AmericanMineralogist, Volume64, pages 317-328, 1979

Synthesisand stability relations of Mn-Al piemontite,Ca2MnAlrSisOrr(OH)

Mlny KrsxlNpNrl,No JUHNG. Lrou

Tuttle-Jahns Laboratory for Experiment.al Petrology Deparlment of Geology, Stanford Uniuersity Stanfo rd, C alifornia 94305

Abstract

Stabilityrelations for the Mn-Al piemontitebulk compositionCazMnAlrSirOrr(OH) were investigatedat I and 2 kbar and temperaturesbetween 200o and 750oC,using cold-seal pressureapparatus and solid oxygenbuffer techniques.Pure piemontite,with averagecell dimensionsa= 8.86(1),b:5.69(l),.c: 10.19(2),4andB:115"42',wasreadilysynthesized from oxidemixtures at 500'-600'c andfoz definedby the cu-cuzo and curo-cuo buffers at I and 2 kbar. The high-temperatureequivalent assemblage is intermediategrossular- -- spessartinegarnet solid solution [Ca,MnAlrSi,O,,ia 11.804(2)4,no = 1.7636(2)]+ fluid for fO, definedby the QFM, HM, CC, and CT buffers.The cell parametersfor synthetic piemontitesand garnetsgrown over a rangeof/O, conditionsshow only smalland random variations;this relationsuggests that compositionalchanges of garnetand piemontitefor the bulk compositionPmssCz., with variation of fO, are limited. Averagemeasured a for the syntheticgarnets is slightly higherthan the calculatedvalue for this bulk composition. In agreementwith evidencefrom natural piemontite-bearingparageneses, reversal runs indicatethat crystallizationof piemontiterequires a high oxygenfugacity. Along the HM buffer,garnet was the only condensedphase stable at all experimentalconditions. Equilibrium reversal for the reaction piemontite : garnet + fluid [CarMnAlrSirOp(OH) CarMnAlrSisOL,+ l/2 HrO + l/4Orl wasdelineated,for 2 kbar at 617o+10"for the CT buffer,at 404"+10' for the CC buffer,and below 250oCfor the HM buffer.At pfluid : I kbar,piemontite is stableup to 591'*l0o for the cr bufferand to 402"r10'c for the cc buffer.The effectof fluid pressureon the stabilityof piemontiteis apparentlyminor compared to that of oxygenfugacity. The sporadicoccurrence of piemontitein a wide variety of geologicenvironments from blueschistto greenschistand amphibolitefacies conditions is mainly controlledby oxygen flugacityin addition to pressure,temperature, and major-elementcomposition of the host rocks.Introduction of Fe into piemontitein naturalcompositions will evidentlyresult in a more complex breakdownreaction and in an extensionof the piemontitestability field to highertemperatures and lower oxygenfugacitres.

Introduction (Ernstand Seki,1967, and others), and manygreen- Piemontite,the Mn3+, has been described schist facies piemontiteshave also been described from a broad spectrumof rock typesand geological (Simonson,1935; Marmo eI al. 1959;Nayak,1969; environments,ranging from blueschistto Taylor and Baer, 1973).A more thorough com- and amphibolitefacies. A common assertionhas pilation of piemontitelocalities, however, reveals a beenthat piemontiteis predominantlythe productof significantnumber of occurrencesfrom rocks meta- low-grademetamorphism (Deer et al.,1962;Makan- morphosedunder upper greenschist and juola and Howie, 1972).Natural examplessupport faciesconditions (Smith and Albee, 1967;Cooper, this view: piemontite-bearingmetacherts and schists l97l; Strensrud,1973). Similarly, the requirementof commonlyoccur in blueschistfacies terranes in Japan a moderatelyunusual, Mn-rich host rock has also been suggestedas an explanationfor the restricted I Presentaddress: Department of Geology, Smith College, developmentof piemontite,a hypothesisencouraged Northampton, Massachusetts01063. by the common appearanceof piemontitein shear w034o4x/79/0304-0317$02.00 317 318 KESKINEN AND LIOU: STABILITY RELATIONS OF PIEMONTITE zonesaffected by "mineralizing" hydrothermalsolu- with Sr, Pb, and lanthanide-serieselements occurring tions associatedwith manganeseore deposits(Hut- in isomorphousallanite. The threenonequivalent oc- ton, 1938;Trask er al., 1942:'Taliaferro, 1943;Bil- tahedralM sitesin the epidotestructure vary in aver- grami, 1956).However, several studies have indicated agebond lengthand degreeof octahedraldistortion, that appropriatemajor-element composition alone, a factorimportant in understandingthe crystalchem- although important, cannot explain piemontitefor- istryand hencethe degreeof compositionalvariation mation (Ernst and Seki, 1967;Smith and Albee, and the stabilityof piemontiteand its coexistingmin- 1967). eralsin naturalparageneses. These three M sitescon- This restrictedand sporadicoccurrence of piemon- tain dominantlytrivalent with minor divalentions. tite suggeststhat factors other than temperature, Aluminumwith very minor iron or (less pressure,or whole-rockcomposition control its crys- than l0 percent) characterizesthe dimorphouszoi- tallization.Several lines of evidencefrom studiesof site-clinozoisitemineral pair; aluminum,ferric iron, naturalparageneses suggest that oxygenfugacity and and minor manganesecharacterize the ;and fluid compositionmay be critical factorsin crystalli- high manganesedistinguishes the piemontites. zationof piemontite.Until recently,no experimental Solid solution betweenCazAlgSisOrz(OH) and work had beendone to verifythe suggestedlimitation Car(Fe,Mn)sSiaOldOH)is not complete; of of piemontiteto rocksmetamorphosed under highly- the epidotegroup are rarely found with greaterthan oxidizingconditions. The presentstudy is part of an 40 percentreplacement of aluminum by ferric iron. experimentalprogram designed to investigatethe sta- The most common extent of solid solution for epi- bilityof piemontitein termsof temperature,pressure, dote is usuallycited as approximately33 percentof and oxygenfugacity, and to examinethe role of man- the pistaciteend-member. Compositional variation ganeseon crystalchemistry and stability of the epi- of natural piemontitesis plotted in termsof the ter- dote-groupminerals in general. nary Al-Fe-Mn end membersin Figurel. This dia- gram shows a generalcompositional restriction of Epidote-groupminerals: chemistry and piemontitesto greaterthan 50 percentAll(Al+Mn+ previousstudies Fe) and to lessthan 35 percentMn/(Al+Fe*Mn), a Piemontiteand associatedminerals are character- somewhatmore extensivecompositional variation ized by compositionalvariations within octahedral than is shownby the epidotes(Strens, 1966). sites in the basic formula ArMaSisOlr(OH).The It hasbeen assumed that the behaviorof Mn and seven-or eight-coordinatedA siteis usuallyoccupied Fe in the epidotestructure is essentiallythe same,at by calciumand perhapsminor Mn2+in the epidotes, least in their effectson the relationshipsbetween chemicalcomposition and propertiessuch as refrac- tive indicesand cell parameters(see discussions by cLtNozotstTE 1966; 1966).However, the {CozAl3 Si3Or2( OH) ) Seki, 1959;Myer, Strens, 3(At) poor correlationof theseproperties with Fel(Fe+Al) or (FetMn)/(Al+Mn+Fe) ratios for piemontites and the markedly different pleochroicschemes for

h,7Psr?Czs7rCorAl, F%sM.'os Siror, (OH) piemontites(even those with low Mn contents)from

(OH) co2at2Fesi3or2 (oH): Pssc267 Pm3.CzE7:Co2AlaMn Si3ora thoseof epidotessuggest that this assumptionis an oversimplification.The reddish-violet-yellow-crim- sonpleochroism is the distinguishingproperty of pie- montite and is attributedto the presenceof trivalent Mn. Until recently,the only informationon the stability of piemontitewas that obtainedfrom naturaloccur- rencesand by comparisonwith determinedPfluid-I- membersof the epidote (3Fe) t3Mn) /O, relationsof the other P ISTACITE PIEI\4ONTITE (Liou, Piemontitewas syn- (Co2Fe!Si3O,zIOH)J (Co,Mn, SLO,,(OH)) series 1973,and others). thesizedby Strens(1964) from oxide mixtures and Fig. I Extentof solidsolution (in termsof Al-Mn-Fevariations glassesof unspecifiedcomposition seeded with epi- piemontites, within threeoctahedral sites) for natural showing53 pres- publishedanalyses. Compositions of theoreticalend membersof dote at temperaturesof 550o and 650oC and epidoteminerals and bulk compositionsof concernto the present suresfrom 2.1to 4 kbar,but thecomposition of these studyare also shown. piemontitesand their stabilitieswere not determined. KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE 319

The resultsof an extensiveinvestigation of syn- conditionswithin the hematitestability field (HM, thesis,stability, and physical properties of the Mn8+- CC, CT) wasselected for stabilitystudies, as compat- bearingsilicates, including piemontite, have recently ible with evidencefrom natural piemontite occur- beenpublished (Anastasiou and Langer,1976,19771' rencesand with the trivalent stateof manganesein Langer et al., 1976).Anastasiou and Langer (1977) piemontite. A few reconnaissanceruns were per- synthesizedpiemontites with Al:Mn ratios ranging formed along the QFM buffer. Abbreviationsand from 5: I to 5'.7, and describedthe extentof solid compositionsof syntheticphases and buffer assem- solutionof Mn for Al in syntheticpiemontites and blagesused in the text and diagramsare listedin the variationsin physicaland opticalproperties for Tablel. their syntheticphases. However, their investigations Starting materialsfor syntheseswere prepared by wereconducted at veryhigh/Oz conditions(MnrO3- mixing appropriateamounts of oxidesand carbon- MnO, buffer)and at 7 and 15kbars fluid pressure. atesin the stoichiometricproportions for piemontite. The presentstudy has attempted to determinethe Thesemixtures were fired at 900"C and one atmo- fOr-T-Pfluid stability relationsfor a singlepiemon- spherefor one hour in order to break down the tite composition,CarMnAlrSisOp(OH), which is rep- carbonates.For synthesisexperiments, these mix- resentativeof a "natural" endmember of thesemin- tures were sealedwith excessHzO in AgroPdrocap- erals,Cz€?Pmr3. In order to correlatethe experimental sules,which in turn weresealed in Ag or Au outer conditionsas closely as possible with thoseexisting in capsuleswith the buffermixtures and 30 to 45 micro- naturalenvironments for piemontiteformation, we liters of water. For reversalexperiments, synthetic havechosen to conductthe experimentsat pressures piemontite and its high-temperatureequivalent as- of oneand two kbar and in a rangeof oxygenfugac- semblagewere mixed in subequalproportions as itiesreasonably close to thosepresent under natural startingmaterials. metamorphicconditions, defined by hematite-mag- All syntheticphases (except for somemetastable netite,cuprite-tenorite, and copper-cupritebuffers. assemblagesdescribed below) were extremelyfine- An iron-freecomposition was chosen for initial study grainedand in manycases poorly crystallized,mak- in orderto determinethe effectsof manganeseon the ing opticalexamination unproductive except for de- stabilityand structure of piemontitewithout the com- terminationof grosshomogeneity within the charges. plicatingeffects of anothertransition metal in the X-ray characterizationusing slow scanswith a No- system.Choice of this compositionalso facilitates relco diffractometer(CuKa radiation, Ni filter) was comparisonof theseresults with the stabilityrela- the solemeans available for phaseidentification; de- tionspreviously obtained in experimentalstudies of termination of the direction of reaction in reversal the epidote compositionCzuTPs* determined by runs wasmade by comparisonof peakheight ratios Holdaway(1972) and Liou (1973). from equivalentX-ray scansof startingmaterials and reversalrun products.The unit-celldimensions of Experimental method syntheticpiemontite and garnet were determined Synthesisand stabilityrelations for piemontiteof from diffractionscans and Guinier powder patterns the compositionCarAlrMnSi3O,r(OH) were investi- and a powderedsilicon internal standard and refined gatedhydrothermally, in standardcold-seal pressure vesselswith water as the pressuremedium. A few reconnaissanceruns were conducted using high-pres- Table l. Abbreviations and compositionsof phasesand surecold-seal apparatus and argon as the pressure componentsusedin textand diasrams medium.Pressure was measuredwith a variety of gaugescalibrated against a 30,000psi Heisegauge, piemontite end nember: ca2Mn3Si 30r 2 (0H) and valuescited are believedaccurate to within * l0 coopositlon 6tudied: ca2MnA12s13ol2 (olt) bars.Temperatures listed in the run tables(Tables 2 pistacite end Dember: ca2Fe3Si3or2 (0H) g (On) and Cz= clinozoisite endsember: CazAl SlaO I z 5), taking into accountaccuracies of the chro- galnet composition studled: Ca2UnA12Si3Or2 mel-alumelthermocouples, temperature gradients, wollastonite B-casio3 and the temperaturefluctuations imposed by the tem- anorthite CaAIzSiz0e peraturecontrollers, are believedaccurate to within fluid

+10'C. The presenceof variable-valenceelements in Hl,1 = hematite-mgnetlte buf f er I'e2O3-Fe3Or piemontiteand in the resultantbreakdown phases copper-cuprite buffer Cu-Cu20 makesit necessaryto control oxygenfugacity by the cuprite-tenorite buf f er Cu2 O-CuO -f ayallte-nagneEite buf f er S102-Fe2S10{-Ie !0q useof solidoxygen buffer techniques. A rangeof/O, 320 KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE usingthe least-squarescell-parameter refinement pro- occurred,thus indicatingat leastan approachto- gram of Applemanand Evans(1973). wards equilibrium hydrogenexchange. The con- Experimentsat such high oxygen fugacitiesmay sistencyof our experimentalresults even at low tem- involvespecial problems in ensuringthat hydrogenis peratures(for example,the conversionof garnet to able to diffuseinto the inner capsuleat a rate ade- piemontiteat 400'C with the Cu-CurObuffer and of quate to establishequilibrium /O, valueswithin the piemontiteto garnet at 250'C with the hematite- charge.As a check as to whetherequilibrium .fO, magnetitebuffer) further suggeststhat run durations conditionswere attained, a seriesof runswas made in weresufficient to lend credibilityto all but the lowest- whichvarious transition metals or theiroxides (e.g., temperature(<300'C) runs,in which the extentof Cu, CurO,CuO, FqOr) weresealed with H2Oin an reactionwas so slisht as to be undetectable. AgPd capsule,then enclosedwithin Au capsuleswith the various buffer mixtures and water and held at Descriptionof syntheticphases 450'C and 2 kbar for three weeks.For theseshort run durationsand low temperatures,extent of reac- Piemonlite tion of the inner chargeto the metal or oxide appro- Piemontitewas readily synthesizedin the temper- priate for the predicted/Oz conditionswas always ature range400o to 600oCat all pressureswith the incomplete,suggesting that low reaction and diffu- CC and CT buffers. Synthesisruns at appropriate sion ratesat temperaturesless than 450orender re- conditions(Table 2) yieldedpure piemontitefrom sults at theseconditions suspect. However, for the oxidemixtures of the bulk compositionstudied, with experimentsin this study,run durationsof 6 to l0 no detectableaccessory phases. X-ray diffractionpat- weeksat low temperaturesfor reversalsare believed ternsidentical to that of naturalpiemontite, complete to havebeen of sufficientlength so as to minimizeif recrystallizationof the charges,the specificbulk com- not eliminatethis potentialproblem. Also, all buffer position,and the deeprose-pink color of thepowdery assemblageswere X-rayed at the end of the run to chargesled to positiveidentification of the synthetic ensureboth that the bufferwas not exhaustedand productsas piemontite,A few scarce,slightly larger that someconversion of onephase to the otherhad (20 microns maximum length) piemontitegrains which showedthe characteristicpinkish-violet to yel- Table 2. Synthesis run data for piemontite (PmrrCz.r) bulk low piemontite pleochroismwere seen in several composition*excess H2O from oxide mixtures charges.These grains were clear with no visiblein- clusions.The more usual,finer-grained charges ap- S.rp1" Pfluld Temp. o,,r.-.--'--- ";,---:* condensed Run ProducLs No. (Kbar) ("C) ( oays ) pear to be optically homogeneouswith an average

H1-I CT 1.0 650 5 Pm + An + minor Gt grain sizeof lessthan 2 microns.Cell parametersfor PI_69 CT r.0 745 30 cE + Wo + An + Mn2o3 the syntheticpiemontite and for the other synthetic P7-47 CT 1.0 749 7 Pm + Wo + l{n2o3 Pl-44 CT 1.0 75L 7 Wo + An + Mn203 phasesare listedin Tables3 and 4. P1-55 CT 1.0 154 7 Wo+An+Mn2O3+Ct P1-r10 cr 1.5 723 14 Gt + wo + An * Mn203 As shownin Table 2, crystallizationof piemontite PI_54 CT 2.O 524 7Pn from the oxide mixturesat I and 2 kbar was essen- P1-16 CT 2.0 sso PL-7 CT 2.0 550 36 Pm tially completewhen CC and CT bufferswere used. PI-43 CT 2.O 569 7Pn PI_34 CT 2,O s69 At the oxygen fugacity of the hematite-magnetite P1-5 CT 2,0 600 23 Pn P1-88 CT 2.O 747 11 Coarse gr. Gt+ninor llo buffer, garnet was synthesizedunder all conditions. P1-89 CT 2,0 750 11 Coarse grained ct Theseresults are in somedisagreement with the find- P1-28 CT 6.3 570 23 Pm + GE + minor Mn2O3 P1-12 CT 6. 3 600 3I Pm + minor Gt ings of Anastasiouand Langer (1976),who found P1-9 CT 6.3 600 25 Pm that piemontiteof Al: Mn of 2 : I couldnot besynthe- P1-3 CT 7,3 600 31 Pm + mlnor Gt 7.9 610 24 Pm + minor Gt sizedat 7 kbar and at oxygenfugacities lower than P1-1 CT 8.0 550 25 Pn P1-10 CC 2.0 550 29 Pm those of the MnrOn-MnrOsbuffer, which represents P1-13 CC 7.9 57s 30 ct more oxidizingconditions than thosedefined by the PL-46 None 2.0 57r 14 ct + minor Pn P1-63 HI,t 2.O 500 25 ct CT or CC buffers.Several 6 to 8 kbar synthesesfor Pt-62 HIl 2.O 522 L2 Gt the presentstudy also yielded a mixtureof piemontite Pt-26 I{M 2.O 569 27 ct PI_15 HM 2.0 575 23 cr plus garnetat temperatureshigher than 600'C. Cell ?1-14 HI,l 2,0 600 23 ct ?I-11 Hll 2.O 600 29 cr parametersfor the piemontiteof this bulk composi- P1-8 HU 7,7 500 25 ct tion synthesizedby Anastasiouand Langer are in- P1-4 HU 1.8 500 2a cr P1-6 1II.1 7.8 600 16 ct cludedin Table3 for comparison.The averagevalues P1-20 HI'I 8.2 558 31 Gr of a :8.86(l), b : 5.69(l),c: 10.19(2),{,and p = KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE 321

Table3. Celldimensions of syntheticpiemontites [Ca,AlzMnSLO,{OH)] from presentand previousstudies

Temp. Pfl.id (A) b. (i) ..(E) volume (i3) Sample No. Buffer ". ("c) (bars) (1.01A) (r.01A) (r.o2E) (il.7i3) (18' )

P1-10 cc 550 2000 8.85 5.68 10, 20 462.L 115'35' r PI-63!4 CT 524 2000 8.86 5.70 10, 19 40c. c 115'33 | YI-)4 CT 528 2000 8.87 5.7r 10.18 464.5 115040 | PI-1 CT 550 8000 8.85 5.69 10. r-9 465.7 115'40 PL-34 CT 569 2000 8.87 5.70 LO.2L 464.3 115"48' PI-43 CT 569 2000 8.87 5.70 10.19 464.r 115"48' PL-27 CT 598 2000 8.8s 5.68 10. 20 464.4 1r5'48' Anastasiou and Mn203-MnO2 800 15000 8.839(3) 5.644(2) 10.166(4) 4s9.0 (4) 115"61(3)' Langer, L977.

ll5o42', obtainedfor the piemontitesin our study, spessartinecomponent of the syntheticgarnet would are consistentand are comparableto their synthetic decrease,with a concurrentformation of minor unde- valuesand to thoseof naturalpiemontites. tectableamounts of one or more compositionally- compensatingphases such as MnzOsor CaSiOr,at Garnet high oxygenfugacities. However, in the presentsys- For this bulk composition,the stablebreakdown tem, becauseof the simplenature of the reaction assemblageof piemontiteand the product of syn- [basicallyone phase breaking down to another thesisexperiments at temperatureshigher than 608' isochemical(except for oxidation state) singlecon- alongthe CT bufferand 575oCalong the CC bufferat densedphase] this complicationof a stepwisedecom- two kbar consistsof intermediatespessartine-grossu- positionis apparentlyavoided. As a checkfor com- lar garnetsolid solution*fluid. Along the HM buffer positional inhomogeneities,cell dimensions of curve, garnet was the only phase obtained at all syntheticgarnets crystallized over a rangeof buffer temperature-pressureconditions. Garnet-bearing conditionswere determined, and the resultsare listed chargeswere usually very fine-grained(less than 2 in Table4. A plot of cell edgesof garnetsagainst/O, microns)and whitishpink in color.At high temper- of formationis shownin Figure28. atures (750'C) some charges yielded relatively It is apparentthat the cell edge,which can be coarse-grained(50-100 microns), euhedral, wine-red closelycorrelated to compositionin garnets,varies garnetswith a typicalrefractive index of 1.7636(2)in only slightlyand randomlywith fOz of crystalliza- Na light.Essentially complete recrystallization of the chargesto homogeneousgarnet with no detectable accessoryphases suggests that the syntheticgarnet is Table 4. Cell edges of synthetic garnets for composition CazMnAlzSLOrz*fluid probablyon compositionwith the startingmixtures. Mn3+-bearinggarnets have been synthesized only at Sample Tenp. Pftuld CeII edge^(a.) ceu volse (i3) 6urrer pressureshigher than 25 kbar (Strens,1965; Ni- No. r.cj (bars) ( r.oorfi) ( tl.o [3) shizawaand Koizumi, 1975),and are rarely re- P1-90 CT 2000 tL.794 164I P1-88 CT 747 2000 11.803 t644 ported from natural rocks;hence it may be assumed Pt-66 CT 740 2000 rr.799 r642 P1-13 cc 7900 1r.817 1650 that a charge-balancedgarnet of the composition P1-6 HM 600 7500 11.804 1644 CazMnAlzSi.O,,would contain predominantly PI_I8 HM 600 2000 11.798 L642 Mn2+. P1-15 H},I 2000 11.788 1638 For garnetssynthesized over such a wide range P1-26 HM 569 2000 L7.794 1640 PL-20 Hl,1 558 8200 11.825 r654 of fO2 conditions, it might be suspectedthat the P1-61 I{M ))o 2000 LL.787 1638 'Mn8+ P1-63 HM 500 2000 rr,795 L64r Mn2+ ratioswould vary. It hasbeen observed, ?1-R1 QTM 500 2000 11.811 r648 most pertinentlyby Holdawayin his 1972study on (eee Fe-bearing epidotes, that gradual compositional Ptedlcted oaLres fot thie eonposition tqt): AsBWLng Linea teiatiq changesof reactantand product phasesalong a bebnea end nenbe?s 11.775 A 1633 breakdowncurve may yield a divariant field rather Uaing legresaion eqwtion (Noodk & Gibbs" 7977) 11.770 A 1631 than a univariantline. If this were the casehere. the 322 KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE

CELLEDGES- SYNTHETIC GARNETS

og IJ oI r2.ro A. LLI CorMnrSiaO,, J J r2.m ttl il.90 F Co.A I, Si. O', UJ (GROSSULAR) (rz il.80 (' 1r.70I ) usAr2si;q2 lt. (7" END MEMBERCOMPONENTS) .tsEASuREo CELL ED6€S sPEssAR?rNE: r.62r il._ er or GRossuLAR: tt.e5t iJ(ut"t 'reoz' CA3mnzsi3otz: t? 060i (Nishizowo,1975)

-5 B. 4 -to I

o -t5 o -o -20 o -25 !Jcrrcurareo rnom o l- | REGRESSIONEOUATION {Ndot o -30 r97t)

-35 ;-PR€orcrEo vALuE F FoR SP33GR66

| .74 il 76 11.78 11.80 11.82 11.84

GARNET CELL EDGE - {i) "p;rg.EDRcELLsEooEs Fig. 2. Plotsof cell dimensionsof syntheticgarnets Ds. (A) percenttheoretical end member componentsand (B) log/O, of formation. Fig.2A showspredicted cell edge in garnet solid solutionsassuming linear variation betweenspessartine, grossular. and CarMnzSigO,, (Nishizawa and Koizumi, 1975) end members. Measured cell edges of synthetic garnets shown by solid circles. Arrow designates predicted value for garnets of this bulk composition. Fig. 2B shows variation between measured cell edges of garnets from this study versus./O, of crystallization. Dotted lines are placed at calculated values for this bulk composition (see text). tion, with an averagevalue of 11.804(2)A.Com- higher than those predicted for grossular6? parisonof cell parametersof piemontitesynthesized spessartin%s,as shown in Table 4 and Figure 24. at variousoxygen fugacities (Table 3) similarlyshows This discrepancymay be dueto somesmall degree of no variation,with all valuesconsistent to within variationin Mn2+:Mn8+ in comparisonto natural +0.01A. The coarse-grainedgarnets crystallized at garnets,although how both chargebalance and bulk high temperatureshow slight optical zonation, sug- compositioncan be simultaneouslymaintained re- gestingthat somecompositional readjustment may mains an unresolvedproblem. The possibilitythat takeplace during the courseof a synthesisrun. somedegree of hydration(with a resultingincrease in It is of interestto comparethe measuredcell di- cell edge)might occur in the syntheticgarnets, espe- mensionsfor thesegarnets with thosepredicted either cially thosesynthesized at low temperatures,cannot by assuminglinear variationof cell edgewith grossu- be totally discounted.However, the anomalously lar-spessartinesolid solutionusing end-member val- high and consistentcell edgesfor garnetsgrown from uesfrom Deeret al. (1962),or by usingthe regression 500' to 750"C,temperatures above those at which equationgiven by Novak and Gibbs (1971)relating hydrogarnetcomponents of grossularand spessartine garnetcomposition to physicalproperties. The mea- garnetsare usually stable (Carlson, 1956; Hsu, 1968), suredvalues of the syntheticgarnets are significantly suggestthat this phenomenonis most likely not a KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE t23

significantfactor. Therefore, synthetic garnets for the tainty. The experimentalresults are listedin Table 5 reversalexperiments described in the later section and isobaric-fO"-T relationsare shownin Figures3 were preparedat temperatureshigh enoughto pre- and 4 for one and two kbar respectively.The break- cludesignificant hydration of the garnets. down reactionfor this compositionis a combination of dehydration and oxidation-reductionas shown M etastable assemblage s below: Reconnaissanceruns at one kbar initially sug- CazAlrMnSirOrz(OH)e gestedthat the phaserelations for this bulk composi- tion weresomewhat more complex. At 750"Cand the CazMnAlrSirO*t l/2 HrO + l/4 Oz definedby the CT buffer,a fairly coarse-grained /Oz As mentionedearlier, piemontite of this composition assemblageof 0-CaSiOs(wollastonite)*anorthite* MnzOs(+ fluid) crystallizesfrom the oxide mix. At Table 5. Run data for piemontitestability starting from lower fO" conditions the high-temperatureassem- mixtures. (l ) Piemontite*garnet*fluid;(2) Pm+An+Wo+ blageat one kbar is againgarnet*fluid. Anastasiou MnrOr*Gt andLanger (1977) found that braunitesolid solution, Run No. SEarting Tenp. Pghro Buffer DutaLion Results* togetherwith garnet*fluid, rather than MnrOr, oc- Mix ('c) (bars) (days)

curredas one of the breakdownproducts of piemon- P1-Rl (r) 502 2000 qFM 44 Gl P1-49 (1) 329 1000 Hl,l 2a cr tite. However, X-ray differentiationof these two Pr-41 (1) 405 1000 H}T 28 Gr P1-40 (1) 431 1000 HU 2a cr phasesin chargesof poor crystallinitycannot be de- P1-75 (1) 2r2 2000 HIl 57 No reacE. finitive. In subsequentexperiments, the three-phase P1-58 (1) 254 2000 HM 50 ct P1-45 (1) 301 2000 Hll 2a cr chargeswere reground and rerun.Under all experi- P1-48 (1) 329 2000 Hlt 28 ct P1-39 (1) 357 2000 Hl{ 33 Gt mentalconditions, garnet or piemontitewas found to P1-38 (1) 383 2000 Hil 2A GE grow at the expenseof this assemblage,indicating Pr-37 (1) 401 2000 HM 2A GE Pr-36 (1) 429 2000 HM 28 Gr that the assemblage wollastonite*anorthite* Pr-2r (1) 455 2000 HM 22 Gr P1-23 (1) 474 2000 HM 22 cr MnrO3*fluid is metastable.The metastableassem- Pr-24 (1) 502 2000 HM 22 cr blageprobably crystallized as the resultof low diffu- PL-2s (1) 530 2000 HU 22 ct Pr-81 (1) 547 2000 HIl sion rates of hydrogenthrough the capsulewalls at Pr.-91 (1) 750 2000 HM 26 Gt P1-70 (1) 275 5000 Hll 65 No react. high oxygen fugacitiesin short-durationruns. The P1-53 (1) 353 5000 HU 35 cr Wo*An*MnzOg assemblagemay form metastably P1-94 (1) 37t 1000 cc 42 Pn P1-95 (1) 388 1000 cc 42 No react. early in the run under anomalouslyhigh oxygen- P1-1r.5 (1) 388 1000 Pr-L24 (1) 392 1000 cc 12 No react. fugacity conditionswhich would result from the P1-112 (1) 399 1000 cc highly oxidizedstate of the air-fired oxide mixture. Pr-r21 (1) 406 1000 cc 33 No react. Pr-153 (1) 410 1000 cc 66 cr The buffer may later be able to imposeequilibrium Pr-t44 (1) 416 1000 cc 68 Gr P1-102 (1) 749 1000 cc 22 cr conditionson thecharge, and recrystallization to gar- P1-98 (1) 757 1000 cc net would garnets Pr-68 (1) 352 2000 cc 44 Pn begin.Such recrystallized might be P1-83 (1) 37a 2000 cc 29 Pm more coarse-grainedas a resultof the availabilityof Pr-84 (r) 396 2000 cc 29 Pn Pr-92 (r) 402 2000 cc fewer nucleationsites than werepresent in the origi- Pt-72 (1) 408 2000 cc 48 ct P1-60 (1) 411 2000 cc 24 Gr nal finely-groundoxide mixtures. Longer runs at one P1-57 (1) 448 2000 cc 15 ct kbar supportedthis suggestedsequence. Pr-52 (1) 533 2000 cc 15 Gr P1-50 (1) 544 2000 cc 14 Gt P1-51 (1) 579 2000 cc 14 Gr Reversalexperiments P1-139 (2) 593 1000 CT 1) fm P1-136 (2) 602 1000 CT 15 Gt P1-137 (2) 614 1000 CT 15 Gr Reversalexperiments were carried out by using P1-u8 (2) 635 1000 CT 15 Gr finely-groundstarting mixtures of subequalamounts P1-r35 (2) 704 1000 CT 24 Gr Pr-77 (2) 50r 2000 CT 7 Pm+Gt of piemontiteand thebreakdown phase garnet in the P1-86 (2) 505 2000 CT 39 Pn Pr-80 (2) 614 2000 CT 7 Pn+Gt presenceof excessHrO. The chargeswere held at the P1-87 (2> 614 2000 CT 36 Pm temperatureand pressureof interestfor two to four Pr-116 (1) 626 2000 CT 40 Gr P1-108 (1) 629 2000 CT 22 ct weeks at high temperaturesand from four to ten P1-109 (1) 636 2000 CT 22 Ct P1-82 (2) 652 2000 CT 2a Pm+Gt weeksat lower temperatures.For runsat low temper- P1-114 (r) 659 2000 CT 14 Gt aturesand thosevery closeto the equilibriumvalues, P1-93 (2) 661 2000 CT 42 ct 754 2000 CT 36 cr P1-88 <2) 't64 it was sometimesnecessary to regrind and rerun a P1-85 (1) 2000 CT 13 Gr

charge at identical P-T-fO, conditions before the 'et (o" Pd g?tuth at tle espense of other ple9es in the direction of reactioncould be determinedwith cer- st@tinq nittwe, 324 KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE

montite may be restrictedto below 250" at thefOz valuesdefined by theHM bufferand Pfluid :2kbar. With increasingoxygen fugacity, the dehydration temperatureincreases. The reactionwas reversed and bracketedat 617"+10'C for the CT buffer and at 404"+10'C for the CC buffer at two kbar. At one kbar, the reversedbreakdown temperatureswere 591'+ l0'C for the CT bufferand 402o +.l0'C for the CC buffer. The Pfluid-Z slope of the reaction is

REVERSALS nearlyindependent ofpressure, especially for the CC

O GARNETGRowTa buffer.This is consistentwith a calculatedchange in -4.9L0.3 O No REAcfloN volumefor this reactionof approximately O PtEuqrrrE GRowrH cm3,/mole(using values for water at 400oC,I kbar, from Burnhamet al., 1969),which, when combined with the positivechange in entropyfor a dehydration pres- TEMPERATURE.OC reaction,suggests a verticalto slightlynegative sure-temperatureslope for this piemontite break- Fig. 3. Log/Or-T diagram for the piemontitebulk composition, CarAlrMnSirOrr(OH) + excess H2O, at one kbar Pfluid. Open down reaction.The effectof pressureon dehydration circles: piemontite growth at the expense of garnet in reversal temperaturefor piemontiteof this compositionis experiments. Solid circles: garnet growth at the expense of apparentlyvery minor in comparisonto the effectof piemontite. Half-filled circles: no apparent reaction. Oxygen oxygenfugacity. buffer curves in this and subsequent figures were calculated ac- cording to data by Huebner (1969, l97l ) and Kurshakova (1971). Discussion: experimental results Temperatures for reversal experiments are believed accurate to + 10.c. Our experimentalresults confirm the evidencede- ducedfrom naturaloccurrences that oxidationstate during metamorphismrepresents a major factor in was synthesizedonly when buffers more oxidizing piemontiteformation. Comparison of theseresults than HM were used.Reversal experiments in the with the publishedstability data on pureMn-Al pie- range2000-500"C along the HM bufferwere consis- montitesdetermined by Langeret al. (1976)at 7 and tent in showingthat garnetgrew at the expenseof l5 kbar indicatesthat the stabilityfield of piemontite piemontiteeven at temperaturesas low as250oC. The is restrictedto significantlylower temperaturesfor reactionsbelow 250oCare extremelysluggish; there- lower oxygen fugacities.The considerablyhigher fore, we tentativelyconclude that the stabilityof pie- pressureat which they performedtheir studymakes comparisonof their resultsto thoseof the present and of previousepidote-stability studies difficult. P..u,o=2 KILOBARS Their preliminarywork doesconfirm the decreaseof (ra piemontitestability with decreasedfOz. They found (D -to that piemontiteswith compositionsalong the join piemontite-clinozoisite [CarAlr-oMnoSirO'r(OH)] F o -lc plus excesssilica are stableup to meltingtemper- in rangeof 890' to 940oCat 7 kbarand the l atures the r -20 valuesdefined by the MnrOr-MnOzbuffer. For z /O, U piemontitewith Mn:Al of l:2, theirwork indicated >25 X that (l) the breakdowntemperature decreases at 7

O NO REACTION (9 -30 kbar from 940'C at the fO, of the MnrO'-MnOz ,oZ o PrEiloNrrE J buffer to lessthan 900o at the fO, of the MnrOr- MnzOgbuffer; (2) forfOz belowthat of the CT buf- fer, piemontitecoexists with garnet*anorthitefor _40- (3) piemontiteis unstable too 26 300 400 500 600 700 800 900 this bulk composition;and TEMPERATURE,"C with respectto garnet+anorthiteat the fO, of the Fig. 4. LogfOz-I diagram for the piemontite bulk composition, MnrOn-MnO buffer (Langeret al., 1976). CarAlrMnSirO,r(OH)*excess HrO, at two kbars Pfluid. They extendedtheir results by comparing their KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE 325 data on piemontite to epidote-stability studies by to lower temperaturesat oxygenfugacity values(CC, Holdaway (1972),to make the observationthat the HM buffers) which would already qualify as in the Mn3+-bearingphase shows a higher temperaturesta- upper /O, range for natural conditions of meta- bility. This statement is perhaps justified by their morphism. The non-stability of piemontite along the data. However, in the presentstudy, in which experi- HM buffer curve is in agreement with evidence from mental conditions were closer to those used in the natural assemblages,in which piemontite is rarely if epidote studiesby Liou (1973)and Holdaway (1972), ever reported to occur with magnetite.The fact that it can definitelybe seenthat, for geologicallyrealistic natural intermediate Fe-Al-Mn piemontites occur physicalconditions, pure Mn:Al piemontiteis stable over a fairly wide range of metamorphic conditions only at lower temperaturesthan are the Al :Fe epi- suggeststhat increasingsubstitution of Fe3+will de- dotes(Fig. 5). creasethe sensitivityof piemontite to oxygen fugac- The wide temperature range (blueschistthrough ity, will increasethe dehydration temperature,and greenschistto amphibolite facies) over which pie- will shift the stability curve of Fe-Al-Mn piemontite montite may occur in nature requires some ex- towards that of epidote. planation, in the light of the very low stability tem- Addition of pistacite component to piemontite peratures encountered for the piemontite studied would also resultin a more complexbreakdown reac- here. Addition of pistacite component to the pure tion similar to that for epidote and once again less Mn-Al piemontite may expand the stability field to dependenton high/Or. This lesseningoffO, sensitiv- lower oxygen fugacities and to higher temperatures. ity would occur sincethe studiedpiemontite a garnet Our resultson piemontitestability are comparedwith tfluid reaction is strongly redox in nature, whereas those on epidotestability by Liou (1973)in Figure 5. in epidote breakdown ferric iron is present in both It is apparentthat the Mn-Al piemontite is restricted the reactant epidote and the products garnet*mag-

Pr.r,o= 2 KILOBARS

+5

6 E-q e ; -ro F

(9 -t5 l |! z !.1 -zo >< -25 o

J -30

35

t@ 200 300 400 500 600 700 800 900 TEMPERATURE,PC)

Fig. 5. Log/Or-Z diagram for stabilityrelations of piemontite(PmmCz",) and of epidote(Ps*C2""), at 2 kbars Pfluid. Epidote stability by Liou (1973). 326 KESKINENAND LIOU: STABILITY RELATIONS OF PIEMONTITE

netite. Breakdown for an intermediate piemontite groupsto form five-memberedrings. These rings are may involve gradual compositional changesin co- bound by octahedralcations and by Ca in two ap- existing piemontite and garnet along the breakdown proximatelytrigonal sites with seven-or eight-coordi- curve, with a resultantstepwise reaction such as that nation(Dollase, 1968). The M(3) or between-chain suggestedfor epidotesby Holdaway (1972). We are octahedralposition is significantlylarger and more presentlyconducting an experimentalexamination of distortedin its geometrythan the two chainoctahe- the stability at one and two kbar of the intermediate dral sites,M( | ) and M(2).Single-crystal X-ray refine- piemontite compositionCarAlr(Mno.uFeo.u)SirO,r(OH) ments,as well as Miissbauerand polarizedabsorp- (see Fig. I ). Preliminaryresults indicate that both the , tion spectralstudies (deCoster et al., 1963;Bancroft suggested additional complexities and extended et al., 1967;Burns and Strens,1967; Dollase, 1971, stability field appear to be present for this inter- 1973),have indicated that in epidoteand piemontite mediate piemontite composition. Fe and Mn areconfined primarily to the M(3) site,a preferredsubstitution related both to ionic sizecri- Discussion : crystal chemistry teria(Fe and Mn ionshave larger ionic radii thanAl Our experimental results are in agreementwith ions)and to a gainin crystalfield stabilization energy petrological evidence concerning the formation of due to Mn8+ions in the distortedsite. The stability piemontite. They suggestthat oxygen fugacity, in andcomposition of piemontiteor epidoteis therefore addition to temperature,pressure, and major-element highly dependentupon the typesof ions available, compositionof the host rocks, is an important factor which is in turn controlledby the oxidationstate in controlling the crystallization of piemontite in existingat the time of metamorphism.The siteenvi- preferenceto a manganoan epidote of the yellow- ronmentsfor thesetransition metals within other green variety or of another Mn-bearing phase.The phases,such as garnet, also affect intercrystalline cat- relation between high fO2 and Mns+ in the epidote ion partitioningbetween the phases, whether the rela- structure (yielding piemontite) is an obvious one; tionshipbetween them is one of coexistenceor of although many green epidotesin nature may contain progressivereplacemen t. higher Mn contents than piemontites from nearby rocks (e.g. Smith and Albee, 1967),the substitution Applications of this amount as divalent Mn for Ca is believedto The breakdownreaction determined in the present account for the nondevelopmentof the characteristic studyis admittedlygreatly oversimplified in termsof piemontite pleochroic scheme. naturalparageneses. Nevertheless, in naturaloccur- The questionof how oxidation stateaffects the site rences,spessartine-rich garnet is commonlyassoci- partitioning of transition elementsand what minerals ated with piemontite,either as a coexistingphase form as a result is a complex one for the epidote- (Cooper,l97l; Nayak, 1969)or as a breakdown group minerals. The effectsof Mn on the stability product due to progressivelyhigher temperature field of epidote must be separatedfrom those of the and/or increasinglyreducing conditions. addition of Fe to piemontite,inasmuch as it might be Experimentalwork on transition-metal-bearing assumedincorrectly that the effects upon the two geologicsystems has repeatedly indicated the impor- solid solutionswould eventuallyconverge. However, tance of oxygen fugacity as a parameterin petro- the Mn in epidote is probably predominantly diva- logicalprocesses. The occurrenceof alternatingbeds lent, the Fe and Mn in piemontite trivalent. There- of piemontite-and epidote-bearing rocks of verysim- fore, a piemontite and an epidote may be similar in ilar bulk compositionhas been described numerous their cation proportions and yet still be two distinct timesin the geologicalliterature (Mayo, 1933;Gre- species. sensand Stensrud,1977), and this seeminglyper- Crystal-chemicalcriteria have beenused to explain plexingvariation in paragenesishas beengenerally the degreeof substitution of Mn and Fe for Al in attributedto variationsin oxidation state during epidote-piemontiteminerals and the apparentexten- metamorphism.The high oxygenfugacity required sion of the compositional range of these minerals for piemontiteformation is also compatibleas an with increasinggrade of metamorphism,with an "op- explanationfor unusualcation partitioning involving timum" substitution of 33 percent (Fe*Mn) in the Fe, Mn, and Mg in rockslike the amphibolitefacies octahedralsites (Miyashiro and Seki, 1958).The epi- interlayeredpiemontite- and epidote-bearing gneisses dote structure contains chains of edge-sharingocta- describedby Smithand Albee (1967). The presence of hedra parallel to the b axis, linked by SiOoand SirOT hematiteand of high Fe3+contents in muscoviteand KESKINEN AND LIOU: STABILITY RELATIONS OF PIEMONTITE 32'l phlogopite coexisting with piemontite suggestsfor- Deer, W A., R. A. Howie and J. Zussman (1962) Rock-forming mation under highly oxidizing conditions. Adjacent Minerals, Vol. 1. Wiley, New York. (1968) comparison of the struc- epidote-bearinglayers contain more common Fe2+- Dollase, W. A. Refinementand tures of zoisiteand clinozoisite.Am. Mineral., J.l, 1882-1898. rich assemblagesfor amphibolite facies gneisses: - (1969)Crystal structureand cation ordering ofpiemontite biotite, ferrous-iron-rich amphibole, hematite, and A m. M ineral . 54. 7l0-7l'l . garnetswith significantalmandine contents.The ele- - (1971) Refinementof the crystal structuresof epidote, al- mental compositionsof thesegneisses, except for va- lanite, and hancockite.Am Mineral., 56,44'l-464. - (1973) iron distribution in the epi- lence states,are quite similar despite mineralogical Miissbauerspectra and dote group minerals.Z. Kristallogr., 138,4l-63. differences;hence the explanation for thesetwo dif- Ernst, W. G. and Y. Seki (1967) Petrologic comparison of the ferent paragenesesrequires variation of fO, between Franciscan and Sanbagawametamorphic terranes. Tectonophys' the layers at the time of metamorphism, an inter- ics. 4. 463-478. pretation which this experimentalstudy supports. Gresens, R. L. and H. L Stensrud (1977) More data on red muscovite. Am. Mineral., 62, 1245-51. Acknowledgments Holdaway, M. J (19'12)Thermal stability of Al-Fe epidotes as a Thisresearch was conducted under funding provided by NSF function of fO, and Fe content. Conlrib Mineral. Petrol , 37, grantEAR73-06520-A02/Liou. In addition, the first author was 307-340. Al- supported during part of this research by an NSF Craduate Fel- Hsu,L. C. (1968)Selected phase relationships in the system model for garnetequilibria. J, Petrol lowship. Dr Philip M. Fenn, Peter Schiffman,and PeterGordon Mn-Fe-Si-O-H: a ,9,40- are warmly thanked for invaluablemoral and logisticalsupport, 83 (1969) of rhodochrositein the technical assistance,and the proverbial helpful discussions.For Huebner, J. S. Stability relations 54, 457-481. above-the-call-of-dutymaintenance of the all-importantX-ray dif- system manganese-carbon-oxygen Am. Mineral., - (1971) systems at ele- fraction equipment as well as valuable consultationsconcerning Buffering techniques for hydrostatic Techniques collectionand reductionofthe X-ray data cited in this paper, Dr. vated pressures.In G. C. Ulmer, Ed., Research for p Mark Taylor and Keith Keefer are gratefully acknowledged We High Pressure and High Temperature, 123-177. Springer-Ver- also thank Drs. W G. Ernst. L. C Hsu. P. M. Fenn. and D. M. lag, New York. (1938) withamite from Glen Coe, Burt for critical readingsof the manuscript and for their helpful Hutton, C. O On the nature of comments. Scotland. M ineral. Mag., 25, ll9-124. Keskinen, M (1979) Experimental Inuestigation of lhe St.ability References Relations, Crystal Chemistry, and Field Occurrences of the Man' ganeseEpidote, Piemontite Ph.D. Thesis, Stanford University. Anastasiou,P. and K. Langer (1976)Synthese und Stabilitet von (1971 Stabilityfield ofhedenbergiteon the log Piemontit, CarAl, Fortschr Min- Kurshakova, L. D. ) oMno(Si,O"/SiOo/O/OH). -l eral..54.3-4 P6 diagram. Geochem.Int., 8, 340-349. P. Anastasiouand I Abs-Wurmbach (1976)Synthesis, - and - (1977) Synthesisand physical properties of Langer, K., stability, and physical properties of Mn3+-bearing silicateminer- piemon ti te, Ca,A lls- o,M n$+(SirO 7 / SiO,/ O / OH). Contib. M in- Congr Sydney,1976, Abstr. Vol. 2, 578-579. eral. Perrot .60. 225-245 als 25th Inr. Geol. , (1973) Synthesisand stability relations of epidote, Appleman,D E. and H Evans(1973) Job 9214:Indexing and Liou, J. G. Petrol., 14, 381-413. least-squaresrefinement ofpowder diffraction data. Natl Tech Ca,AlzFeSi.O,,(OH) "/. Makanjuola, A. A. and R. A. Howie (1972)The mineralogyof the Inf. Seru , U.S. Dep Commerce, Springfield, Virginia, Document rocks from lle de Groix, PB-2t6 188. glaucophaneschists and associated Mineral Petrol., 35, 83-l 18. Bancroft, G. M., A G Maddock and R. G. Burns (1967)Appli- Brittany, F rance. Contrib. P (1959)The piedmon- cations of the Mdssbauer effect to silicate mineralogy-I. Iron Marmo, V., K. J. Neuvonen and Ojanpere (Italy), (lndia), and Marampa silicatesof known crystal structure. Geochim. Cosmochim Acta, tites of Piedmont Kajlindongri 31,2219-2246. (Sierra Leone). Bull Comm. Geol. Finlande, 184,ll-20. (1933)Discovery of piedmontitein the SierraNevada. Bilgrami, S. A. (1956) Manganesesilicate minerals from Chikla, Mayo, E. B. Rep., 29, 239-243. Bhandara District, lndia. Mineral. Mag., 31,236-244. Cal. Diu. Mines and Geol: and Y. Seki (1958)Enlargement of the composition Boettcher,A L. (1970)The systemCaO-ALOg-SiOz-HzO at high Miyashiro, A. piemontite with rising temperalurc. Am. J. pressuresand temperatures. J. Petrol., I I, 337-379. field of epidote and Burnham, C. W., J. R. Holloway and N. F. Davis (1969) The Sci..256.423-430. H. (1966)New data on zoisiteand epidote.Am. J Sci., specific volume of water in the range 1000 to 8900 bars, 20o to Myer, G. 900". Am. J. Sci..267A.70-95. 264,364-38s. Nayak, V. K (1969)Piemontite from the manganeseore depositof Burns,R. G. and R G. J. Strens(1967) Structural interpretation of Kajfindongri Mine, Jhabua District, Madhya Pradesh.Ind. Min- polarized absorption spectra of the Al-Fe-Mn-Cr epidotes. M ineral. Mag., 36, 204-226 eral.. 10. 174-180. (1975) and infrared Carlson, E. T. (1956) Hydrogarnet formation in the systemlime- Nishizawa, H. and M. Koizumi Synthesis alumina-silica-waIer. J Res.Nall Bur. Stand.. 56. 327-335. spectraof Ca3Mn2SirO',and CdTB,SLO'dB:Al,Ga,Cr,V,Fe,Mn) Cooper, A. F. (1971) Piemontite schistsfrom Haast River, New garnets. Am. Mineral , 60, 84-87. Zealand. Mineral. Mag., 38, 64-71. Novak, G. A and G. V. Gibbs (1971)The crystalchemistry of the deCoster, M., H. Pollak and S. Amelinckx (1963) A study of silicate garnets. Am. Mineral., 56, 791-825 Mdssbauer absorption in iron silicates. Phys. Status Solitli, 3, Seki, Y. ( 1959)Relation betweenchemical composition and lattice 283-288. constants of epidote. Am. Mineral., 44, 720-730. 328 KESKINEN AND LIOU: STABILITY RELATIONS OF PIEMONTITE

Simonson,R. R. (1935)Piedmontite from Los AngelesCounty, Taliaferro,N. L. (1943)Manganese deposits of the SierraNevada, California.Am. Mineral.,20,737-8. their genesisand metamorphism.Cal. Diu. Mines Bull , 125, Smith, D. and A. L. Albee (1967)Petrology of a piemontite- 277-333. bearinggneiss, San Gorgonio Pass, California . Contrib.Mineral. Taylor, F. C. and A. J. Baer( 1973)Piemontite-bearing explosion Petrol.,16,189-203. brecciain Archeanrocks, Labrador, Newfoundland. Can. J Stensrud,H. L. (1973)Does piemontite represent only greenschist Earth Sci.,10, 1397-1402. faciesmetamorphism? Contrib. Mineral. Petol.,40,'19-82. Trask, P. D., I. F. Wilson and F. S. Simons(1942) Manganese Strens,R. C. J. (1964)Synthesis and propertiesof piemontite. depositsof California:a summaryreport. Cal. Diu. MinesBull., Narure,201, l'15-176. 125,5l-215. - (1965)Instability of thegarnet CagMnzSiaO,z and thesub- stitution Mn+3c Al. Mineral.Mag., 35, 547-550. - (1966) Propertiesof the Al-Fe-Mn epidotes.Mineral. Manuscriptreceiued, April 7, 1978; Mag., 35,928-944. acceptedfor publication,September 11, 1978.