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An Empirical Phase Diagram for the Clinozoisite-Zoisite Transformation In

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An Empirical Phase Diagram for the Clinozoisite-Zoisite Transformation In American Mineralogist, Volume 77, pages 631442, 1992 An empirical phasediagram for the clinozoisite-zoisitetransformation in the system CarAlrSirOrr(OH){arAlrFe3 +Si3Or r(OH) Gnnnlno FuNz FachgebietPetrologie, Technische Universitiit Berlin, StraBe des 17Juni 135,D-1000 Berlin 12,Germany JaNr Srr,vERSToNE Departmentof Earthand PlanetarySciences, Harvard University, 20 OxfordStreet, Cambridge, Massachusetts 02138, U.S.A. Ansrnlcr The effects of pressure,temperature, and Fe3* + Al exchangeon the transformation from orthorhombic zoisite to clinozoisite are examined using thermodynamic data and analysesofnaturally coexisting mineral pairs. Theoretical considerationsindicate a steep transformation loop in the T-X sections, with the Al end-member reaction occurring at low temperature and the Fe end-member reaction at high temperature;the orthorhombic phaseis the high-temperatureform. The position of the two-phasezoisite-clinozoisite area in a Z-Xsection varies with pressure.Equilibrium mineral pairs from low- to intermediate- pressureregimes indicate a smaller two-phase area (betweenapproximately 8-15 molo/o AlrFe in zoisite and 25-40 molo/oin clinozoisite) than those from high-pressureregimes (samevariation in zoisite, 35-55 molo/oin clinozoisite). The width of the two-phaseregion also dependson minor elements,such as the rare earth elements.The effect on the phase diagram of a possible low-temperature miscibility gap in the monoclinic series is also discussed.Disequilibrium textures, such as complex zoning patterns and multiple gener- ations of grains, are common features in naturally coexisting clinozoisite-zoisite pairs. Owing to the sluggishreaction behavior of the epidote minerals, these can serve as im- portant petrogeneticindicators of prepeakor relict metamorphic conditions in a wide range of bulk compositions. INrnolucrroN ing temperature,both the monoclinic and the orthorhom- forms become enriched in Fe3*' However' Acker- The epidote mineral group, Iike the pyfoxenesand feld- bic a1d Raase (1973) observed that clinozoisite and spars, exhibits borh chemical and structural ;;i;;r. ry1d prograde core to rim zoning toward Fe- The principal isomorphous substitution Al * FJil;;- zoisite showed poor Prunier and Hewitt (1985) presented tinuous over a wide range of compositions unA op"*t"t compositions' evidence that at high temperaturesboth inbothorthorhombicandmonocliniccrystals.Th;il;: experimental forms are-relatively Fe rich and thus verified Enami and tural transformation from the orthorhombi;i; ,h" Banno's hypothesis' Comparing the loop proposed by monoclinic form thus potentially depends o" pr"rru.L, Prunier and Hewitt (1985) with the data of Enami and temperature, and chemical composition. This ;;i;;- (1980) still leaves considerableuncertaintv about mation is petrologically significant in a variery ;i;;;;- Banno the exact position in P-?"spaceof the two-phaseloop and morphic environments: zoisite is part or tn" rtigi-p."r- its form in T-X and P-x projections' The aim of this sure assemblagethat replaceslawsonite una u"o.tiltJu"i is 10 present new data on coexisting zoisite and is common in many eclogites;epidote -i"".ufr'u." study "U+ clinozoisite mineral pairs in order to construct an emplr- uitous in mafic rocks throughout the blueschisi, fi";- ical phase diagram that stressesthe important influence schist, and epidote amphibolite facies; and ,"iri;;;; ofpressure on the phasetransformation' clinozoisite both occur in calc-silicate rocks rro* u "u- riety of P-Z environments. Despite its petrologic impor- tance. however. the effects of structural and chemical IHEORETTCALCONSTDERATTONS variationson reactionsinvolving epidotegroup minerals This investigation is restricted to the system are not well understood. CarAlrSirO,r(OH)-CarAlrFe3'Si3Orr(OH);the influenceof The controversy concerning the zoisite-clinozoisite other potentially important substitutions involving Mn, phasediagram can be summarized as follows. Enami and Cr, and rare earth elements is only briefly discussed.In Banno (1980) analyzed coexisting zoisite-clinozoisite addition, we restrict it to the Al-rich part of the system, mineral pairs from rocks of different metamorphic grade. where only up to one-third of the total Al is replacedby They arranged the analysesby increasing metamorphic p"rt (and we assumethat Fe2*is not important). Higher grade (i.e., temperature)and suggestedthat, with increas- Fe contents are only rarely reported in the literature (e.g., ooo3404x/92l05064631$02.00 631 632 FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION Al2Al rather low critical temperature or both. Their possible 0 influence is discussedin a subsequentsection. The transformation from monoclinic to orthorhombic forms can be described in terms of two end-member re- actrons: mcl CarAl.SijO,r(OH) : orh CarAlrSi3O,r(OH) (l) mcl CarAlrFeSi3Orr(OH) : orh CarAlrFeSirO,r(OH). (2) The structuresof the orthorhombic and monoclinic forms are similar. They consist of SiOoand SirO, groups linked Fig. l. Nomenclatureof the epidotegroup minerals in the by octahedral chains occupied by Al and Fe3*.The two system CarAl3Si30r r(OH)-CarAlrFeSirO,r(OH).Orthorhombic forms are related to each other by a glide-twin operation, zoisiteincludes the Fe-poorvariety zoisitewith orientationof B but the types of octahedral chains in the two forms are the optic axial planeparallel to (010)and the Fe-richvariety a zoisitewith the optic axial planeparallel to (100).Monoclinic essentiallydifferent: in zoisite only one type ofchain oc- zoisiteincludes clinozoisite proper, which is opticallypositive, curs, whereasin clinozoisite there are two types of chains epidoteproper (optically negative), and the hypotheticalcom- (Dollase, 1968). The transformation can also be de- positionpistacite (not shown),CarFerSirO,r(OH). Because of the scribedas cation shuffieor synchroshear(Ray et al., 1986) highoptic axialangle dispersion ofthe epidoreminerals and the with a rearrangementof (Al,Fe) and Ca on the M3 site. strongeffect of minor elementson opticalproperties, the exact Such a transformation is probably of first order with a positionof the curvesvaries (dashed lines, diagram modified two-phase region in a T-X or P-X section. It cannot be afterTrdger, 1982; Maaskant, 1985). describedas a miscibility gap,which would be a function only of cation ordering. In the epidotes, ordering of oc- tahedrally coordinated Al and Fe3*is almost complete in Bird et al., 1988)and appearto be confinedto low-tem- zoisite(Fe3+ in the A13 site;Ghose and Tsang,l97l) and perature environments; thesemonoclinic high-Fe epidote lessperfect in clinozoisite (Fe3*in the M3 site, with small sampleshave not been reported coexistingwith an ortho- amounts in the Ml site; Dollase, l97l). rhombic member of the epidote seriesand henceare not Both Enami and Banno(1980) and Prunierand Hewitt relevant to the problem addressedhere. (1985)argued that pressurehas no influenceon the trans- We use the following nomenclature, which is basedon formation loop, owing to the very small differencein mo- optical properties and summarized in Figure l. Zoisite is lar volumes of the orthorhombic and monoclinic forms the orthorhombic form. The optical varieties with differ- (V"",-- 135.9cm3lmol, V^": 136.2cm3lmol, Helgeson ent optic axial orientations and a changefrom positive to et al., 1978; V,"t : 135.58, V"t": 136.73,Holland and negative character (see Myer, 1966; Vogel and Bahezre, Powell,1990; V*,: 135.88,V*:136.76, Berman,1988). 1965;Maaskant, 1985),which is stronglydependent on However, the P-T slope of a reaction depends on both Fe content, are consideredas one phase,since there is no AV and,AS, and since the entropy values for the epidote indication that thesechemical (and optical) variations are minerals are also similar (S,", : 296.1 J/K.mol, S.o : discontinuous or that they are related to structural dis- 295.1 J/K.mol, Helgesonet al., 1978;S,.1 : 295, S.,, : continuities. There is also no clear indication of structur- 291, Holland and Powell, 1990; S,", : 297.58,5.,": ally different types in the monoclinic series(Deer et al., 287.08,Berman, 1988; &' :296.58,,S"k: 294.06,Gott- 1986).A continuousrange of compositionsfrom low to schalk, 1990), it is possible that pressureexerts a signifi- high Fe contentsexists at certain P-?"conditions, and the cant effect on the position and shape of the two-phase monoclinic seriescan also be representedby one phase. region. The reported molar-volume value with the small- We use the term "clinozoisite" throughout this paper to est error in determination of the lattice constantsfor syn- refer to monoclinic forms and reserve "epidote" for the thetic orthozoisitewas given by I-angerand Lattard (1980) whole mineral group; "pistacite," which is also a term as 135.77cm3/mol, which is in good agreementwith the usedfor Fe-rich epidote, may be usedfor the hypothetical data from the above mentioned data sets. No reliable end-memberCarFerSirO,r(OH). The Fe-freemonoclinic data for synthetic Fe-free clinozoisite are known. Pisto- end-member is known from experimental studies (e.g., rius (1961) reportedidentical values for zoisite and cli- Jenkinset al., 1985).Several studies on natural samples nozoisite(within the limits of error) of 136.4+ 0.7 cm3/ suggestthat miscibility gapsoccur within the monoclinic mol. Extrapolation of data for Fe-bearing clinozoisites series (Hietanen, 1974; Raith, 1976; Selverstoneand presentedby Myer (1966),Bird and Helgeson(1980), and Spear, 1985).These proposed miscibility gapsare located Seki
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