the five reactions Storre and Nitsche con- + V C L. Curve (3) is for the reaction An + V = L. See Goldsmith(1981) for completediscussion. structeda P-T diagramwith severalinvariant points and a number of univariant reactions related to margarite stability. One of the invariant points placed ry up to 17 kbar, where the curve for the water- at 650' C and 9 kbar is the upper P-T limit saturatedmelting of albiteterminates at an invariant for the calculated reaction point as jadeite albite breaks down to and quartz. 5CaAl2Si2O8 + zHzO :: 2Ca2Al3Si3Or2(OH) join The anorthite-waterhas been investigatedby anorthite zoisite Newton and Kennedy (1963), Newton (1966), + Boettcher(1970), and Goldsmith(1981); there is a CaAlaSi2Oro(OH)z+ 2SiO2 quartz subsolidusreaction involving zoisite, as well as an margarite incongruent melting reaction that also produces which would constrainthe reaction zoisite.There has not, however,been any extensive 4anorthite + investigationpublished on the subsolidusalbite- H2O ?2zoisite anorthite-water (plagioclase-H2O)relations. * kyanite + quartz (Fig. 1) join The CaAl2Si2O6-HzO is important to an to a pressurerange of stability of little more than I understandingof the behavior of the more complex kbar. Chatterjee(1976) calculated equilibria involv- plagioclase-water system. Figure I is a diagram ing margarite,and in the absenceof other gasesthat (from Goldsmith, 1981)illustrating some of the p-Z would lower the activity of H2O, locatedthe invari- relationsin the join. (l) Curve is for the reaction4 ant point where the Zo + Ky + Qz assemblage anorthit'e + H2O =: 2 zoisite + kyanite * quartz. changesto Zo + Ma + Qz at 570"C and 7 kbar. Curve (2) is for the reaction zoisite * kyanite * Perkins et al. (1980), using new thermochemical quartz + vapor ? liquid, and curve (3) for anorthite data, also calculatedreactions in the systemCaO- + vapor P liquid, the latter delimiting the fields of AI2O3-SiO2-H2O,and point out that the margarite anorthite + vapor and zoisite * kyanite + liquid + and zoisite used by Storre and Nitsche contained GOLDSMITH : PLAGIOCLASE STABILITY 655 significant amounts of impurities that may have The starting materials (approximately 6 mg) were sealed in affectedtheir results.The univariantpoint calculat- platinum capsulescontaining approximately 2 mg of water, and a run any capsulethat did not ooze water when ed by Perkinset al. is at approximately610' C and on completionof openedwas discarded. Two side-by-sidecapsules were generally just over 8 kbar. run in a single assembly, and a chromel-alumel thermocouple Although notice has been taken of the possible tipped with a thin layer of Al2O3was nestled betweenthem; effectof Na as a margaritesubstituent by Perkinset temperatureswere controlledto +2'C. a/. (1980),little information exists on the effect of Starting materials Na on mica-producingreactions. Franz and Althaus A variety of starting materials were used-several sets of (1977)considered the stability relations of the as- synthetic plagioclase,several natural plagioclases,and several semblage paragonite-zoisite-quartz in the water- reversal mixtures of a synthetic plagioclaseplus other phases. saturatedsystem Na2O-CaO-Al2O3-SiO2, and state L Synthetic plagioclases that the addition of paragonitecomponent to marga- (a) A seriesfrom An16to Aneq(wt.Vo) in l0 percentintervals, approximately30 yearsago by J' F' Schairer, rite, which ultimately forms a paragonite-richsolid plusAn5, supplied each crystallized from glasseswith multiple heatingsand crush- solutionproduces a reactionofplagioclase to parag- ings. The crystallization of the albitic compositionswas a lengthy onite (solid solution) * zoisite + quartz that takes process(An5 was held at 1090-1100" C for a total of 170days). placeat higher pressuresthan the reactionAnlso * (b) A numberof syntheticplagioclase glasses supplied from the H. S' Yoder, and H2O = Zo + Ky + Qz. Franz and Althaus also stock of the GeophysicalLaboratory by at 700" C and 2 kbar H2O pressure. point out that Storre and Nitsche (1974) used a crystallized hydrothermally (c) A seriesof synthetic plagioclasescrystallized by two differ- margaritecontaining "substantial amounts" of pa- ent methodsfrom glassesmade in this laboratory,(l) hydrother- ragonite and ephesite component, and that the mally at 700" C and l-2 kbar water pressure' and (2) dry' at invariant point should be at a lower pressure, 1200'C and 20 kbar. perhapsattheT kbar and 570"C value calculatedby (d) Glassessupplied many years ago by N. L. Bowen' of (molar proportions) also crystallized hydro- Chatterjee(1976). AbsAnl and AbrAn5 thermally and dry, as in c), above, plus plagioclaseof An25 Franz and Althaus determined experimentally compositioncrystallized by Bowen. Thesewere usedby Bowen that at 9 kbar or somewhat higher pressure, and in his l9l3 determination of the Ab-An phase diagram. temperaturesbelow 500'C a synthetic plagioclase (e) Anorthite (An1so)prepared by reacting the oxides at approxi- of compositionAn3s is completely decomposedto mately 1450'C with several cycles of heating and crushing. plagioclases,all large, rather clear crystals, low in mica * zoisite + quartz. At 9 kbar and tempera- II. Natural K and Fe: turesbetween 500 and 600' C, Aq6 is incompletely (a) Anrs, MonteagleTownship, Ontario, Canada decomposedto mica, zoisite, and quartz, and that (b) Anzos, Roan Mountain, North Carolina some feldspar of unknown composition remains. (c) Anzz5, Bakerville,North Carolina Detailsof their experimentsare not given. The role (d) Anro, Corundum Hill, North Carolina IIL Reversal mixes, in which the starting plagioclasecompo- of margarite in the present investigation will be sitionswere Anso,An+o, and An16.The proportions and sourceof briefly consideredin a later section. the phasesin the three mixes are: (a) 4Anso+ 2Zo + lKy + lQz. The bulk compositionis An75' Experimental methods The An56was hydrothermally crystallized, the zoisite was from All the experimentswere carried out in piston-cylinder appara- Buck Creek, North Carolina, kyanite from Litchfield, Connecti- tus, using0.75" diameter pressure chambers. Both NaCl and talc cut, the quartz from Lisbon, Maryland. were usedas pressuremedia: the designof the two assemblies (b) EAnm + 2Zo + lKy + lQz. Bulk compositionis An66.The was the same, and similar to that described by Newton in 4r!6 w?s crystallized at 1200' and 20 kbar, the zoisite was Johanneset al. (971'l.. No "frictional" correction was made for synthetic,reacted from the oxidesand hydroxidesat 750'and 15 theNaCl assemblies(Johannes et al., l97l), and experiencewith kbar. comparative runs in this and other investigations (Goldsmith, (c) 8Anro I 2Zo + lKy + lQz. Bulk compositionis Ana6.The 1981)indicate a l0 percent correction for talc relative to NaCl, Anlq was crystallizedat 1200'C and 20 kbar, the zoisite was with the following procedures. The NaCl assemblies were synthetic, as above. The kyanite and quartz in (b) and (c) were as pumped to a predetermined pressure before heating based on in (a). prior experience;the large thermal expansion of the salt during The plagioclase,zoisite, kyanite, and quartz were carefully heating brought the run to the desired pressure, assuring a blended under acetone in the proportions indicated, by mulling "piston-out" condition. Talc expandsvery little on heating, and four times to dryness in an agate mortar and pestle. The mixes initial pumping was close to the desired final pressure. Signifi- are designatedin the run tables as An56p,Ana6q, and An16a.At cant initial overshooting in pressure was avoided in order to leastfive X-ray diffraction scanswere made of each to character- minimize any tendency to produce zoisite "on the way up", ize the rangesof relative intensities of the major peaks and thus since this phase may persist metastably particularly in low- to establish what constitutes a significant difference in propor- temperatureruns, as discussedbelow. tions of phases.The use of a different technique, not dependent 656 GOLD SMITH : PLAGIOC LAS E STABI LITY

proportion on the ofphasesalone was usedin the reversalruns, and unexchangedportions are distinct, and if enough of the K- and will be describedbelow. exchangedmaterial is present, it is readily measurable.Bimodal Run and reversaltechniques or smearedreflections do appearin exchangedrun products, but as a result of compositional gradation or variation produced by Reversed runs, as normally used to bracket an equilibrium incompletereaction during the high pressureexperirnent and not boundary,are not easily done in systemswhere the reaction by partial exchangewith KCl. involves a changein the composition of a plagioclasefeldspar. The reversalreactions may go either in the direction offorming The hydrothermal breakdown reaction of the end-memberanor- zoisite (+ kyanite + quartz) and a lesser amount of a more sodic thite (Goldsmith, -t- l98l), 4 anorthite + vapor ? 2 zoisite plagioclase from the starting plagioclase, or in the opposite kyanite + quartz was reversed at temperatures of 550. C and direction by reacting zoisite (+ kyanite + quartz) with the above in runs of less than 3 weeks, for the reaction chiefly original plagioclaseto produce an increased amount of a more involvesgrowth or dissolutionof anorthiteand zoisite,without calcicfeldspar. If the reactionwere to go to completion,i.e., the change in the AVSi ratio of either phase. The reaction in the equilibrium configuration, the plagioclase would represent the system under considerationis one in which any shift in the most calcic compositionstable under the imposedP-T condi- plagioclase-zoisite equilibriumrequires a changein the albite_ tions, neglectingany metastabilityconsequent upon unattained anorthite ratio plagioclase. of the The extreme sluggishnessof structural re-adjustments.Determination of the plagioclasecom- this process (Goldsmith, is well known 1952;Johannes, l97g). position in the reversal runs by K-exchangewas much more Locating the equilibrium boundariesin this investigationwith sensitiveand revealing than attempts to determine the direction runs all delineating full reversals of the equilibrium would be ofreaction by relative changesin X-ray ditrraction peak intensi- unrealistic, but an adequatenumber were made to validate the ties. The magnitude of the change as well as the direction was resultsof the numerousunseeded runs. determinablein a single run. One other variant of the reversal In order to establishthe direction of a reactioninvolving the reaction was tried-attempted rehomogenizationof zoisite pro- reversalmixes, it is necessaryto determinethe relativechange in ducedfrom plagioclasein a previousrun. proportion of the phases involved, or to observe a change in Most of the runs were not reversal, but "half reversal" composition of the plagioclase,or both. If the reaction pro_ experiments,starting with the syntheticplagioclases, some with gresies far enough, determination of the change in the ratio of the natural crystals, in capsules sealed with approximately 25 phasesmay be relativelyeasy. If, however,rather little change wt.Vo water. All of the run products were examined under the takes place, even though it may be significant, determination petrographic microscope. Zoisite is readily detected in the with certainty based on relative intensities of X-ray diffraction presenceof plagioclaseeven in very small tracesor as very small peaksmay be ambiguous.A numberoffactors, including orienta- crystals becauseof the large diference in relief. Optical exami- tion, grain size and crystallinity can influenceintensities and nationin this caseis a very sensitivemethod of detection;if one introduce ambiguities, so that differencesof at least 10-20Voor had to depend on the presenceor absenceof zoisite ditrraction more are required. On the other hand, determination of the peaks in an X-ray pattern to establish a phase boundary a composition of plagioclases from X-ray powder diagrams is considerableerror would result. Some of the runs produced fraught with difficulty, and differencesof l0-20Vo in An content rather large crystals of zoisite which were observable under a cannotbe routinely detected. binocular microscopeat very low magnificationor even with the Viswanathan(1971) has shown that K may be exchangedfor unaidedeye. A numberof run productswere examinedand the Na in plagioclase at approximately 850. C in molten KCl. The phasessemi-quantitatively analyzed with a not- JsM-35Scanning exchangedproduct, unlike the original plagioclase,has a varia_ Electron Microscope(SEM), equippedwith energy dispersive tion in volume and lattice constant d that varies significantlywith analysis(EDX). composition, i.e., with the Na/Ca ratio of the original plagio_ Two capsulesrun together with plagioclasesthat composition- clase.The 201 spacingis particularlyuseful, and a number of ally bracketedthe boundarycurves, although unseeded with the synthetic plagioclases were exchanged and the tOl spacing reaction products, consistentlyproduced zoisite in the more calibratedagainst KBrO3, as a checkon Viswanathan'sdetermi_ calcicof the two plagioclases,and nonein the more sodic.The nativecurves. Agreementwas quite good, and the composition resultswere remarkablyconsistent and reproducible,and the of exchanged runs in sample sizes of 5-10 mg was determinable identification of the curves as equilibrium rather than kinetically to +2 percent An. There is a problem with X-ray determination controlled or synthesisboundaries was validated by those rever- of spacings plagioclases of more sodic than An1s, possibly sal runs that were successfully carried out. The sensitivity of becauseof a nonlinear change in lattice constants of silicate detectionis illustratedby run PLl44a (Table5). The reversalmix solid solutions near the end membersas discussedbv Newton did not show a detectable reaction, yet the companion run of and Wood (1980). An75alone (PLl44b, Table 2) producd "abundant" zoisite in the As Viswanathan points out, the more calcic plagioclasesare unseededruns. more diffcult to exchange completely, but no diffculty was The structural state of the synthetic plagioclases used as experienced with calibration samplesand run products in com_ startingmaterials is presumedto be the high-temperature(largely positionsup to Anr6. The fine grained run products were held in disordered)modification, particularly in those samplescrystal- molten KCI for three hours at 820-850. C. Significantly longer lized,at 20 kbar and l2m" C. The plagioclasesthat were crystal- exchangetimes induced breakdown of the feldspar to leucite, lized dry (J. F. Schairer) produced very poorly resolved X-ray kaliophilite, and possibly wollastonite. The K-exchangeis an all difraction patterns (especially the more sodic compositions) in or nothing process-one does not observeintermediate spacings whichonly the strongestpeaks were observable.However, even produced by partial replacement of Na by K. Thus even if the for these,at the completion of a run the X-ray pattern was crisp; exchangeprocess is incomplete, the spacingsof the exchanged "crystallinity" was greatly improved, even if significantrecrys- GOLD SMITH : PLAGI OCLAS E STABI LIT Y 657

pLAGTOCLASE(+vApOR PLAGIOCLASE{+VAPOR] ) 6t5"C

r'"" o O ZOISITE+ PLAGIOCLASE ZOISITE+ PLAGIOCLASE O {+KYANITE+ OTZ +VAPOR) (+KYANITE + OtZ + VAPOR) o

0204060 80 r00 NoAl Sir 0n CoA l2S i20s Mol% An CoAl2 S i20g : NoAl Si306 Fig. 2. Phaserelations at PH2o 8 kbar on the NaAlSi3OB- Mol % An CaAl2Si2Osjoin. Selecteddata from Table l. Solid circles : Fig. 4. Phaserelations at Pgrq : 10 kbar on the NaAlSi3Os- plagioclaseonly present.Open circles : zoisite + plagioclase : CaAl2Si2Osjoin. Selected data from Table 3. Solid circles (kyanite and quartz not observed, see text). Half-filled circle : plagioclaseonly present. Open circles : zoisite t plagioclase trace of zoisiteobserved. Arrow indicating615'C is basedon (kyanite and quartz not observed, see text). Half-filled circles : reversedreaction in pure An (seecurve (l), Fig. l). The region trace of Zo observed. Arrow indicating 715" C is based on under the boundary curve is not on the Ab-Anjoin (see text). reversedreaction in pure An (seecurve (l)' Fig. l). The region under the boundary curve is not on the Ab-An join (see text)' tallization was not apparent under the petrographic microscope The structural state of the plagioclases in the run products at 15kbar and 400' C, the product was almost certainly clinozoi- remainsto be determined. site, althoughthe difraction pattern was not well resolved. A run at 15kbar and 500' C produced what appearedto be a mixture of Identification of phases zoisiteand clinozoisite.Zoisite- or clinozoisite-producingruns Almost all of the runs were made on compositions and at were carriedout in the presentstudy at 400'C at 8, 9, l0' and temperaturesrather close to the boundary curve separatingthe 10.8kbar (Figs.2-5), but the amountspresent are inadequateto field ofplagioclase + vapor from that containing zoisite, in order produce an X-ray pattern. At 400" C the crystals are very thin to delineateit. Relatively little of any phasesother than a rather needles without observable birefringence, and their symmetry small amount of zoisite and a plagioclaseonly very slightly more could not be determinedon the basis of optical propertiesor sodic than the starting composition could be expected. Rather morphology. few runs produced enough zoisite to be recorded in an X-ray At temperatures> 600'C, especiallyin runs where partial difraction pattern.In severalat 650'C, 10.8kbar, the pattern, melting had taken place, the zoisite generallycrystallized as although quite weak, appeared to be orthorhombic zoisite' Crystals produced from oxides and hydroxides or from glass of the composition2CaO{l2Al2$'3SiO2 at l5 kbar and 600-700' C are zoisite, which is clearly identifiable from its diffraction pattern.In severalsynthesis runs madeby Matthewset al. (1981)

ZOISITE+ PLAGIOCLASE PLAGIOCLASE(+VAPOR ) 665 (+ KYANITE+ OtZ + VAPOR) .rt ; ;7; zotstTE+ PLAG|oCLASE iE o (+KYANITE+orz+VAPoR) 0 NoAlSi306 Mol % An Fig. 5. Phaserelations at Psr6 = l0'8 kbar on the NaAlSi3os- : CaAl2Si2Osjoin. Selected data from Table 4. Solid circles present. Open circles : zoisite -f plagioclase NoAlSi30s CoAI2Si206 plagioclaseonly Mol % An (kyanite and quartz not observed, see text)' Open squaresare in f Fig. 3. Phaserelations at Psr6 = 9 kbar on the NaAlSi3Os- region of partial melt (zoisite + corundum * plagioclase 730' and CaAl2Si2OEjoin. Selected data from Table 2. Solid circles : liquid). The solid arrows along the An16gborder at join plagioclaseonly present. Open circles = zoisite t plagioclase 770"C delimit the region of partial melt at l0'8 kbar on the (kyanite and quartz not observed, see text). Arrow indicating An-H2O (see Fig. l). The question mark adjacent to the half- presenceor 665"C is basedon reversed reaction in pure An (see curve (l), filled circle at 400' C indicatesuncertainty about the partial melt Fig. l). The crossesindicate no reaction, at 350"C. The region absenceof zoisite. The curves delimiting the fields of under the boundary curve is not on the Ab-Anjoin (see text). are dashedbecause they were not closely determined' 658 GOLDSMITH : PLAGIOC LASE STABILITY

Table l Data at 8 kbar In runs with small amounts of reaction products a detailed

Jf,arIl n0 SEM study is neededto seek out any phasessuch as quartz in a l:l t ,materiai ro. Time, Resutts sea of plagioclase,or to distinguish kyanite or margarite from (PL-l Anc source ' " hrs. (plusp'lagioclase) zoisite; neither identification can be readily made optically in the 90b 89.4 s 625 288 No Zo very fine-grainedcharges. 92a 79.0 s 610 99 Sma'llamount Zo t5aa I I HT 625 100 No Zo Quartz and margarite were identified as reaction products in 96 48.5 s 625 94 Very rare (tiny xls) Zo two samples(PL22la 92b 48.5 610 99 Rare Zo and b, Table 3). The compositionof the 'I mica is unknown; identification was by (002)and (004)diffraction szb 40 n 625 100 No Zo - 177a 33 HT 610 192 No Zo peaksat 20 9.2" and 18.5'respectively(CuKo radiation),and 188a 33 HT 570 220 Sparse Zo optical examinationrevealed the presenceof a phasewith higher t77b 30 HT 510 192 No Zo 185 30 HT 590 239 No Zo birefringencethan plagioclase, quar\z, or zoisite. '188b Glassis not easily detected in small amounts, although it may 30 HT 570 220 Sparse Zo 85b 28.8 s 610 165 No Zo appearas a brownishindistinct massunder the microscope.In 114 25 HT 580 147 No Zo somewhatlarger quantitiesit was observedas isotropic material, '126bI 26a 25 HT 550 192 No Zo 2s 550 192 No Zo occasionallycoating or including plagioclaseand zoisite. At this stage the runs were also I 73a 25 HT 530 196 Very sparse Zo somewhat crunchy when crushed. '173bI 20a 25 HT s00 335 very sparse Zo Globules, at times agglomerated into dumbbells or roelike 22.5 Ba 530 I 95 No certain Zo 120b 20 HT 500 335 No Zo masses,occasionally surrounding small crystals, were generally I 99a 20 HT 450 337 Very rare Zo presentin the runs made at 700' C or more, whether melted or l91a 20 HT 400 743 SparseZo not. Theserepresent quenched vapor, and are fairly distinctive. I loa l9.l 550 167 No Zo Hexagonal-appearingplaty crystals were also observed in a l07a ig.l H 500 240 No Zo I lob 15 n 550 I 67 No Zo* number of the runs associated with melt. These high-index r07b l 5 H 500 240 lloderate Zo* crystals,isotropic when viewed normal to the hexagonaloutline, l99b 15 HT 450 337 No Zo were analyzedin runs PL36a and b, PL43a and b, and PL46a and tglb 15 HT 400 (tiny 743 Rare xls) Zo b (Table 4) with the SEM, and proved to be corundum. 214a l5',l HT 400 672 Sparse Zo 214b I .l HT 400 672 No Zo

loooo 38,6 s No Zo Experimental results ALL ws i,n NaCL assemblies. S = SeVnire? czvstals Plagioclase-waterreactions were carried out at 8, - H Hydtothml crAatallizqtion (?00o , 1-2 kb@) EI = High temperuture c"Astallizatim (7200", Z0 k:bad 9, 10 and 10.8 kbar. The data are contained in Ba = BakersuilLe x A4ISH giues spu?iou' results (see tert), Tables1-4; the reversalruns in Table 5. The largest v Rtn at 8,5 kbap numberof runs were made at9 and 10.8kbar, most of them with talc pressuremedium. The appropriate data are plotted in Figures 2-5. fairly thick, blocky prismatic crystals, with well-formed termina- tions. At lower temperatures a needleJike, or elongated habit Ramberg(1949) assumed a continuouschange in was more common, although run duration may also afect the plagioclasecomposition with temperature (Gold- morphological axial ratio. In the larger crystals birefringence smith, 1982,Fig. 1), and Noble (1962)suggested a appearednormal, without anomalouscolor, but the crystals were flattening of a portion of the curve due to metasta- usually not thick enough to demonstrateit. An occasionalrun, bility, but Figures 2-4 indicate that within suchas PL6a (An,,o,9 kbar, 650'C, Table 2) producedacicular experi- zoisitecrystals up to 0.5 mm long, visible to the unaidedeye. mental error, plagioclase compositions from ap- Occasionallyclusters or rosetteswere observed,but for the most proximately Anaqto Anlso react with water to form part the zoisite crystals were isolated from each other and from a zoisite-bearingassemblage at the same tempera- the parent plagioclase. Zoisite was not observed growing in ture. Such a horizontal line in the subsolidusdia- plagioclase, but crystallized by extraction of the appropriate gramat 8,9, and 10kbar would result if the elements from the feldspar with independent nucleation and activity growth of the zoisite. When abundant, the zoisite crystals are of anorthite componentin plagioclasewere nearly commonly an order of magnitude longer than the birefringent constantover almosttwo-thirds of the composition- units(presumed crystal size) ofthe plagioclase.The extractionof al range-a most remarkablesituation. A discussion CaO, Al2O3,and SiO2from the plagioclaseto produce zoisite (+ of possible explanations and implications will fol- kyanite + quartz?)and a more sodic plagioclase,at pressures of low, but it is apparent that all plagioclases 8 kbar or more, results in a recrystallized plagioclase with a more sharp and in some cases notably improved X-ray diffraction calcic than Ana6 break down to a more sodic pattern. plagioclaseplus zoisite (and presumably kyanite No attempt was made to quantify the amount or proportion of and quartz) at the samepressures and temperatures zoisite in the run products; zoisite was generally too sparsefor as pure anorthite,along curve (l) in Figure l. This analysis by X-ray ditrraction intensities, and optical estimates curve can be described (Goldsmith, are biased by variations in size and shape by the equation of the crystals in : -4590 : different runs (e.g., a flood of very fine needles, or fewer 1981)P + 20.47(P bars,T : "C). blockier crystals). Tables l-4 contain simple qualitative state- Plagioclasesare stable relative to zoisite in the ments about the run products. presenceof water in the region above the bound- GOLD SM ITH : PLAGI OCLAS E STABI LIT Y 659

Table 2. Data at 9 kbar Table 2. (continued)

Startj ng Starti ng Run# material Time, Results Run material Time, Results '" # I L (PL-) An- source hrs. (plus plagioclase) (PL- An source hrs. (plus plagioclase) ) x '19al8a 89.4 s 680 47 No Zo 89.4 s 660 69 Rare Zo l84v 25 HT 560 286 SparseZo 18b 79 s 680 47 No Zo I 08b 20.5 RM 575 143 Sparse Zo 89a 79 s 680 24 No Zo I 06a 20.5 RM 550 ]68 very sparse Zo 23a 79 S 650 45 SparseZo (large xls) I 02a 20.5 RM 500 210 Fairly abundantZo l4b 20.5 RM 450 197 Sparse Zo r4ob: 77 HT 650 216 Fairly abundantZo 't ' 't44b 75 HT 630 192 AbundantZo 127b 20 HT 575 120 No Zo 12 75 HT 550 22 AbundantZo I 06b 20 H 550 I 68 No Zo I 7a 68.7 680 70 No Zo l12b 20 HT 550 22 No Zo ]9b 68.7 660 69 No Zo 122a 20 HT 530 ]68 No Zo I 38a 20 HT 530 I 66 No Zo 23b 68.7 650 45 Sparse Zo 'I 6a. 68.7 650 72 LargeZo xls (0.02") HT 500 335 Rare zo '130a25a 20 6b: 58.6 650 72 Large Zo xls HT 450 430 Zo present 'l0b 20 98b" 58.6 650 7l Zo present l9.t 550 9l No Zo lza 52 600 46 Abundant Zo 500 I 39 No Zo 'll1llb l9.t 19.1 500 3l I Zo present I 50bv 50 HT 690 263 No Zo l7b 48.5 s 680 70 No Zo '19.1 s00 210 Zo present I 02b'l4a '19 7a 48.5 s 650 49 Rare Zo .1 450 197 Zo present 7b 38.6 s 650 49 No Zo 25a l9.l 400 336 Zo present 84a 38.6 s 650 75 SparseZo (o 1/4 kbar high) l6a l9.r 350 366 No Zo (kinetics) 122b t5 HT 530 168 No Zo 79a 38.6 s 630 75 Zo present 9a 38.6 J 600 73 AbundantZo (large xls) I l6a l5 H 500 332 Fairly abundantZo* 84b 33 H 650 75 SparseZo (r l/4 kbar high)* HT 500 335 No zo 'tI 25b l5 86a 33 H 650 67 Zo present* HT 450 430 Sparse Zo '136a30h l5 79b JJ H 630 75 Sparse Zo* HT 400 407 SparseZo 'l 15 37a l5 HT 400 408 Sparse Zo 74a 33 n 600 73 Zo present* '15 142a 30 HT 600 142 Very sparse Zo HT 400 505 Zo present 'I',lI 04b '15 12b 30 CH 600 46 Very sparse Zo 500 554 Sparse Zo 'l5b 'l 6b 0 30 CH 600 99 No Zo 35b 10 HT 400 407 No Zo 86b 28.8 s 650 67 No Zo I 37b l0 HT 400 408 No Zo 25h 9.5 s 400 336 No zo 9b 28.8 s 600 73 No Zo l5a 28.8 s 600 99 No Zo zoa 100 680 46 AbundantZo 70b 28.8 600 47 No Zo 20b 95 680 46 Abundant Zo 64a 28.8 575 97 Zo present (large xls) 21a 100 700 68 Zo present l0a 28.8 550 9t zo present (large xls) 21b 95 700 68 Zo present ?2a 100 720 43 Zo present lla 28.8 500 I 39 Zo present see text 142a ?5 600 142 NoZo 22b 95 720 43 Zo present 64b 25 575 97 Zo present** 24a 100 740 96 Zo present I 08a ta 575 ]43 NoZo 24b 95 740 96 Zo present tzta aa 575 120 No Zo 26a 100 760 44 Zo present 26b 95 760 Zo present

'100 2l a 780 42 Zo present 27b 95 780 42 Zo present aries in Figures 2-4 up to temperaturesat which 28a 100 780 0.5 Large Zo xls 28b 95 780 0.5 No Zo field beneaththe curves repre- 680 2 Zo Present*** meltingbegins. The 29a 100 see plagioclase text present*** sents the region where compositions 29b 95 680 '182 Zo 30a 100 720 Large Zo xls*** more albitic than Anas along the down-dipping 30b 95 720 18 Rare Zo*** portion of the curve are in equilibriumwith zoisite, 31a ]00 760 23.5 Large Zo xls, corroded*** 3tb 95 760 23.5 No Zo*** kyanite (or margarite) and quartz. Figures 2-5 are 34 I00 800 4 Zo, embayedfragments

projectionson the water- (10 pet@nt not binary diagrams,but Rms i,n talc assemblies, at rcniml presaure of 10 kbar saturatedNaAlSi3Os-CaAl2Os join in the 5-compo- " |ri cti.wl " corcec ti on ) ctt'= nnro, corundw HiLL, Nc; tu = An20.s, Roan Mounltain' NC; nent system.Although the boundarycurve itself is o. = Anry Monteagle rdp-' ontario; L = rabradorite on the join, the phasesin equilibrium with it (be- v Runs in NaCLassqblies at 9 kbar S = Sclnire? cy,Jstdll neaththe curve) are not, althoughthe composition H = Hudrotheml crustQLlization (700"' 1-2 kbq!) nf ="ntslt tenpenaire ctystalLization (1200"' 20 kbaP) of zoisiteis fairly close to that of anorthite(3An + B = Btuen-lu-a""inL.i7. mstals : 1 etgstallizal:ion' epurLous leeu'lta (eee tert) CalOHlz Zo). ** off carcositin (see lert) * * * P*;4 hLating , ini Lia'l I kbat' Melting at 10.8kbar Figure 5, at 10.8kbar showspartial meltingfrom approximatelyAn35 to An1es.At 10.8kbar on the indicatedby the dashedlines. The lower boundary join CaAl2SizOrHzO the field of zoisite * kyanite of the field containingliquid is not well determined, + vapor + liquid is intersectedfrom 730" to 770' but it is bracketed in the region near An35 at (Fig. l) so that a band of partial melt extendsinto approximately710J20'C, only 10-20' below that Figure 5 from the anorthite end of the diagram, as of the 730"C melting of pure anorthite. Thus, like GOLD SM ITH : PLAGIOC LAS E STABILITY

Table 3. Data at 10 kbar Stewart(1967) observed what he called "B-AI2O3" 5!arEt ng of unknown composition, particularly in runs in Bun t material Time, Results (PL-) An-- source hrs. (plus olaoioctasel anorthite-richcompositions at l0 kbar, decreasing

78b 89.4 s 710 69 Zo present in quantity to 5 kbar, and not observed below 5 221a 83 HT 500 623 Zo, Qz, Ma 'B-AI2O3' 148b 79 s 730 48 No Zo kbar. Stewart stated that "The arose 91a 79 s 725 49 Very rare Zo from the incongruentsolubility of anorthite in the l5lb 79 s 705 71 SparseZo (sma1lxls) gas, which was enriched in silica." Goldsmith 88a 79 s 700 73 Large Zo xls 221b 77 HT 500 623 Zo, Qz, Ma (1980)observed the developmentofcorundum from 209a 50 HT 700 48 Very sparse Zo 147b 50 H 690 169 Large Zo xls anorthitein the absenceof water at pressuresof 9 9',Ib 48.5 s 725 49 Very rare Zo kbar and above, at temperatureswell below melt- 88b 48.5 s 700 73 Large Zo xls 202a 42 HT 700 24 No Zo ing. This "exsolution" of Al2O3 to form a non- 204a 42 HT 675 72 No Zo stoichiometricanorthite (Goldsmith, 1980)has 206a 42 HT 675 47 Largish Zo xls not 215 40 HT 720 47 No Zo beenspecifically studied in the systemCaAl2Si2Os- 202b 40 H 700 24 Rare Zo H2O, but severalobservations indicated the pres- 209b 40 HT 700 48 Sparse Zo 2A4b 40 H 675 72 Zo present enceof corundumor "B-AI2O3"(Goldsmith, 1981); '195aZUbO 40 HT 675 47 Largish Zo xls JJ HT 620 92 Zo present whetherit is an "exsolution" product as in the case

210 30 HT 670 71 No Zo ofthe anhydroussystem or a consequenceofincon- 208a 30 HT 650 48 Zo present gruent solubility ZUJA 30 HT 630 48 Zo present enhancedby high pressuresis not I 95b 30 HT 620 92 Zo present known. 208b 25 HT 650 48 Very rare Zo (1 xl?) 205 25 HT 630 48 Zo present Reversalexperiments 200 25 HT 575 96 Abundant Zo 207 20 HT 630 72 No Zo 212 20 HT 600 96 I Trny zo xl The runs madewith the reversalmixes are listed 200b 20 HT 575 96 Very rare Zo in Table 5, and illustrated in part in Figures 6-8. 211a 20 HT 540 95 Sparse Zo 2llb l5 HT 540 95 No Zo The arrows emanatefrom the composition of the ztJa l5 HT 450 623 Moderate amount Zo startingplagioclase in the mix, and the headsof the 2l 6a 'll.It5 HT 450 815 liloderate amountZo 216b HT 450 8l 5 Sparse Zo arrowsindicate the final compositionsof the plagio- 2l8a II.l HT 400 704 Moderate amount Zo clase,as determinedon the K-exchangedfeldspars. 2l8b 5.0 HT 400 704 No Zo At 8 kbar (Fig. 6) the 650"C run of the Anls starting A77. nne in IIaCL assmbT,ies. S = Sclni.rer czjsta'ls mix had a very broad201 peak, indicatinga rangeof = H HAdyotheruL ezgetallizatim (?00" , 1-2 AJ>ar) compositions, portion Hf = Hi.gh tempepatw crgstall,irutim (1200", 2A kba!) but the strongest represented a composition of Ar33, rnost of the way to the ultimate (bulk) compositionof Anas.The An5smix the plagioclase+ H2O breakdown reaction, the reacted very little, to approximately An53. At curve representingthe beginningof hydrousmelting 580'C, the run on the An16mix showeda bimodal of plagioclasesis remarkably flat in the region 201 peak, representingcompositions of -An16 and An35-An1ss.At 10 kbar pure albite melts in the An2s.It would appearthat someof the startingmix presence of water between 680' and 690. C remainedunreacted, but that the plagioclasethat (Boettcherand Wyllie, 1967,1969), and only slight- did react went to a composition very close to the ly lower at 10.8kbar. A dashedline from the albite curve as drawn. The plagioclaseof the run on the melting point is connectedto the An35region, but An56mix was unfortunatelydestroyed by holdingit there are no data involving melt in this region. too long in the molten KCl, but an X-ray diffraction The boundaryin Figure 5 representingthe upper pattern has been made prior to the exchangeproc- temperaturelimit of the field of partial melt emanat- ess,and it clearly showedzoisite growth relative to ing from anorthiteis shownrising with temperature, plagioclase.An arrow of undeterminedlength end- as indicatedby the limited data, althoughthe phase ing in a questionmark is thereforeshown on Figure relationsin the S-componentsystem are unknown. 6. Partialmelting releases zoisite from the plagioclase, Most of the reversalswere carried out at 9 kbar and blocky, well-formed crystals are observed at and are shown in Figure 7. Several reversal runs the onset of melting. Hexagonal plates of corun- were made before the K-exchangetechnique was dum are also observedin runs that display partial used, and although effectively repeated using K- melting. Similar crystals were observed in the exchange,the results in terms of growth or deple- CaAl2Si2Os-H2O(Na-free) join (Goldsmith, l98l). tion of phasesare describedhere. The An56mix was GOLD SM IT H : PLAGIOC LAS E STABI LIT Y 661

Table 4. Data at 10.8kbar Table 4. (continued)

5tartl ng Starti ng Run# naterial Time, Results- '" Run# materi a l I lme Resu I Es (PL-) hrs. (plus p'lagioclase) T.C An, source (PL-) An source hrs. (ptus plagioclase) 49a 89.4 s 800 23 Melt + xls' 49b 79 s 800 23 Melt + xls 65b 25 B 690 22 Sparse Zo** 38a 79 780 21 Melt + xls 6lb 25 B 675 l9 Sparse Zo** I 45b 79 S 775 7 Melt + xls H 675 48 No Zo l'l5a',l7a 25 36a 79 s 750 56 Melt + xls I 25 n 6s0 117 Very rare Zo I 28a 25 HT 650 77 Rare Zo 'rlo3v 79 S 750 75 [,lelt + xls 4]b 75 750 54 Me'lt + x'ls reTal 25 HT 650 99 No Zo 143b 75 H 750 48 Me'lt + xls 194a' 25 HT 600 100 Moderate Zo 38b 68.7 s 780 21 Melt + xls I l5b 22.5 Ba 675 48 No Zo 36b 68.7 S 750 66 Melt + xls I lTb 22.5 Ba 650 ll7 Very rare Zo I 28b 22.5 Ba 650 77 Sparse Zo 5a 68.7 s 650 46 Zo present l3la 60 HT 600 238 MuchZo I 09b 20.5 Rt! 690 73 NoZo 58.6 43a 750 45 llelt + xls Ir9b 20.5 RM 530 72 NoZo 5b 58.6 650 46 Zo present 44b 20.5 RM 600 ll8 Verysparse Zo 48.5 57 775 21 Melt + xls 97a 20.5 Rtl 575 96 Very rare Zo 95a 20.5 Rt'l 550 158 Very rare Zo 43b 48.5 750 46 Melt + xls 47a 48.5 750 44 Melt + x]s r3b 20.5 RM 550 69 AbundantZo 404 48.5 730 67 Melt + xls 94a 20.5 Rt4 525 166 Small amountZo 4a 48.5 650 68 Zo present 123b 20 HT 650 47 No Zo 56 38.6 775 24 Me'lt + x'ls 20 HT 630 72 Zo present 'l2laI l9a 20 HT 600 70 Small amountZo l83a 38.6 160 40 + Melt xls '100 47b 38.6 750 44 Melt + xls I 94bv 20 HT 600 small amountZo 46b 38.6 730 67 Melt + xls 8b ]9.l S 650 'Io6 No Zo 32a 38.6 700 43 SparseZo (large xls) 500 l8 No Zo 44. '19l9.l s 4b 38.5 650 68 Zo present tJa .l s 550 69 AbundantZo 50a t9.l s s00 166 AbundantZo l83b 33 H 760 40 Melt + xls 77a 33 H 740 76 Melt + x]s 93a l9.l s 500 242 Zo present l80a 33 HT 730 49 Melt + xls I l3b l5 H 610 70 Sparse Zo* '192a'72a. 33 n 710 75 Moderate Zo* I 05a l5 H 580 168 Moderate Zo* HT (see 33 700 23 No Zo I l8b 15 f, 575 120 AbundantZo* llSa) l0lb 15 H 565 143 Sparse Zo* 201av 33 HT 700 ?4 Sparse Zo 77b 30 CH 740 76 Melt + xls H 550 l7'l Very sparse Zo* '180b 'l2l99b l5 30 HT 730 49 Melt + xls b" 15 HT 600 70 No Zo '198a' 87b. 30 CH 700 73 SparseZo l5 HT 600 146 No Zo l92b' 30 HT 700 23 No Zo I l8a t5 HT 575 120 No Zo (see IlSb) 124a t5 HT 550 168 No Zo (very very rare?) 72b 28.8 s 710 No Zo 32b 28.8 s 700 43 No Zo I 33a l5 HT 530 ]44 Moderate Zo 8a. 28.8 s 550 68 Zo present (large xls) 129a l5 HT 500 350 l4oderateZo (trace 201b' HT 700 24 Rare Zo melt?) I 05b l5 0 580 ]58 No Zo 109a 25 690 No Zo 97b l5 0 575 '14396 Very rare Zo l0la l5 0 565 No Zo

95b l5 0 550 168 Very rare Zo 0 550 l7'l very rare Zo run at 675' C (PLl38a), 10"C above the curve, and 99a '15l5 '168 t24b 0 550 Very sparse Zo at 650"C (PL140a),15'C below the curve. Slight 94b t5 0 525 166 Very sParse Zo 93b l5 0 500 242 Moderate Zo plagioclasegrowth relative to zoisite was observed I 33b il.'l HT 530 144 No Zo 'I 'll.I Zo in the former, slight zoisite growth relative to 29b HT 500 '168360 Sparse '134a '11.1 Zo HT 480 '146 Sparse plagioclasein the latter. No certain reaction was I 98b l0 HT 600 No Zo observedin a run at 630'C (PLl44a). At 690'C 134b 10 HT 480 158 No Zo (PL150a)the An56 mix showed good growth of 50b s 500 166 No Zo 35b 9.5 s 400 331 Sparse Zo SDarseZo plagioclase,which when K-exchanged,gave a value 63a 9.5 J 400 336 63b 5 s 400 Nb zo (very very rare Zo?) of An65.This mix was also run at 690"C (PLl47a) (10% and l0 kbar (the only 10kbar reversal-mixrun that Rune in talc assemblies, at nomircL presswe of 12 kb@ "frLetioml" corceetim) was made) and showed zoisite growth relative to 'l Runs in NaCL ass@blies' at 10.8 kbdt cH = /!ttro, corundm HiLL, NC; Bo = Anzz,s, BakevsoiLLe' NC; plagioclase.This run is not plotted, but referenceto : An1s, Mmtedgle ra? ' ' ont@io RM = An-20. y R@ Mwtain' Nc; o Figure 4 indicates that 690"C is 25' below the S = scLnirev ergstals H = Hudrotherui craastal'l,izqti 4 (?00" , 1-2 kban) (1200"' curve,as opposedto the 25'above the curve at 9 nf ="u'tgl, tenperaa&e crystaLlizatin 20 kbar) B = Boaen erustale (aee kbar; thus a clear-cutreversal is indicated. 1 uudrothe"ml ezustallization, sputious lesults teet) At 625" (PL168a) the An5s mix showed in- ^^ oif cwositLa ('ee tert) C . zblt"Lti *oa eowdw pvesent in we pattially melted creasedzoisite; the 201 peak was bimodal,with the principal peak shifted to Ana3. The Anls mix at 625' C (PL168b)also showeda bimodal peak, with showedzoisite growth and a sharp201 peak repre- compositionsof Anls (unreacted)and An32.Anoth- sentingAn36. The companionrun (PL175b)of Anls er reversalrun at 625"C (PL175a)with Ana6 mix mix showedzoisite to be depleted,and the 201 peak 62 GOLD SMITH : PLAGIOC LAS E STABI LITY

Table 5. Reversal runs

Starti ng Run material D percent # Toc nme, mo1 kbar hrs. An of oroducts 174a 50 R (+Qz) Little chanqe(An-^ 650 -^) I 74b i0 R 359 201 smearedl'Anrcu-cs' '155a 50R Some growth (Sample '10 580 363 Zo lost in KCI) I 55b R Bimodal201. An.,O+An*

I 50a 50R 690 9 263 GoodPlag growth (AnUU)

138a 50R 675 9 144 Slight Piag growth

183a 40R An, 675 360 t I 83b l0 R An]i (Bulk comp.= Anoo) '140a 50R 216 Slight Zo growth

I 82a 50R Anaa 640 385 I B2b 40R No ieaction

144a 50R 630 192 No certain reaction

I 68a s0 R (+Qz) bimodal. Anr". Zo growth 'I oz3 330 !01-201 68b l0 R bimodal. nnji+nn'

I 75a 40R Z0l sharp. Anea. Zo qrowth 'I bz5 473 75b l0 R 201 bimoilal. Ailio+Rnro-Zo depleted I 76a 40R 657 9.5 359 No certain reaction I 76b l0 R 201 sharp. AnrU. Plag growth

I 47a 50R 690 t0 169 Zo growth '10.3- 189a 40R 700 2?1 (see text) i 89b l0 R 10.8 *ltz^"35 '190a i00 + 42 700 719 N;-;;;;;;;-i;;;-;;;j---- l90b 42 + 15 Signi ficant homogenization PLl4a(Plag+Zo) 2l7a 6En ??6 No apparentresorption 217b Pl35b(Plag+Zo) Completelyresorbed (see text)

ArSf = 4AnS0 + 2Zo + yrJ + Qz (BuLk cornpostition = Anrr) Anqf = SAnnO + 2Zo + y'A + Qz (BuLk cornposition = AnUO) Anlf = 8An10 + 2Zo + Ky + 8z (BuLk conposition = AnOO)

was a doublet indicatingcompositions of Anls (un- showedplagioclase growth, with a sharp 201 peak reacted)and An36,a rather good bracketingof the at An35. At 9.5 kbar, An35 is very close to the curve. At 640"C the An5emix (PLl82a) moved to boundarycurve. Ana6and the Anas mix (PL182b)showed no reac- Figure 8 shows a single pair of reversal runs at tion. The Ana6composition at 640' C is rather close 10.8kbar. The pressureduring this run fluctuated, to the equilibrium boundary, and there is little and ran somewhatlow for most of the time. It is driving force for the reaction. listed at 10.3-10.8kbar. Anas mix (PLl89a) had a At 675"C, above the boundary, the Anas mix sharp 201 peak, shifted to An37. The Anls mix (PL183a) reacted to An47, and the An16 mix (PL189b)ended up at An35.The bracketingis quite (PL183b)all the way to An3e@ulk compositionis close, and what appears as an overshooting of the An.). Another run previously made at 675'C had equilibrium by the Anle mix may well be the been overpressedand ran very close to 0.5 kbar appropriate reaction at the somewhat lower pres- high, or 9.5 kbar. The Anas mix (PL176a)showed sure of the run. no reaction;at this pressure,the equilibriumbound- In two of the runs (PL168aandPLlT4a) addition- ary is very closeto 675'C. The An16mix (PLl76b) al quartz was added to the An5s r€v€rSal mix as a GOLDSMITH : PLAGIOC LASE STABILITY

PLAGIOCLASE(+V )

?4 ZOISITE+ PLAGIOCLASE ZOISITE+ PLAGIOCLASE (+Ky+02+V) (+l(y+02+V)

NoAlSi3os CoAl2Si2os 0204060 80 r00 NoAlsisos CoA12Si20t Mol% An Mor% An = (see Fig. 6. Boundary curve dt Psr6 8 kbar Fig. 2) with Fig. 8. Boundary curve at Pgr6 = 10.8 kbar, with dashed (see arrows indicating reversed reactions text). Tail of arrow solidus (see Fig. 5). Arrows indicate reversed reactions (see represents initial composition of plagioclase in reversal mix, text). Tail ofarrow representsinitial composition ofplagioclase, arrow head is final composition. Question mark denotes tip of arrow head is final composition. Data in Table 5. uncertain final composition.

boundary curve bracketed the curve, in agreement check as to whether or not any melting was taking with its position as determinedby the zoisite crys- place under the conditionsofthe reversalruns. The tallizationsfrom the unseededplagioclases. validity of the reversals might be questioned if The arrowlengths in Figures 6-8 indicate that the partial meltingis involved. Quartzis the solid phase reactions involving compositional change in the that is eliminatedwhen hydrous melting occurs in plagioclasesrun more readily in the direction of an (Goldsmith, anorthiteat > 10 kbar 1981).If quartz increasedanorthite content. This may at least in remains after reaction there can have been no part be due to greater reactivity of the more sodic melting and the additional quartz makes the obser- compositions, but it may also be controlled by vation more certain. No melting was apparent in mechanismsassociated with the Na-Ca and Al-Si theseruns. exchangeprocess. This result nrns counter to the The direction of reaction of the runs made with intuitive feelingthat it might be easierfor zoisiteto the reversalmixes is clear and unambiguous.All of be crystallized from plagioclasethan to be resorbed the mixes run in the field of plagioclase * vapor in it. On the other hand, the reaction involving the grew plagioclase (plus relative to zoisite kyanite + change from Alvr in zoisite to Alrv in plagioclase quartz), and the plagioclasebecame more calcic. may be faster than the converse;AIIV is the higher All of the mixes in the field of plagioclase * zoisite entropy state. Orville (1972)noted that at 700"C + quartz kyanite + grew zoisite relative to plagio- and 2 kbar the reaction 2NaAlSi3Os -l CaClz ? plagioclase clase,and the becamemore sodic. Sev- CaAlzSizOs+ 2NaCl + 4SiO2 proceeded much eral runs close to the boundary showed little or no more rapidly in the right-hand, quartz-producing reaction. Several of the reversal runs that ap- direction,and closelyapproached equilibrium with- proached the downward-dipping portion of the in one day. The reaction proceeded much more slowly in the left-hand,quartz-consuming direction, and equilibrium was not approachedwithin 30 days. Johannes(1978), in discussing the extraordinary slownessof reaction rates in the system Ab-An- H2O at 5 kbar says, "It also appearsthat reaction rates are even slower for experimentalconditions ZOISITE+ PLAGIOCLASE (+Ky+Qz+V) where an An-rich plagioclaseshould react to an Ab- richer plagioclasethan vice versa." Some insight in the process of compositional changewas given by an experiment in which ho- NoAlSi306 CoA12 Si206 mogenizationof plagioclaseswas attempted. Me- Mol % An chanical mixtures of equal weights of powdered = (see Fig. 7. Boundary curve at PHro 9 kbar Fig. 3) with crystallineAna2 and An15and of Anlesand arrows indicating reversed reactions (see text). Tail of arrow synthetic representsinitial composition of plagioclase,tip of arrow head is Anez(PL190a and b, Table 5) were sealedin cap- final composition. Data in Table 5. suleswith water and run at 700'C and 9 kbar for 664 GOLDSMITH : PLAGIOCLASE STABILITY

719 hours. K-exchangeof the initial mixtures pro- A pair of runs were also made to determine duced well-resolvedand readily measured201 re- whether or not the re-solutionof zoisite in plagio- flections of the two plagioclases(0.37'20 separa- clase could be used as a variant of the reversal tion). The run product of the An15* Ana2mixture technique.An optical observationof the absenceor after K-exchange showed a broadened701 peak diminution of zoisite in plagioclase in a re-run with two distinct maxima at compositionsof An22 sample previously held in the zoisite-containing and An33(approximately 0.13'20 separation). Thus field, if feasible, would be simpler than the K- the reaction went more than half-way to complete exchange method. Run numbers PLl4a (Anzo, homogenization,and apparentlyby a mechanismof 450'C, 9 kbar), and PL35b (Anro, 400'C, 10.8 diffusiveinterchange, rather than by one involving kbar) were re-run with H2O asPL2lTa and b (Table dissolution and reprecipitation of the equilibrium 5). at 500"C and 8 kbar for 336hours. Both PLl4a composition.This homogenizationdiffers, howev- and PL35b had needlesand small prismatic zoisite er, from the reversals,for two feldsparsrather than crystalsof approximatelythe same size and about a feldspar and zoisite are involved. There was no the samequantity. The An26composition at 550'C observablereaction in the An42* Anlss mixture. is 50' C abovethe curve (solid triangle,Fig. 6), and This may be related to the ultimate stability of the An16 composition is approximately 200'C plagioclasesolid solutionsin this compositionrange aboveit (opentriangle, Fig. 6). The Anls composi- at this temperature as discussed in a following tion showed complete resorption of the zoisite, section. whereas the An26 appearedessentially the same Considering the sluggishnessof the reactions under the microscopeas the originally run sample; involving compositionalchange in plagioclases,the no certain re-solutionwas apparent.Whether this resultsof the reversalexperiments are rather grati- differenceis due to a difference in reactivity be- fying, and the location and the interpretationof the causeof the differencein plagioclasecompositions, curvesas equilibriumboundaries seems reasonable. or becauseof the relative locations with reference The relatively high pressuresused in this study are to the equilibriumboundary was not determined.It probably responsiblefor the approachto equilibri- appearsthat reversalsbased on re-homogenization um, and the kinetic advantageof high water pres- of plagioclase-zoisite(plus kyanite and quartz) as- suresis illustratedby Goldsmithand Newton(1974) semblagesmay require too large a degreeof super- and Clayton et al. (1975).It is unlikely that sarisfac- heatingto provide adequatelysensitive results. tory data could have been obtainedat much lower pressures.Johannes (1978) stresses the observatipn Effect of starting materials that experimentsin the systemsAb-An-H2O and Differencesin behaviorin severalstarting materi- Qz-Ab-An-H2O demonstrate that reaction rates als were observedas the investigationproceeded. and attainment of equilibrium compositions are Syntheticplagioclases crystallized (dry) at tempera- extremelydependent on temperature.Time studies turesoffrom 1090"to over 1300'C by J. F. Schairer in the Ab-An-H2O system at 5 kbar show that at were used in the initial experiments. Additional 1000'C a constant (and presumably equilibrium) runs were madeon plagioclasescrystallized hydro- compositionof plagioclaseis reached within one thermallyfrom glassesat l-2 kbar and 700"C, and hour, whereasat around 700'C (an extrapolated) on severalnatural plagioclases.A sampleof An25 10,000years are needed to reach equilibrium. Jo- crystallizedfrom a glass (conditionsunknown) by hannesfound that at 5 kbar and 900' C run times of N. L. Bowenin 1912-1913was also used.Inconsis- approximately 200 hours were necessaryto ap- tent resultsbegan to be observedin severalof the proach compositional equilibrium in the plagio- preparations;hydrothermally crystallized An15 and clases,and at 800'C the reaction is too slow to An33(JRG) and N. L. Bowen's An25in particular attain constant compositions within reasonable produced zoisite at temperatureswell above the times. It remains to be determined whether the boundarydetermined with other preparations,act- differencein the rate of reactions observedat 8 kbar ing as if they were more calcic than the presumed and above in the system Ab-An-H2O at approxi- (synthesized)composition. For example,in an ex- mately 700" C in this study and the failure of periment to check the aberrant behavior, An15 reaction (Johannes, 1978)at 800" C and 5 kbar is crystallized hydrothermally and crystallized from solely attributable to the 3 kbar (and greater) differ- the sameglass dry at 1200' C and 20 kbar were run encein pressure,or to this plus other differencesin together(PL118a and b, 10.8kbar, 575oC, Table4). experimentalmethods and type of reaction. The high-temperatureplagioclase remained all feld- GOLDSMITH : PLAGIOCLASE STABILITY spar, whereas the hydrothermal plagioclasepro- 1200'C and 20 kbar for periods of approximately3 ducedrather abundantzoisite. Some of the hydro- hours. thermally crystallizedsamples seemed to give rea- sonableand consistentresults. however. whereas a The nature of the zoisite-producingreaction few were suspect.Hydrothermal crystallizationof The anorthite * H2O breakdown reaction (curve plagioclasesat l-2 kbar takes place fairly rapidly at 1, Fig. 1) to zoisite,kyanite, and quartz is terminat- about 700'C, and there is no compellingreason to ed at 10.2 kbar by partial melting, and is also assumethat the first crystals to form would repre- constrainedat lower temperaturesand pressuresby sentthe bulk compositionof the feldspar.Although the reactioninvolving margaritein placeof kyanite: 700'C is far enough below the solidus that one 5An + zHzO e2Zo + Ma + 2Qz might assumethat the equilibrium-type relation that produces a more calcic feldspar than the bulk The current investigation does not define the compositionat super solidus temperatureswould exact nature of the unseededhydrothermal break- not hold, it is not unlikely that such Ca-enriched down reaction of plagioclase. Zoisite is always plagioclasescould be formed metastably.Parsons evident as a product, and although the originally (1969) observesjust this behavior in the alkali- blendedkyanite and quartz are evidentin the rever- feldspars;when gels and glassesare crystallized sal runs, and with the exception of quartz in hydrothermally below their solidus the crystals PLZ2la, b, to be describedshortly, neither kyanite presentwhen crystallizationis incomplete do not nor quartz have been observedin the products of have the sameNa:K ratio as the starting material the unseededplagioclase runs. This is not to say but bear a relation to the starting material analogous that they areabsent, but that ifpresent, they are not to that between crystals and liquids coexisting at readily detectable.Kyanite might not be expected the liquidus. A similar effect could be producedby to spontaneouslynucleate and grow in an unseeded hydrothermalleaching of sodaand silica-richmate- run under the P-T conditionsused. Newton (1969) rial from the plagioclaseglass immediately prior to showed that the Al2SiO5 triple point was at a crystallization.At 700"C re-equilibrationof plagio- significantlylower P and T than determined by clasesto a homogeneouscomposition will not easily Althaus (1967) in part because of the extreme takeplace, and compositionalvariation is not readi- grindingof the kyanite used by Althaus that effec- ly detectable in ordinary X-ray powder diffraction tively destroyed the kyanite seeds. Newton was patterns.What appearsto be a well-resolvedsharp unableto crystallize kyanite in its stability field at pattern could representsomewhat inhomogeneous pressuresbelow approximately11 kbar at 750"C in plagioclase.It is likely that at least some plagio- a kyanite-sillimanitemix when kyanite seedshad clasescould havecrystallized enough material more beenhighly degradedby grinding. calcic than the bulk (presumed)composition that Goldsmith (1981)reversed the reaction 4An * observablezoisite would be produced at tempera- H2O = 2Zo + Ky + Qz at7 kbar, but the criteria tures higher than those expected. used for determination of reaction direction de- Analysis of the An25composition crystallized at pended upon relative amounts of anorthite and high temperatureby N. L. Bowen with the electron zoisite, not upon growth or diminution of kyanite microproberevealed significant Na-deficiency; the and quartz. It is possible that the reaction could material was no longer a stoichiometricfeldspar, producemargarite, even in the presenceof kyanite althoughit is close to An4 in composition.Incon- seeds,and its presencemay be missed. If so, the sistent experimental results were also obtained presumptionof kyanite-producingreaction would from the natural plagioclases,again possibly the be erroneous.A reversalrun at 6.9 kbar and 550'C result of slight compositionalinhomogeneities, al- (plottedin Fig. l) was carefullyexamined under the though the effect of structural state cannot be petrographicmicroscope, with no sign of a mica. discounted. The An15 sample from Ontario was Zoisite, plagioclase,kyanite, and quartz, all orig- observed to be a low-temperatureform (Wenk, inally present,were evident. 1969).Because of uncertaintiesas to the effect of All of the runs that define the curves of Figures 2- structural state of natural plagioclases,the run data 5 were examinedoptically, and in noneof them was are not plotted in Figures2-5, althoughthe data are there any indication of a highly birefringent,mica- listed in the tables. The most consistentand trust- ceousphase. This does not prove that the equilib- worthy starting material was the same as that used rium reactiondoes not involve margariteor parago- by Newton et al. (1980),plagioclase crystallized at nite solid solutions,particularly at the lower pres- GOLDSMITH : PLAGIOCLASE STABI LITY suresand temperatures,for the nucleationof brittle plagioclasesfrom approximately Anas to Anlse in mica in the absenceof seedsmay bejust as difficult the P-T range studied. as the nucleationof kyanite. The effect of feldspar stoichiometryand the effect of vapor composition The crystallizationbehavior of zoisite on the amounts and nature of the product phases Early attemptsto synthesizezoisite at pressures are unknown. The vapor tends to be silica-rich, up to 2 kbar invariably failed (Ehlers, 1953),and which may help to diminishthe quartz, especiallyin pressures above 2 kbar were not generally attain- the case of the small amount of reaction that takes able until the early 1960's.Below approximately4 place near the boundary curves, and the precise kbar zoisite does not nucleate,and even in seeded compositionsof the other principal phasesare un- runs at 3-5 kbar it is very sluggish in reacting and known but also might influence the nature of the crystallizing(Fyfe, 1960;Merrin,1962; Liou, 1973). product phaseswhen in minor amounts.Anorthite In this study at 8 kbar and above,zoisite grew in its hasbeen shown to be alumina-deficientat pressures stabilityfield from unseededplagioclases with such of at least 9 kbar and above (Goldsmith, 1980, consistencythat thesesynthesis runs were success- 1981),but no informationis availableon plagioclase fully used to define the field in which plagioclase solutions. and zoisite coexist. There seemedto be no excep- There is thus no indicationin the current study in tion to this observation in runs of adequatelength at runs close to the boundary curves for any reaction temperaturesabove 400' C. Runs of 366 hours at 9 -r other than An * H2O ? Zo Ky + Qz. There are kbar and 350"C on An1e.1and Ane.5(PL16a and b, no obvious breaks or discontinuitiesin the bound- Table 2), failed to crystallizezoisite. Both runs are ary curves. In the present work, only one experi- plotted in Figure 3 and althoughthe Ane.5composi- ment was carried out at conditionssimilar to those tion is essentiallyon the curve, Anrs.r is well in the describedin a previous section by Franz and Al- field of zoisite stability. No further attempts were thaus (1977), well below the plagioclase-zoisite made to crystallize zoisite from sodic plagioclases boundarycurve. Aq3 and An77w€re run at 500'and at temperaturesbelow 400'C. 10kbar for 623hours (RunsPL22la and b, Table 3). Compositions close to anorthite nucleate and Both producedzoisite * plagioclase* margarite* crystallize zoisite so readily that Anlso and Ane5 quartz, all detectable in the X-ray diffraction pat- could not be used to define the boundary curve terns. The proportions appeared to be appropriate between the field of plagioclase * H2O and that of for the equilibrium ratios; the run with An77 con- plagioclase* zoisite * kyanite * quartz. These tainedmore of the (sodic)plagioclase than did Aq3, compositionsproduced large zoisitecrystals during as would be expected.These are the only runs in the heatingprocess, even when the run was brought which either margarite or a margarite-paragonite from room temperature to approximately 700" C in solid solutionand quartz were observed.It is obvi- 15-20seconds. This is illustratedin Table 2 by runs ous from runs PL221aand b and the work of others PL20aand b,Zlaandb,22a andb,26aand b,30a that brittle mica replaceskyanite at moderate to low and b, 3la and b and PL34. Run PL28awas brought temperatures,and it is likely that at least the lower up quickly, and held for 30 minutes. Large zoisite temperatureportions of the curves for sodic compo- crystals persisted, although at 780'C it is 115' sitions represent metastable equilibria. The P-T above the boundary curve. Runs PL30a and b, conditionsfor the transitionfrom the kyanite to the brought up to 720'C rapidly and initially pumped margarite reaction even in the Na-free system are only to 7 kbar, well into the anorthite * vapor field, uncertain, and the relations in the Na-containing also showedlarge zoisitecrystals. Run PL3la, held system are completely unknown. The determina- at 760' C for 23Vzhours,showed corroded zoisite- tion of the reactionsinvolving plagioclaseand the seemingly dissolving or converting to anorthite. margarite-paragonitephase or phasesis quite im- PL34, at 800'C, superheatedby 135'C, showed portant for a full understanding of metamorphic fragmentsand embayedzoisite crystals, surely con- relations,but is outsidethe scopeof this study. An verting to anorthite. Anss was somewhatslower in experimentalstudy at the low temperaturesof the producingzoisite, but the synthesistechnique for equilibria, although needed, will be tedious and locatingthe curve could only be used with plagio- time-consuming.None of these considerations, clasesof Aneqand more sodic compositionswhich however, affect the flat breakdown boundaries of alwaysnucleated, but did so slowly enoughthat the Figures 2-4 that mark the region of instability of run could be brought to temperature before the GOLDSMITH : PLAGIOCLASE STABILITY

his cation exchange experiments at 700"C. The dotted curve is calculated from an ideal solution model. A similar configuration with a concave oc upward portion in the Ca-rich region, with an inflection and downward dipping region in the Na- rich portion, is obtained by plotting activity of anorthite in plagioclase at an arbitrary constant temperatureagainst mol fraction. 0 80 t00 (and NoAl Sis 0s CoA 12 Si20s What would be the effect of Al-Si Na-Ca) Mol% An orderingon plagioclasereaction behavior? Figure 2 Fig.9. Solid line is boundarycurve for P"ro = l0 kbar, from of Newton et al. (1980)shows a maximum excess Figure 4. Dotted and dashed curves are calculated (R. C. enthalpy'of mixing in high-stateplagioclase of ap- Newton, unpublished)from thermochemicaldata (Newton et al., proximately1.5 kcal. The enthalpyof Al/Si disorder l9E0). Dotted curve calculated from an ideal solution model. in albite, on the other hand, is about 3 kcal, Dashed curve based on Margules reduction of the calorimetric greater mixing. data plus entropies of mixing predicted by the Al-avoidance significantly than the enthalpy of model (Kerrick and Darken, 1975).The area above the curves is Although there are no data on enthalpy of disorder region of plagioclase stability relative to zoisite, the region of plagioclasesother than albite (and no measure- beneaththe curves of plagioclase+ zoisite + kyanite + quanz. ments of the entropy of intermediateplagioclases exist),it is likely that order-disorderrelations in the plagioclaseswould overpower the effect of enthalpy zoisite crystallized.The location of the pure anor- of mixing, and thus havea largeeffect on the curves thite terminus of the curve was necessarilydone of Figures2-4. with reversalmixes (Goldsmith, 1981,and seeFig. Newton et al. (1980)were unable to use hydro- 1).Goldsmith (1976,1980) also discusses the prob- thermally crystallized plagioclasesin their calori- lem of not being able to suppressthe development metric determinationsof enthalpies of solution. of zoisite at high pressures if even a trace of Hydrothermal crystallization at 700-750"C and moistureis present. 1000bars for 3-5 days gave feldspars with good X- ray patterns but which resulted in very scattered The configuration of the isobaric curves-Discussion calorimetric data, presumably becauseof uncon- The flat portionsofthe curvesat 8, 9, and 10kbar trollable variations in structural state and the pres- are, within experimental uncertainty and in the enceofvapor-solution products in the charges.The limited pressurerange studied, sections of a planar scatter was always in the direction of increased surface in P-T-X space. This behavior is remark- enthalpy of solution (Newton, personalcommuni- able, and not readily explainableon the basis of cation), as would be expectedof partially ordered known thermodynamicdata. As drawn, Figures2-4 feldspars. are compatible with unit activity of anorthite in Eberhard (1967) hydrothermally annealed syn- plagioclasefrom approximatelyAna6 to An1so.This thetic plagioclaseat temperaturesfrom 500-800" C cannotbe reconciledwith the activity data obtained at one kbar, and in the range Ans-An66 observed a by Orville (1972)from ion-exchangeexperiments at rangeof intermediatedegrees of order basedon the 700" C and 2 kbar, which show nearly ideal An 2g(l3l)-20(131) indicator of structural state (see, activity in plagioclasein this compositionrange. for example,Ribbe, 1975).MacKenzie (1957)had Figure 9 contains the l0 kbar curve (solid line earlier reported partial ordering of pure albite and from Fig. 4), along with two curves calculatedby Martin (1969),crystallization of highly ordered al- R. C. Newton (unpublished)with dataderived from bite. Orville (1972)reports structural state changes high-temperaturesolution calorimetry on high-tem- for Ablqe and AbTsat 700"C and 2 kbar for 1000 perature synthetic plagioclases(Newton et al., hoursmuch like thoseobserved by Eberhardand by 1980).The dashedcurve is basedon the Margules MacKenziefor albite, and statesthat this structural reduction of the calorimetric data plus the entropies state,as measuredby A20(13f-131),is lessthan a of mixing predicted by the Al-avoidancemodel of quarter of the way from high albite toward the low Kerrick and Darken (1975).The Gibbs energy of albite structure,and values of 420(131-131)for mixing at 970 K using this model agreesquite well experimentsof 6 days have nearly reached the with Orville's (1972)mixing function derived from constantlimiting value characteristicof the equilib- 668 GOLDSMITH : PLAGIOCLASE STABILITY rium structuralstate at 700"C. Wenk (1978)heated ate plagioclasesand a combination of X-ray diffrac- a volcanic plagioclase(Anoo) with diffuse e-reflec- tion and electron microscopy have recently made tions at 870' C and 2 kbar for 50 days, and observed inroadsand given insights into the variegatednature ordering of the intermediate superstructure and of plagioclases.The simplest schemeby which a sharpeningof the satellite reflections. All these plagioclasecan be "ordered" is to haveinterleaved indicationsof increasedorder, even by treatmentat regionsof pure anorthite and pure ordered albite; l-2 kbar indicate the likelihood of a significant only the "walls" or connectingregions would not effect on the breakdown reaction as comparedto havecomplete Al-Si order. This conceptin a sense high structuralstate plagioclases. goes back to the early days of feldspar structural The effect of introducing partial order in the work by Taylor (Chao and Taylor, 1940),and has plagioclasesin a model calculation seemsfruitless been rethought and refined by many feldspar spe- without prior knowledgeof the oidering scheme,its cialistsover the years(see discussion and review by degreeand its variationwith composition.Although Smithand Ribbe, 1969;Smith, 1974).Morerecently the entropy differencecould be more than adequate transmissionelectron microscopy (TEM), including in a quantitativesense to accountfor the disagree- direct lattice imaging in intermediateplagioclases ment betweenthe observedand calculatedcurves, has revealed antiphaseboundaries (APB's) delin- it could well go in the wrong direction, for increas- eating anorthite or anorthite-like bands, and in ing the degree of order in the plagioclaseof the labradoritesalternation of albite-likeand anorthite- calculatedcurve would tend to stabilizeplagioclase like bands (see for example Kitamura and Mori- relative to zoisite, and thus drop it further in moto, 1977;Nakajima et al., 1977). temperature,assuming the ordered plagioclaseis If plagioclasesorder or segregateAl and Si and more stableat this temperaturethan the disordered Na and Ca into detectablecompositional and struc- plagioclase.Under any circumstances,enthalpy of tural units of anorthite and albite, they obviously solution data on plagioclasestreated at the pres- displaychemical heterogeneity, even if on a rather suresand temperaturesof the experimentalcurves fine scale.Segregation into discrete (incoherentor are needed, and such a program is planned as a "unmixed") phasesis not essentialto define the continuationof the presentwork. The observation anorthiteportion of the structure.The zoisite-form- of enhancedreaction rates at H2O pressuresof 8-10 ing behavior in the range Ana6-An1s6is as if the kbar as comparedto 2 kbar makes it appearlikely plagioclasewere a mechanicalmixture of thosetwo that Al-Si orderingmay also be enhanced,and this end-members.This is intuitively in accordwith the hasbeen observed in KAlSi3Os(Goldsmith, unpub- structure of intermediateplagioclases as revealed lished data). Data on enthalpy of solution of vari- by TEM observationsin which segregatedunits ously treatedplagioclase will supply more quantita- occur. Although the APB's seenwith TEM indicate tive information than empirical evidenceprovided coherencyof the structure, the protons or hydrox- by latticeconstants or indicatorssuch as A20(131- ide ions that becomeintimate with the plagioclaseat 13l). elevatedtemperatures and pressurescan hardly be In the absenceof any informationon the structur- expectedto have been educatedin the nicetiesof al state of the feldsparsof this investigation,or of coherencyversus incoherency.Runs PL190a and the effect of structural state on the zoisite-produc- 190b(Table 5), previouslyconsidered, may provide ing reactionI would like to speculateon a possible additionalevidence for the view that solid solutions explanationfor the flat region of the plagioclase between Anas and Anlss ar€ not stable in the breakdown curves. Simply stated it could be the neighborhoodof 700"C, irrespectiveof their stabil- result of segregationof anorthitein the structureof ity relativeto zoisite.The mixture of An15and Ana2 the plagioclasesat a scalesuch that H2O at elevated reactedat 700"C and 9 kbar in7l9 hours more than pressuresand temperaturescould react with the half of the way to a homogeneousplagioclase. anorthite component as if it were in a separated There was no observablereaction in the mixture of phase.Such behavior would require a substantial Ana2and Anroo.Failure to react provesnothing, but amountof organizationand orderingin the synthet- the differencein behavior is suggestiveof possible ic plagioclasesduring the course of the experi- instability. ments. If the interpretationof separatedomains is cor- A number of models have been proposed to rect, the phaserelations could be as drawn in Figure explainthe structuralcomplexities of the intermedi- 10. The simple schematic solvus of Figure 10a GOLDSMITH: PLAGIOCLASE STABILITY 669

Plagioclasestability and metamorphic rocks PLAGIOCLASE 1 PLAGI0CLASEz / The relative importance of reactions involving //2 plagioclasein a heterogeneous(chemical) sense in / PLAGIOCLASES metamorphismas opposedto reactionscontrolled 2 PLAGIOCLASES /:' PLAGTOCLASE+ chiefly by internal or structural ("homogeneous") ,.. ZOISITE+ KYANITE propertiesof the plagioclaseshas been reviewed + OUARTZ from field and laboratory viewpointsby Goldsmith (1982). o*ontoron' Certainly the presence or absence of a feldspar,or the rangeof compositionspresent, can Fig. 10. (a) Simple schematic solvus in plagioclasefeldspars be relatedto the bulk composition(including vola- indicating inherent instability of solid solutions; unmixing nor tiles)as well as P and T in the system.On the other realized because of kinetic barriers. (b) Solvus transected by hand,the structuralstate of a particularplagioclase zoisite-producing reaction in the presence water of at an may appropriatepressure. The upper(dashed) portions ofthe solvus, also influence its stability and therefore its as in (a), generally not observed to develop separatephases for persistence.These factors apply to pre-existing kinetic reasons.This schematicportion ofthe diagram is shown igneous or earlier metamorphic plagioclasesand asa morecomplex region by Smith(1972, 1974).The dotted lines perhapsto a lesser extent to metamorphicplagio- are metastableextensions of the solvus cut-off by the zoisite- claseproduced by reaction,dependent upon wheth- forming reaction, which is shown quantitatively in Figs. 2-4. er the plagioclasecrystallized metastably in a high- temperaturestate or in equilibrium with the envi- ronment(Christie, 1962;Rutland, 1962). would be transected by the zoisite-producingreac- Wenk and Wenk (1977) presented a tentative tion in the presence of water at an appropriate diagram showing variation of plagioclase composi- pressure,as in Figure 10b, with an upper dashed tion as a function of metamorphicgrade and rock solvus region that, for kinetic reasons,is not ob- compositionbased on examination of plagioclase servedto develop phases. separate An attempt to from a number of localities. A slightly modified resolvethis problem in the region above the hori- versionpresented by Wenk (1979a)is reproducedas zontal curves of Figures 2-4 is currently underway. Figure 12. The temperature scale (metamorphic The initial depression of the curve at the An side of grade) is based on Wenk's estimates from the the diagram, required by Raoult's Law, appearsto Tertiary metamorphicregion in the Central Alps. be too small to have been detected. The data of Tables 1-4 and Figures 2-5 can be used to illustrate the P-T behavior of the plagio- Zo+Ky clasesin termsof breakdownto zoisite + kyanite + Zo+Ky+oz+V l;i- qtrartz.Figure 11is a projectionon the P-Zplane of the breakdowncurves of Anles and An3s,An26 and An1s,the compositionsselected from the curves of Figures 2-5. The Anlqq curv€ and its associated L boundarycurves delimiting the field of partial melt on the An-H2O join are taken from Figure 1. Plagioclasesfrom approximately Anas to Anlse under the conditions of this investigationall react with water to produce zoisite * kyanite + quartz 300 400 too ,oo 700 800 - alongthe An16sP-? curve. The spacebetween the IU"^ An3sand Anlss cufves thus representsthe regionin Fig. ll. CaAl2Si2Osisopleths of plagioclasestability as function of P"ro and T. The curve labeled An166is taken from which plagioclasesbetwen An3eand Anaebecome Fig. l, andrepresentsthereaction4An + V c2zo + Ky + Qz + unstable.The greater stability of the more sodic V. The region labeled Zo + Ky + V + L is the field of partial plagioclasesis illustratedin Figure 11.Although the melt for the composition An166.The solid circles are points lower temperature reaction or reactions are un- arbitrarily taken at An16, An26, and An36 from the isobaric known, if margarite were to replace kyanite the boundarycurves of Figs. 2-5 at 8, 9, 10, and 10.8 kbar. The appropriate CaAl2Si2Osisopleths plagioclase region to the right of the isopleths represents conditions of of stability of plagioclasemore calcic than that of the appropriate would be deflectedto highertemperatures, decreas- isopleth; the region to the left, ofbreakdown ofthat composition ing the stability of plagioclaserelative to zoisite. to zoisite + kyanite + quartz. 670 GOLDSMITH: PLAGIOCLASE STABILITY

gioclaseor multiphaseassemblages producing clus- tersof solvus-likeinverted festoons, complex X-ray diffractionpattterns including e-satellitereflections associatedwith superstructuresof various wave- lengths,complex TEM patternsshowing APB's, all thesethings with confusingdifferences as reported by various investigators. Nothing approachinga generallyagreed-upon (equilibrium) phase diagram is at hand. Compositionaland structural equilibra- tion seemsessentially impossible even on a geologi- cal time scale in the absenceof a flux such as H2O F that aids in breaking strong Al-O and Si-O bonds c) E and fosters diffusion and structural rearrangement. o O o-reflectionsonly It is generallyappreciated that the complexity and (9 - e sotellifes (Smith Ribbe, diffuse20 50A confusionis due to metastability and 1969).There are no clearly defined solvi in the Ab 20 40 60 80 An plagioclases,and no well-developedunmixed tex- Fig. 12. Diagram from Wenk (1979) indicating structural tures akin to the perthites. The alkali feldspars complexities of plagioclases from a variety of metamorphic show relatively clear-cut exsolution for two rea- localities, as revealed by single-crystal X-ray diffraction sons: (a) Al-Si rearrangementin the network is analysis, plotted as a hypothetical temperature-composition unnecessary,and (b) both end member feldspars diagram. Isolines show the variation of wave length of the are stable in the presenceof water at "metamor- intermediate plagioclase superstructure, as observed from e- satellitereflections. The ordinate representsmetamorphic grade, phic" temperaturesand pressures.The Al-Si diffu- or temperature.Wenk's analysis from field data indicates a sionalbarrier in plagioclaseis significant,and anor- temperaturerange of 400-600"C for region C, 600-800'C for thite is not stable in the presenceof water at the regionB, and 700-800'C for A. The region of interestfor this more elevated P and T ranges where unmixing investigation "no is labeled metamorphic plagioclase", being would be kinetically favored. analogousto the area under the curves of Figs. 2-4. Smithand Ribbe (1969), Smith (1972,197 4 , 1975) , and McConnell (1974a,1978) have suggestedthat Wenk notesthat'for eachcomposition, except pure the variously ordered intermediate plagioclase albite,there is a minimum temperaturebelow which statesare metastablewith respectto orderedalbite no metamorphicplagioclase has been found. The and anorthite which are not observedat low tem- configuration of the area with no metamorphic peraturesonly becauseof kinetic barriers. Kinetic plagioclasein Figure 12,particularly the almostflat barriers, important as they are, are not the whole region from approximatelyAn35 to Antoo, is very story. The equilibrium unmixing and/or ordering much like that outlined by the plagioclasebreak- statesin plagioclaseare not observedin metamor- down curves. Figure 2 at 8 kbar would match the phic rocks beausethe processesare too slow in the upper temperaturelimit shown in Figure 12, and absenceof water, and in the presenceof water reasonably lower metamorphic pressures would zoisiteis formed in the place of calcic plagioclase. producea goodtemperature match. The association Figures 2-5 and Figure 11 indicate that, even of An37a2with AnsT-e2observed by Spear (1977, neglecting a peristerite gap, extrapolation of the 1980),where the systematicpartitioning of Na and curves representingthe breakdown reaction (to Ca betweenhornblende and plagioclaseis evidence zoisite * kyanite * quartz) producesrather sodic for an equilibrium association,is compatiblewith plagioclases,although the Ca-content would be the resultsof this investigationand with the immis- increasedat lower water pressures.No specific cibility gap illustratedby Smith (1974,p. 6). experimental attempt was made to observe the The complexity of the intermediateplagioclases phenomenain the presumedregion of the peristerite as illustrated by Figure 12 covers most of the gap, but it is not at all unlikely that the lower- compositionalrange. In this and other representa- temperature portions of the curves of Figures 2-5 tions (Smith, 1974,p. 6; McConnell, 1974a)a vari- are metastableextensions into the regionof the gap. ety of phenomenaare described;Huttenlocher and At 10.8 kbar the plagioclasein equilibrium with Bpggild intergrowths and other reported two-pla- zoisiteat 400' C is approximatelyAn6; at 8 kbar it is GOLDSMITH: PLAGIOCLASE STABILITY 67r

albite. If Crawford's estimateof the solvuscrest at 450-500"is correct, and the generallyaccepted limit of the oligoclaselimb at -An2a is accepted,there is a limited range of temperature in which 2 phases

l-+MlxE0 LAYTRPLAGI0CLASES can be in equilibrium:it is 50" C or lessat 6 kbar. If

F SoDICHIGH PtAGT0CtASES(-Anza- the right hand limb is steeper than indicated, the temperatureinterval in the gap is increased,but the LoW + // ALBTTE Zo,efc compositionof the oligoclasein the limb cannot reach An2a.At 8 kbar the breakdown curve is at NoAl Sr306 Mot % An CoAl2Si2Ot approximately 540" C at An2a,and if the gap is as Fig. 13.A hypotheticalrepresentation ofplagioclase stability indicated in Figure 13, the temperature range is relations, at 6 kbar, with respect to the peristerite gap. The reducedessentially to zero. If the oligoclaselimb is boundary curve separating the field of plagioclases from steepenedso that say, at 400" or lower it reaches plagioclase * zoisite (with horizontal portion at 520'C) is An24,Ps,6 would have to be significantly less than extrapolatedfrom Figs. 24.The representationof the peristerite gap is arbitrary, partially basedon the work ofCrawford (1966), 6 kbar to avoid cutting off the limb by the break- and may extend to somewhat higher temperatures. The region down curve. Crawford's text and diagrams clearly above the 6 kbar boundary curve is a presumedregion of mixed- expressthe effect of P11,qon the intersection of the layer plagioclaseswhere ordered layers or domains of anorthite oligoclase limb of the solvus with a presumed are interposed with domains of disordered, sodic plagioclase. calcium mineral-plagioclaseequilibrium curve, and The region just below the boundary represents one of ,1974) the likelihood of this equilibrium of plagioclasesfrom -An2a to Ana6with zoisite (+ Orville also mentioned kyanite and quartz). At the intersection of the boundary curve phenomenon. with the calcic limb of the peristerite gap the boundary curve is It is thus possiblethat the plagioclasebreakdown ''smother" cut-off, and a region of ordered albite (with but little Ca) in relationseffectively or engulf the peris- equilibrium with zoisjte is produced. A second(or higher) order terite gap at 8 kbar and above. If a binary loop is phasetransition from ordered (low) to disordered(high) albite is the presumed.The inset diagramrepresents the peristeritegap as rhe involved, the somewhathigher temperatureof result of a first-order phase change in albite. This diagram albite order-disorderrelations would permit alarg- indicates how the gap is encroached upon with increasing er temperature range of coexistence of two plagio- pressure.It may be even less evident than shown here at 6 kbar. clases.The relative rarity of peristeritesmay well be related to this effect. At lower pressureswhere the gap might be more in evidence,the kinetics of approximately An1a. In the New Zealand rocks phase separation, already hampered by low tem- studied by Crawford (1966)the first appearanceof peratures,would not have the pressure-enhanced two plagioclaseswas at the almandine isograd, reaction advantage.At higher pressurethe gap may whereasthe first appearanceof two plagioclasesin be effectivelyobliterated. The effect of P161a1onthe the Vermont rocks is below this isograd. Crawford configuration of the gap itself, whether a solvus or (1966)located the crest of the peristerite solvus an inversionloop, shouldbe very small, and can be between the almandineand staurolite isogradsin neglectedfor the purposesof this discussion.Ex- the Vermont rocks, and suggesteda temperature of trapolation of the present data would indicate that 450-500"C. Wenk and Wenk (1977)suggested that peristeritesor oligoclasein associationwith albite the peristerite gap closes in the staurolite zone, might not be found in metamorphic rocks crystal- possibly50-100'C higher than the crest suggested lizedat pressuresabove 6-7 kbar. Ifthe crestofthe by Crawford. At 8-10 kbar the sodic portion of the solvusis less than 500' C, and if a spinodalthat is plagioclasebreakdown curves clearly transgresses significantly narrower than the presumed gap is a large portion of the presumedperisterite region. necessaryfor the processto take place,plagioclase Figure 13 is a representationof plagioclase-H2O breakdown will effectively limit phase separationat relations at 6 kbar, in part basedon extrapolation of even lower pressures. the plagioclase+ HzO a Zo * Ky * Qz break- An indication of the effect of pressure on the down relations, and in part a representation of the discontinuousjump from albite to oligoclase is peristerite gap as a solvus modeled largely after shownin the work of Laird (1980,1981), who found Crawford (1966).Also indicatedis a possiblerepre- that in mafic schists containing epidote from Ver- sentationofthe gapas a "binary loop" producedby mont the discontinuity takes place in the biotite first-order phase relations between high and low zone (or even lower grades)in low-pressuresam- GOLDSMITH : PLAGIOCLASE STABILITY ples, while it occurs in the garnet zonein medium- morphic terranes in Japan. Under low-pressure pressure schists. In high-pressuremafic schists contact metamorphic conditions, Miyashiro indi- (Laird and Albee, 1981)containing sodic amphibole cates that labradorite may be present. Miyashiro and omphacite, the P and T estimates are 9t2 kbar recognizesthat his amphibolereference point does and450o-r 100' C, and the plagioclaseis albite,Ans.2 not representa definite temperature,but that the to An1. relationsdescribed suggest the possibility that the It is generallyassumed in the literature that the formation of a more calcic plagioclaseis promoted peristerite gap representsan equilibrium feature. by lower rock pressure. At 10 kbar and 400'C Viswanathan(1973) doubts that peristeritesare two Figure 4 indicates a plagioclase composition of phases,and considersthem modulatedstructures, -Anz. At the extrapolated value for 6 kbar (Fig. not unlike intermediateplagioclases. The develop- 13), it is -An2s. At pressuresnot much below 6 ment of two resolvable plagioclasesin the range kbar a value of Anzs-rois not unreasonable. An6An25 in some rocks is a progradeand not an Carmichael(1978) considers a numberof diagnos- unmixingphenomenon, and could representa meta- tic mineralassemblages in pelitic schiststhat define stableassociation. The field relationsof rocks con- a seriesof "bathograds" that are temperatureinde- taining peristerite are not well defined, and the pendent. Five invariant points within the system phase relations related to peristerite phenomena SiO5AI2O3-FeO-MgGNa2O-K2O-H2O permit the remainhighly speculative(Carpenter, 1981). ordering of relative pressuresin metamorphicter- Not all natural plagioclases of the appropriate ranes.Plagioclase is not used in the reactions,but compositionunmix (Sen, 1963).In the absenceof Carmichael notes that CaO, as the most important phase separationfor whatever reason, the break- "impurity" componentcould changethe equilibri- down curves,which dip more steeplywith decreas- um pressuresof two of the invariant points. He ing temperaturein the sodic compositionalrange suggeststhat the composition of plagioclase in have a configuration and location in reasonably rocks be routinely determined so as to be able to goodagreement with petrologicobservation. Miya- evaluate the effect of CaO on the bathograd deter- shiro (1973,p. 249)notes that the reactionof sodic mination. I would suggest that the composition plagioclasewith epidoteshould be influencedby T, might provide an additional index or check on the P", PHro, Ps, and by the minerals occurring in bathograddetermination considering the pressure- associationwith the albite and epidote,and that the compositionbehavior of the plagioclaseif in equi- rate of compositiopalvariation of plagioclasewith librium with epidoteor zoisite. rising temperature should differ in different meta- No considerationhas been given to the effect of morphic terranes. He claims that petrographic ferric iron on the plagioclase-zoisiterelations. experience,however, has shown that the rate of Ramberg(1949) noted that increasingthe Fe3*/Al compositionalvariation is usually gentle up to 30 ratio will displace the equilibrium toward epidote percentAn, abovewhich it becomesvery fast. For inasmuchas epidoteincorporates Fe3* in its struc- this reason(Ramberg, 1952), aplagioclase composi- ture. Strens(1963), Holdaway (1966,1972)andLiou tion of 30 percent An in equilibrium with epidote (1973)report experimentalwork and Miyashiro and was taken as the boundary between the epidote- Seki (1958)a field study indicating that increasing amphiboliteand amphibolitefacies. Fe3+ stabilizesepidote with respectto zoisite and In addition to the composition-temperaturebe- clinozoisite.A thermodynamicanalysis by Bird and havior, Figure 11 indicatesthat plagioclasecompo- Helgeson(1981) shows a dramatic increaseof epi- sition is quite sensitiveto Ps,6. Again aside from dote relative to plagioclase and an aqueous phase peristerite complications metamorphic pressure with increasing Fe3* substitution. Experimental must be evaluated if temperature estimates are to work on the system Ab-An-H2O with Fe3* as an have any meaning.Miyashiro (1973)notes that at addedcomponent becomes more complicatedand the changeof calcic amphibolesin metabasitesfrom an additional mineral containing ferric iron is neces- actinoliteto hornblende,the compositionof meta- sary as a buffer. In nature at least several minerals basiteplagioclase is about 5 percentAn in the high can and probably do play this role; an amphibolefor pressure terrane of the Sanbagawa belt and the examplecan contain severalpossible buffering spe- Barrovian region of the Scottish Highlands, but is cies, such as Fe, Al, and Ca. The use of an 20-30percent An in the low-pressureregional meta- amphibole in experimental systems makes for diffi- GOLDSMITH : PLAGIOCLASE STABILITY 673 cultieswith reactionand equilibrium,however, and aqueoussolutions with epidote-feldsparmineral assemblages somethingas simple as magnetitemay be neces- in geologic systems. II. Equilibrium constraints in metamor- sary. phic/geothermalprocesses. American Journal of Science,281, It is likely that 576-614. substitution of Fe3+ in epidote Boettcher,A. L. (1970)The system CaO-AlzOrSiO2-H2O at enlargesits field of stability relative to the essential- high pressures and temperatures. Journal of Petrology, ll, ly Fe-freeplagioclase. This is tantamountto raising 337-379. the isobaric curves of Figures 2-4 and Figure 13. Boettcher, A. L. and Wyllie, P. J. (1967)Hydrothermal melting Doing so would produce more sodic feldspars in curves in silicate-water systems at pressuresgreater than l0 equilibrium kilobars. Nature, 216, 572-573. with epidote in the absenceof a peris- Boettcher,A. L. and Wyllie, P. J. (1969)Phase relationships in terite gap, and would also further encroach on the the system NaAlSiO4-SiO2-H2O to 35 kilobars pressure. region of the gap, acting in the sameway as does an American Journal of Science. 267. 875-909. increasein H2O pressure.Examination of field data Carmichael, D. M. (1978)Metamorphic bathozonesand batho- in the light of the observed effect of P11,6and ferric grads: a measureof the depth of post-metamorphicuplift and of Science, iron content plagioclase erosion on the regional scale. American Journal on compositionsand phe- 278,769-:797. nomenarelated to "peristeritic reactions" might be Carpenter,M. A. (1981)A "conditional spinodal" within the fruitful. peristerite miscibility gap of plagioclasefeldspars. American Althoughinvestigation of meltingphenomena has Mineralogist, 66, 553-560. beenrestricted to a limited portion of the solidusat Chao, S. H. and Taylor, W. H. (1940)Isomorphous replacement plagioclase 10.8 kbar, and superlattice structures in the felspars. Royal it is apparent that the zoisite-forming Society(London) Proceedings, series A, 175,76-87. reaction can produce a relatively sodic melt from Chattet'ee,N. D. (1971)Preliminary results on the synthesisand rather calcic plagioclasesat temperaturesfar below upper stability limit of margarite. Naturwissenschaften,5E, thoseof the plagioclasemelting loop. In the region 147. above the dashedsolidus of Figure 5, melts more Chatte{ee, N. D. (1976) Margarite stability and compatibility pressure- sodicthan An3e relations in the system CaO-Al2O3-SiOz-H2Oas a ore formed at little over 700'C. temperatureindicator. American Mineralogist, 61, 699J 09. Christie, O. H. J. Feldspar structure and the equilibrium be- Acknowledgments tween plagioclaseand epidote (discussion).American Journal This research was supported by NSF grant EAR 7813675 of Science.260. 149-153. (Geochemistry Program). I am indebted to many friends and Clayton,R. N., Goldsmith,J. R., Karel, K. J., Mayeda,T. K. (1975) pressure colleaguesfor aid and encouragementover a long period of time. and Newton, R. C. Limits on the effect of on isotopic fractionation. Geochimicaet CosmochimicaActa, 39, Discussionswith Hans Ramberg,beginning in 1947, acted as the impetus for hydrothermal experiments with plagioclase in the 1197-120t. (1966) plagioclase late 1940'sand early 1950's,none of which producedzoisite. Crawford, M. L. Composition of and associat- N. L. Bowenand J. F. Schairerkindly suppliedme with synthet- ed mineralsin someschists from Vermont, U.S.A.. and South peristerite ic plagioclasesand some glasses,later supplementedby several Westland, New Zealand, with inferencesabout the samplesdonated by H. S. Yoder from the GeophysicalLabora- solvus. Contributions to Mineralogy and Petrology, 13,269- tory supply. Robert C. Newton provided advice and stimulation, 294. (1967) and some of the plagioclaseshe crystallized for thermochemical Eberhard, E. Zur Synthese der Plagioklase. Schweizer- work were used in this study. I am also indebted to Mark D. ische Mineralogische und PetrographischeMitteilungen, 47, Barton, Dexter Perkins III, and J. V. Smith for thoughtful 385-398. . (1953) discussions,to H. R. Wenk and Ian Steele for natural plagio- Ehlers,E. G. An investigationof the stablityrelations of group. clases. Ian Steele supplied electron microprobe data on these the Al-Fe membersof the epidote Journal of Geology, and a number of synthetic crystals and glasses, and Ian D. 6l, 231-251. (1977) Hutcheon kindly examinedseveral run products with the TEM. I Franz, G. and Althaus, E. The stability relations of the am continuously indebted to William Moloznik, toolmaker, who paragenesisparagonite-zoisite-quartz. Neues Jahrbuch Min- keeps the high-pressurelaboratory operational, and to Cassan- eralogieAbhandlungen, 130, 159-167. (1960) dra Spooner, whose translation of my miserable script into Fyfe, W. S. Stability of epidote minerals.Nature, 187, typescript requires extraordinary skill and patience. 497-498. (1952) plagioclase The manuscript was significantly improved after critical read- Goldsmith, J. R. Diffusion in feldspars. Jour- ing by R. C. Newton, Dexter Perkins III, Mark Barton, and nal of Geology, ffi, 288-291. David M. Jenkins. Goldsmith,J. R. (1976)Scapolites, granulites, and volatiles in the lower crust. Bulletin of the GeologicalSociety of America, E7,l6l-16E. References Goldsmith, J. R. (19E0)The melting and breakdown reactions of Althaus, E. (1967)The triple point andalusite-sillimanite-kya- anorthite at high pressuresand temperatures.American Min- nite. Contributions to Mineralogy and Petrology, 16,29-44. eralogist, 65, 272-284. Bird, D. K. and Helgeson,H. C. (1981)Chemical interaction of Goldsmith,J. R. (1981)The join CaAl2Si2OgH2O(anorthite- 674 GOLDSMITH : PLAGIOCLASE STABILITY

water) at elevated pressures and temperatures. American tion field of epidote and piemontite with rising temperature. Mineralogist66, I 183-1188. American Journal of Science, 256,423430. Goldsmith, J. R. (1982) Review of the behavior of plagioclase Nakajima, Y., Morimoto, N. and Kitamura, M. (1977)The under metamorphic conditions. American Mineralogist, 67, superstructureof plagioclasefeldspars. Electron microscopic 643-652. study of anorthite and labradorite. Physics and Chemistry of Goldsmith,J. R. and Newton, R. C. (1974)An experimental Minerals,1,213-225. determinationof the dkali feldspar solvus. In W. W. MacKen- Newton, R. C. (1966)Some calc-silicate equilibrium reactions. zie and J. Zussman,Eds., The Feldspars,p. 337-359.Man- American Journal of Science, 264,204-222. chester University Press, Manchester Newton, R. C. (1969)Some high-pressurehydrothermal experi- Holdaway, M. J. (1966) Hydrothermal stability of clinozoisite ments on severely ground kyanite and sillimanite. American plus quartz. American Journal of Science, 2U, 643-667. Journalof Science,267,278-284. Holdaway, M. J. (1972)Thermal stability of Al-Fe epidote as a Newton,R. C., Charlu,T. V. and Kleppa,O. J. (1980)Thermo- function of /6, and Fe content. Contributions to Mineralogy chemistry of the high structural stateplagioclases. Geochimica and Petrology, 37, 307-340. et CosmochimicaActa, 44,933-941. Johannes,W. (1978)Melting of plagioclasein the system Ab- Newton, R. C. and Kennedy, G. C. (1963)Some equilibrium An-H2O and Qz-Ab-An-HrO at P11r6: 5 Kbars, an equilibri- reactionsin the join CaAlzSizOe-H2O.Journal of Geophysical um problem. Contributions to Mineralogy and Petrology, 66, Research.68. 2967J983. 295-303. Newton, R. C. and Wood, B. J. (1980)Volume behavior of Johannes,W., Bell, P. M., Mao, H. K., Boettcher,A. L., silicatesolid solutions.American Mineralogist, 6, 713-:745. Chipman,D. W., Hays, J. F., Newton, R. C. and Seifert,F. Noble, D. C. (1962)Plagioclase unmixing and the lower bound- (1971)An interlaboratory comparisonof piston-cylinder pres- ary of the amphibolitefacies. Journal of Geology, 70, 234-240. sure calibration using the albite-breakdown reaction. Contri- Nord, G. L., Jr., Hammarstrom,J. andZen, E-an (1978)Zoned butionsto Mineralogyand Petrology,32,4-38. plagioclaseand peristerite formation in phyllites from south- Kerrick, D. M. and Darken, L. S. (1975)Statistical thermody- westernMassachusetts. American Mineralogist, 63, 947-955. namic models for ideal oxide and silicate solid solutions with Orville, P. M. (1972)Plagioclase cation exchangeequilibria with applicationto plagioclase.Geochimica et CosmochimicaActa, aqueouschloride solution: results at 700'C and 2000 bars in 39, t43l-1442. the presence of quartz. American Journal of Science, 272, Kitamura, M. and Morimoto, N. (1977) The superstructureof 234-272. plagioclasefeldspars. A modulatedcoherent structure ofthe e- Orville, P. M. 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