American Mineralogist, Volume 77, pages 631442, 1992

An empirical phase diagram for the clinozoisite-zoisite transformation in the systemCarAlrSirOr r(OH){arAlrFe3 + Si3Or r(OH)

Gnnnlno FuNzFachgebiet Petrologie, Technische Universitiit Berlin, StraBe des 17 Juni 135, D-1000 Berlin 12, Germany

JaNr Srr,vERSToNEDepartment of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, U.S.A.

Ansrnlcr

The effects of pressure, temperature, and Fe3* + Al exchange on the transformationfrom orthorhombic zoisite to clinozoisite are examined using thermodynamic data andanalyses ofnaturally coexisting mineral pairs. Theoretical considerations indicate a steeptransformation loop in the T-X sections, with the Al end-member reaction occurring atlow temperature and the Fe end-member reaction at high temperature; the orthorhombicphase is the high-temperature form. The position of the two-phase zoisite-clinozoisite areain a Z-Xsection varies with pressure. Equilibrium mineral pairs from low- to intermediate-pressure regimes indicate a smaller two-phase area (between approximately 8-15 molo/oAlrFe in zoisite and 25-40 molo/o in clinozoisite) than those from high-pressure regimes(same variation in zoisite, 35-55 molo/o in clinozoisite). The width of the two-phase regionalso depends on 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 alsodiscussed. 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 sluggish reaction behavior of the epidote minerals, these can serve as im-portant petrogenetic indicators of prepeak or relict metamorphic conditions in a wide rangeof bulk compositions.

INrnolucrroN ing temperature, both the monoclinic and the orthorhom-

The epidote mineral group, Iike the pyfoxenes and feld- bic forms become enriched in Fe3*' However' Acker-

spars, exhibits borh chemical and structural ;;i;;r. ry1d a1d Raase (1973) observed that clinozoisite and

The principal isomorphous substitution Al * FJil;;- zoisite showed prograde core to rim zoning toward Fe-

tinuous over a wide range of compositions unA op"*t"t poor compositions' Prunier and Hewitt (1985) presented

inbothorthorhombicandmonocliniccrystals.Th;il;: experimental evidence that at high temperatures both

tural transformation from the orthorhombi;i; ,h" forms are- relatively Fe rich and thus verified Enami and

monoclinic form thus potentially depends o" pr"rru.L, Banno's hypothesis' Comparing the loop proposed by

temperature, and chemical composition. This ;;i;;- Prunier and Hewitt (1985) with the data of Enami and

mation is petrologically significant in a variery ;i;;;;- Banno (1980) still leaves considerable uncertaintv about

morphic environments: zoisite is part or tn" rtigi-p."r- the exact position in P-?"space of the two-phase loop and

sure assemblage that replaces lawsonite una u"o.tiltJu"i its form in T-X and P-x projections' The aim of this

is common in many eclogites; epidote -i"".ufr'u." "U+

study is 10 present new data on coexisting zoisite and

uitous in mafic rocks throughout the blueschisi, fi";- clinozoisite mineral pairs in order to construct an emplr-

schist, and epidote amphibolite facies; and ,"iri;;;; ical phase diagram that stresses the important influence

clinozoisite both occur in calc-silicate rocks rro* u "u-

ofpressure on the phase transformation'

riety of P-Z environments. Despite its petrologic impor-tance. however. the effects of structural and chemical IHEORETTCAL CONSTDERATTONS

variations on reactions involving epidote group minerals This investigation is restricted to the systemare not well understood. CarAlrSirO,r(OH)-CarAlrFe3'Si3Orr(OH); the influence of

The controversy concerning the zoisite-clinozoisite other potentially important substitutions involving Mn,phase diagram can be summarized as follows. Enami and Cr, and rare earth elements is only briefly discussed. InBanno (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 replaced byThey arranged the analyses by increasing metamorphic p"rt (and we assume that Fe2* is not important). Highergrade (i.e., temperature) and suggested that, with increas- Fe contents are only rarely reported in the literature (e.g.,

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632

A l 2 A l0

Fig. l. Nomenclature of the epidote group minerals in thesystem CarAl3Si30r r(OH)-CarAlrFeSirO,r(OH). Orthorhombiczoisite includes the Fe-poor variety B zoisite with orientation ofthe optic axial plane parallel to (010) and the Fe-rich variety azoisite with the optic axial plane parallel to (100). Monocliniczoisite includes clinozoisite proper, which is optically positive,epidote proper (optically negative), and the hypothetical com-position pistacite (not shown), CarFerSirO,r(OH). Because of thehigh optic axial angle dispersion ofthe epidore minerals and thestrong effect of minor elements on optical properties, the exactposition of the curves varies (dashed lines, diagram modifiedafter Trdger, 1982; Maaskant, 1985).

Bird et al., 1988) and appear to be confined to low-tem-perature environments; these monoclinic high-Fe epidotesamples have not been reported coexisting with an ortho-rhombic member of the epidote series and hence are notrelevant to the problem addressed here.

We use the following nomenclature, which is based onoptical properties and summarized in Figure l. Zoisite isthe orthorhombic form. The optical varieties with differ-ent optic axial orientations and a change from positive tonegative character (see Myer, 1966; Vogel and Bahezre,1965; Maaskant, 1985), which is strongly dependent onFe content, are considered as one phase, since there is noindication that these chemical (and optical) variations arediscontinuous or that they are related to structural dis-continuities. There is also no clear indication of structur-ally different types in the monoclinic series (Deer et al.,1986). A continuous range of compositions from low tohigh Fe contents exists at certain P-?"conditions, and themonoclinic series can also be represented by one phase.We use the term "clinozoisite" throughout this paper torefer to monoclinic forms and reserve "epidote" for thewhole mineral group; "pistacite," which is also a termused for Fe-rich epidote, may be used for the hypotheticalend-member CarFerSirO,r(OH). The Fe-free monoclinicend-member is known from experimental studies (e.g.,Jenkins et al., 1985). Several studies on natural samplessuggest that miscibility gaps occur within the monoclinicseries (Hietanen, 1974; Raith, 1976; Selverstone andSpear, 1985). These proposed miscibility gaps are locatedat relatively high Fe contents of the system or have a

FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION

rather low critical temperature or both. Their possibleinfluence is discussed in a subsequent section.

The transformation from monoclinic to orthorhombicforms 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 structures of the orthorhombic and monoclinic formsare similar. They consist of SiOo and SirO, groups linkedby octahedral chains occupied by Al and Fe3*. The twoforms are related to each other by a glide-twin operation,but the types of octahedral chains in the two forms areessentially different: in zoisite only one type ofchain oc-curs, whereas in clinozoisite there are two types of chains(Dollase, 1968). The transformation can also be de-scribed as cation shuffie or synchroshear (Ray et al., 1986)with a rearrangement of (Al,Fe) and Ca on the M3 site.Such a transformation is probably of first order with atwo-phase region in a T-X or P-X section. It cannot bedescribed as a miscibility gap, which would be a functiononly of cation ordering. In the epidotes, ordering of oc-tahedrally coordinated Al and Fe3* is almost complete inzoisite (Fe3+ in the A13 site; Ghose and Tsang, l97l) andless perfect in clinozoisite (Fe3* in the M3 site, with smallamounts in the Ml site; Dollase, l97l).

Both Enami and Banno (1980) and Prunier and Hewitt(1985) argued that pressure has no influence on the trans-formation loop, owing to the very small difference in mo-lar volumes of the orthorhombic and monoclinic forms(V"",-- 135.9 cm3lmol, V^": 136.2 cm3lmol, Helgesonet al., 1978; V,"t : 135.58, V"t" : 136.73, Holland andPowell, 1990; V*,: 135.88, V*:136.76, Berman, 1988).However, the P-T slope of a reaction depends on bothAV and, AS, and since the entropy values for the epidoteminerals are also similar (S,", : 296.1 J/K.mol, S.o :295.1 J/K.mol, Helgeson et al., 1978; S,.1 : 295, S.,, :291, Holland and Powell, 1990; S,", : 297.58, 5.,":287.08, Berman, 1988; &' :296.58,,S"k: 294.06, Gott-schalk, 1990), it is possible that pressure exerts a signifi-cant effect on the position and shape of the two-phaseregion. The reported molar-volume value with the small-est error in determination of the lattice constants for syn-thetic orthozoisite was given by I-anger and Lattard (1980)as 135.77 cm3/mol, which is in good agreement with thedata from the above mentioned data sets. No reliabledata for synthetic Fe-free clinozoisite are known. Pisto-rius (1961) reported identical values for zoisite and cli-nozoisite (within the limits of error) of 136.4 + 0.7 cm3/mol. Extrapolation of data for Fe-bearing clinozoisitespresented by Myer (1966), Bird and Helgeson (1980), andSeki (1959) yields a value of 136.2 cm3/mol. Linearlyextrapolated data from H0rmann and Raith (1971) in-

FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION 633

dicate a slightly higher value of 136.5, but they suggestthat a linear extrapolation is probably not correct, andtheir given line would extrapolate toward a value of 137.5cm3/mol.

Reaction I has a negative slope in P-Zspace, althoughthere is a large range in the magnitude of the slope ob-tained with different data sets (-124 bars/"C, Berman,1988; - 13.3 bars/"C, Helgeson et al., 1978; -34.8 bars/oC, Holland and Powell, 1990). If an average value of-50 bars/'C is accepted, a pressure increase of 2 GPawould shift the equilibrium temperature for Reaction Iby 400 'C. No thermodynamic data for Reaction 2 areavailable. If we assume that the change in entropy arisingfrom the exchange of Fe3* for Al is essentially the sameregardless of whether the exchange occurs in an ortho-rhombic or a monoclinic crystal, the P-T slope of Reac-tion 2 depends only on the molar volumes and the entro-py contribution from order-disorder phenomena. Bird andHelgeson (1980) compiled data on lattice constants andcalculated molar volumes. In their Figure 2 it can be seenthat, with increasing Fe3+ contents, the difference in themolar volumes of the orthorhombic and monoclinic formsmay get smaller.

Ifthere is significant order-disorder ofAl and Fe in theepidotes, it is likely that the degree ofdisorder in clino-zoisite is higher than in zoisite (Dollase, 197 l; Ghose andTsang, 197 l), which results in a decrease in AS for Re-action 2. Bird and Helgeson (1980) give an entropy dif-ference for ordered-disordered epidote of 0.3 J/K.mol.Changing the value of AS and a decreasing A Z will havea large effect on the slope of the reaction, but unfortu-nately uncertainties in the available entropy and volumedata and lack ofdata concerning their sensitivity to sub-stitution and order-disorder phenomena do not allow usto distinguish with confidence between a positive andnegative slope for Reaction 2.

For the construction of a schematic P-T-X diagram, itis also necessary to have some information about thetemperature range in which Reactions I and 2 are locat-ed. Jenkins et al. (1985) determined Reaction I experi-mentally and located it at temperatures below 350'C;calculations with the data of Berman (1988) yield a tem-perature near 200'C at I bar. Despite the uncertaintiesin the absolute temperatures, there is no doubt that theorthorhombic form is the high-temperature form. Reac-tion 2 is probably metastable because it must lie at tem-peratures well above 600'C. This was demonstrated ex-perimentally by Holdaway (1972) and empirically by thecommon occurrence of monoclinic epidote near its upperstability limit in amphibolite facies rocks.

Figure 2 shows I-X sections for Reactions I and 2,with the assumption of both positive and negative slopesfor Reaction 2 and extreme temperature differences be-tween the two reactions.ln a T-X diagram the limbs ofthe two-phase region are very steep, and their positionchanges with pressure and the changing slope ofReaction2.If anegative slope for Reaction I is correct, it is clearthat pressure has a large effect on the composition ofco-

X n,f" Xotf"

Fig.2. Schematic T-X phase diagram for the transformationfrom clinozoisite to zoisite. (A) The two possibilities of either apositive (+) or a negative (-) P-7 slope for the AlrFe end-mem-ber Reaction 2 for two isobaric sections at low pressure Pl andhigh pressure P2. (B) An enlarged part of A, showing the possiblezoning trends of coexisting minerals (squares : zoisite, dia-monds : clinozoisite) during prograde and retrograde develop-ment (paths a-d, see text). The range of compositions of theorthorhombic and the monoclinic types during such a cycle mayoverlap, and Fe-poor clinozoisite may therefore coexist with rel-atively Fe-rich zoisite in complexly zoned minerals.

existing phases. With the assumption that the model witha positive slope for Reaction 2 is correct, coexisting zois-ite and clinozoisite would exhibit different types of zon-ing, depending upon the P-f path they had followed dur-ing metamorphism: a simple prograde path withsimultaneous burial and heating would result in increas-ing Fe contents from core to rim ofboth phases (path ain Fig. 2B), whereas isothermal uplift (path b) and iso-baric cooling (path c) would result in decreasing Fe con-tents; heating during uplift (path d) would also cause arimward decrease in Fe content of both minerals. If Re-action 2 has instead a negative P-Z slope, the two-phaseregion is shifted with increasing pressure toward lowertemperatures. Qualitatively, the change in compositionof coexisting pairs would be the same for the P-I pathsdiscussed for Figure 2B, but the change would be muchmore pronounced for both minerals, in contrast to thesmall changes in zoisite composition illustrated in Figure2B'

NlTrlRAll.y coExrsrrNc zolsrrE{LrNozorsrrE PArRs

In order to constrain the position ofthe two-phase re-gion, we analyzed coexisting minerals from a variety ofP-Tregimes. This approach has some inherent problems.The first problem is that epidotes, especially Fe-rich cli-nozoisite, can preserve extremely complicated zoningpatterns, which show up clearly in backscattered electronimages and indicate slow diffusion rates at metamorphictemperatures. In theory, slow Fe3*-Al diffusion shouldmake the epidote minerals suitable for an empirical studyof the two-phase region because their sluggish reactionbehavior inhibits retrograde reequilibration. In practice,however, epidotes can form over a wide range of P-T

o

or l ' o

Fe2O3

E N13-3

O RM71. NR125+ RWl03r FT1Atr DTBD^ 85.1

634 FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION

o 51 0

o 2

I

R o r

c 0 2

;e

oP O

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O 6.c

5

=

0 1 2 3 4 5 6 7

wt % in zoisiteFig. 3. Distribution of FerO, between clinozoisite and zoisite

(only analyses ofgrains in contact with one another are plotted);clinozoisite is strongly enriched in FerOr.

conditions between the lower greenschist facies and theeclogite facies, which at times makes it difficult to deter-mine if coexisting zoisite and clinozoisite grew during thesame event. We selected grains that appeared to belongto the same fabric generation and worked with backscat-tered electron images to analyze only grains in mutualcontact and with no evidence for late-stage growth orreequilibration.

Uncertainties in the microprobe analyses give rise toan error on the order of + 1 molo/o AlrFe. Assuming anerror of 0. I absolute wto/o for an Fe-bearing zoisite with1.75 wto/o FerO, results in +0.6 molo/o AlrFe. An error of1.25 relative wt0/o in SiO, has no influence on the calcu-lated AlrFe component, whereas an error of 1.25 relativewto/o in AlrO. results in +0.3 molo/o AlrFe. The presenceof minor elements such as Cr or Mn results in only aslight overestimation of the AlrFe component. In most ofthe analyses the sum ofminor octahedral cations per for-mula unit is below 0.05, which has little influnce on themolo/oAlrFe,calculatedas 100[Fe,.,/(-2 + Al ., + Fe,",)].

All analyses were carried out with automated Camebaxelectron microprobes at Harvard University and theTechnische Universitiit Berlin, with simultaneous ener-gy-dispersive analysis to check for minor elements, andwith backscattered electron images to avoid mixed anal-yses on adjacent grains or on different zones within thegrains. Despite different standards and correction pro-grams (Harvard-natural minerals, Bence-Albee coffec-tion; TUB-natural minerals and synthetic metals, PAPcorrection), analyses on the same materials are identicalin the range mentioned above.

Distinction between orthorhombic and monocliniccrystals was made in thin section by determination of theextinction angle in grain sections showing the a-c plane.

E o

Cr2Og..:"

o l 0 2

wl % in zoisiteiri rrn roi.ilj n %in zoisite

Fig. 4. Distribution of the minor components TiO', MnO,MgO, and CrrO, between clinozoisite and zoisite (same data setas Fig. 3; overlapping analyses not shown). The l:l slope line isgiven as a reference line; for TiO, a reference line with a slopeof 2.5:I is also shown.

In most samples, other optical features, such as anoma-lous interference colors, crystal shape and size, and inclu-sion patterns, were also helpful in distinguishing betweenzoisite and clinozoisite. Additional transmission electronmicroscopy was done on two samples; a detailed reporton this work is in preparation (Smelik and Franz).

Compositions of coexisting phases

The FerOr, TiOr, MnO, MgO, and CrrO, contents ofall coexisting clinozoisite-zoisite pairs from all rock typesare given in Figures 3 and 4. All analyses are compiledin Table l.r Figure 3 shows that FerO. is preferentiallyincorporated into the monoclinic form. TiO, (Fig. 4) isalso preferentially incorporated in clinozoisite, up to amaximum value of 0.4 wto/o. MnO and MgO are alsoenriched in clinozoisite (up to 0.2 wto/o), whereas CrrO,partitioning behavior is unclear (Fig. a). Preferential in-corporation of the minor elements into the monocliniccrystals will contribute to the uncertainty in position andslope of the two-phase loop in the Al-Fe system, but be-cause the absolute amounts are small, they are probablyof minor importance. This is not the case, however, ifthese elements are present in large amounts, as is dem-onstrated for the rare earth elements. Sample BP I 19(metagranite from the Granatspitzkern Zentralgneis,Tauern Window, Austria; Fig. 5) contains zoisite blastswith small (approximately 20- 1rm) clinozoisite inclusions,which have variable amounts of REE. The zoisite has aconstant composition with 6-8 molo/o AlrFe, but the Fe

' Table I may be ordered as Document AM-92-496 from theBusiness Office, Mineralogical Society of America, 1130 Sev-enteenth Street NW, Suite 330, Washington, DC 20036, U.S.A.Please remit $5.00 in advance for the microfiche.

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Fig. 5. Analytical results for the investigated samples of cli-nozoisite-zoisite pairs; squares : orthorhombic; diamonds :

monoclinic; rotated squares : structural state uncertain; trian-gles: rare earth-bearing clinozoisite. Lines connect coexistingminerals; analyses were made a few micrometers away from thegrain boundary. Sample BP I 19: REE-bearing and REE-free in-clusions ofclinozoisite in zoisite; the width ofthe two-phase areawidens with increasing REE content. N l3-3: Matrix and veinassemblage in a low-grade amphibolite; filled symbols indicatecore compositions. Sample NR 125: Tourmaline-epidote segre-gation from contact metamorphic environment; filled symbolsindicate core compositions. Sample RW 103: Calc-silicate nod-ule from a polymetamorphic granulite to amphibolite facies ter-rane. Solid symbol indicates Fe-poor granulite facies zoisite withovergrowth of Fe-richer zoisite during amphibolite facies meta-morphism. Sample 1266:. Law-grade calc-silicate rock. Closedand open squares represent core and rim analyses, respectively,of grains determined optically to be orthorhombic; closed andopen diamonds are core and rim analyses from clinozoisite, witha continuous range of intermediate compositions (no analysispoints shown). Closed diamonds (lower two rows) representmonoclinic intergrowths (two crystals determined with TEM)that contain 8-32 molo/o Al"Fe in one case ard 22-46 molo/o

6 8 r 0 1 2 1 4 t 6 t 8 2 0

r 0 1 2 ! 4 1 6 30 34

mol7o AlzFe

AlrFe in the other. The compositional gap between 15 and 24molo/o in one crystal does not indicate a miscibility gap, sincemicroprobe analyses of other crystals fall into this range. Sam-ples 85-l and 85-2: Pegmatoid vein in eclogite with formationofclinozoisite in core ofzoisite, secondary clinozoisite near andat the rim ofzoisite, secondary clinozoisite in cracks in zoisite.The samples show both equilibrium and disequilibrium pairsand also the alteration ofprimary zoisite into clino(?)zoisite withincreasing Fe contents. Newly formed crystals with < I molo/oAl,Fe are too small for structural determination; an orthorhom-bic structure is consistent with the proposed phase diagram, butnot proved. Sample St l: Quartz-zoisite vein from eclogites,showing the same type of alteration of primary zoisite as insamples 85-1 and 85-2. Core compositions of the zoisite (filledsymbols) is Fe rich, rims are Fe poor, but near to alterationproducts Fe content increases. Samples DT 8 and FT I are fromeclogites, filled symbols indicate core compositions. Both cli-nozoisite and zoisite show a wider range of compositions com-pared to the low-P rocks. Dashed line connects core composi-tions of two adjacent crystals. Sample RM 7l is an epidote +garnet + albite vein in an eclogite. The high Fe rim ofclinozois-ite is only 5-10 pm wide (open diamonds), inside of this rim(filled diamonds) the clinozoisite is homogeneous.

6 1 4 2 0 2 4

l - - - - - - -

636

Fig. 6. Photomicrograph of equilibrium intergrowth of cli-nozoisite (34-36 molo/o AlrFe) in the core of a zoisite (12 molo/oAlrFe), together with (sericitized) albite (sample 85-1, crossedpolars).

content in the clinozoisite varies from 25 molo/0, wherethe REE are below the detection limit (0.5 wto/o), to 5lmolo/0, where the REE total is I I wto/o (estimated fromCa deficiency and deviation of ideal anhydrous total of98.5 wto/o; semiquantitative energy dispersive analysis).The REE end-member of the epidote group, allanite, con-tains Fe2* in large proportions (Dollase, I 97 I ), and there-fore these high Fe contents do not reflect the true AlrFemol0/0, and clinozoisite of this type cannot be used toconstrain the two-phase loop.

Petrographical description

Owing to the problems mentioned above (complexzoning, growth at diferent P-Z conditions) each sampleis different and must therefore be described separately.

Sample N l3-3 (Fig. 5) comes from an ophiolite com-plex in the Gebel Rahib area, Darfur, northwestern Su-dan (Abdel Rahman et al., 1990). Z-"- and P-.. are rough-ly estimated for this area as 400 + 50'C and <0.5 GPa,respectively, from the mineral assemblage amphibole +chlorite * epidote * plagioclase + quartz + calcite. Ret-rograde shearing in the rocks is recorded by transforma-tion of amphibole with 6 wto/o AlrO, into actinolite with3 wIo/o AlrOr. The sample shows coexisting zoisite andclinozoisite in the matrix and in a small, deformed vein-let. In the matrix, both minerals have the same grain sizeand morphological features and show only minor varia-tion in chemical composition from core to rim. In theveinlet, zoisite is larger in grain size than clinozoisite andshows sector zoning (variation from 5 to 9 molo/o AlrFe).In the veinlet, coexisting mineral pairs are zoisite with 7-9, clinozoisitewith20-25 molo/o AlrFe, compared to ma-trix zoisite with 9-14 molo/o and clinozoisite with 27-28molo/o AlrFe.

Sample NR 125 (Fig. 5) is a tourmaline-epidote seg-regation. It comes from the contact of a granitoid withmetasediments, the so-called Hiillserie ("Seettirl" locali-

FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION

Fig. 7. Photomicrograph of zoisite with 10-12 mol0/o AlrFewith alteration zones parallel to (100) (dark because ofanoma-lous interference colors) with 15 molo/o AlrFe, intergrown withalbite (ab; sample 85-1, crossed polars).

ty, Granatspitz Zentalgneiskern, Tauern Window, Aus-tria). Although these rocks subsequently underwent Ter-tiary Alpine metamorphism, the Paleozoic contactmetamorphism is well preserved and is characterized bythe mineral assemblage calcite + wollastonite + quaftz* grossular + diopside. Temperatures near 550 "C at0.2GPa are assumed to represent the equilibration condi-tions. There is no significant zoning in the epidote min-erals, and tourmaline inclusions in the epidote cores aresimilar in composition as the matrix tourmaline crystals.Zoisite contains l0-13 molo/o AlrFe, clinozoisite 32-37molo/0.

Samples 85-l and 85-2 (Figs. 5, 6, 7) are qtrartz-al-bite(An,o)-zoisite-phengite segregations in the Weissen-stein eclogite, Miinchberg Massif (Germany). The eclo-gite itself formed at high pressures of at least 1.9 GPa attemperatures near 620 "C (Klemd, 1989). The segrega-tions and associated pegmatoids probably formed duringuplift when the P-Ipath just crossed the minimum melt-ing curve (Franz et al., 1986) at pressures of approxi-mately 0.9 GPa. Crystallization of a granitic melt at theseconditions yields zoisite and potassium white mica in-stead of potassium feldspar and anorthite component (Jo-hannes, 1985). Within the centimeter-sized zoisite crys-tals are 0. l-mm-wide lamellae of clinozoisite (Fig. 6)together with pure albite; the lamellae have straight grainboundaries, typically occur in the cores of the zoisite crys-tals, and are oriented parallel to (100) ofzoisite. They areinterpreted as equilibrium intergrowths that formed dur-ing cooling of the segregations. The zoisite crystals arezoned with rims slightly lower in Fe content (10 molo/oAlrFe) compared to the cores (12-13 molo/o). Coexistingmineral pairs give consistent values of I 2- l3 molo/o AlrFeand 35-36 molo/0, with one exception (14 and 4l molo/o).Texturally similar clinozoisite located at the rim of thelarge zoisite crystals shows a larger variation in AlrFecontent from 35 to 42 molo/o.

FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION 637

A second type of clinozoisite (with 3l-53 molo/o AlrFe)occurs along irregular cracks in zoisite (see Fig. 9 in Ham-merschmidt and Franz, 1992) and also parallel to (100)and (010) planes of the host zoisite (Fig. 7), where it isaccompanied by a slight change in interference colors ofthe adjacent zoisite. The clinozoisite formation and mod-ification ofthe zoisite took place at a later stage, probablybelow 350 oC at low pressure. A late (postmetamorphic)hydrothermal stage that might have been responsible forthe features shown in Figure 7 has been documented byHammerschmidt and Franz (1992) for the MiinchbergMassif. The modified zoisite crystals vary between 12and26 molo/o (analyses at points where interference colorschanged or immediately adjacent to the secondary cli-nozoisite). We were unable to determine by optical meth-ods whether these crystals are monoclinic or orthorhom-bic, but preliminary transmission electron microscopyresults show deviations from orthorhombic symmetry inthese areas (a detailed description ofthese results will bepresented elsewhere). We were also unable to determinethe symmetry of small crystals approximately 5 pm indiameter that are located in the albite matrix parallel to(100) of zoisite (Fig. 7) and in altered pegmatitic plagio-clase next to the pegmatitic zoisite crystals. These smallcrystals have AlrFe contents near I mol0/0. Assuming thatthe latter are orthorhombic and the zoisite modificationsare monoclinic, a compositional gap between I and 12molo/o exists. The rocks from this locality provide an ex-cellent example of the variety of epidote minerals thatform during cooling. Only the primary intergrowths wereused in determination of the transformation loop, al-though the secondary clinozoisite crystals give some con-straints about the position ofthe loop at low P and L

Sample 1266 (Figs. 5, 8) is also from the Miinchbergarea, but from the border zone adjacent to very low-gradesediments (locality Schwingen, near Schwarzenbach/Saa-le, Germany). This so-called prasinite-phyllite series didnot experience temperatures in excess of 350 'C (the sta-ble assemblage is stilpnomelane * actinolite + chlorite+ calcite + albite; pressrues are not determined, but thereis no indication of high pressures). The specimen is acalcite-phengite rock with minor amounts of epidote, ti-tanite, and tourmaline, from a gteenschist-metapelitecontact. Zoisite occurs as needle-shaped crystals 5-20 pm,with 4-12 molo/o AlrFe mostly restricted to the calcite-rich part of the sample, whereas large clinozoisite grains(300 pm) wfih 7-46 molo/o AlrFe occur mainly in thephengite-rich part of the sample. They were not found inclear grain contact but were within a few micrometers ofeach other. Closer inspection of these crystals with back-scattered electron images revealed zoning from core torim toward Fe-rich compositions in both the monoclinicand the orthorhombic grains, although zoning in the cli-nozoisite was less regular (Fig. 8). The monoclinic natureof those parts of the crystals with very low Fe content (8molo/0, determined by analytical electron microscopy) wasverified by selected area diffraction analysis of two crys-tals. The analyses clearly demonstrate that, at low P-T

21.-26/'' -

/ >

n +

50 r lm

Fig. 8. Zoning in clinozoisite (drawing after backscatteredelectron image, sample 1266) in a calcite + phengite matrix(cleavage indicated) showing central compositions with low Fecontent (6 mol0/0, as indicated in the figure) increasing towardthe rim in irregular zones (separated in the figure by dotted andsolid lines according to their sharpness) up to 3l molo/o; all in-termediate compositions were found. Some of the Fe-rich zoneshave REE present in small amounts. Small crystals are zoisite,ranging from 4 to 12 molo/o AlFe from core to rim, and subor-dinate titanite (stippled).

conditions, low to intermediate AlrFe clinozoisites exist,indicating that there is no obvious miscibility gap, andthat there can be an overlap in compositions of zonedclino- and orthozoisite that grew during a prograde burialand heating event.

Sample St I (Fig. 5) is another example of modifica-tions to primary zoisite during retrograde development.This sample comes from the eclogite zone (Knappenhauslocality, Frosnitztal, Tauern Window, Austria), from azoisite-quartz segregation in a banded eclogite (Thomasand Franz, 1989). Following eclogite facies metamor-phism, these rocks underwent subsequent alteration inthe P-T range from the lower amphibolite facies down totemperatures below 350 "C (Spear and Franz, 1986). The

*20 -

2 0

\

-*-r-r-

I

638 FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION

Fig 9. Backscattered electron image ofclinozoisite and zois-ite from eclogites equilibrated at 2 GPa (sample DT-S), togetherwith omphacite (O) and amphibole (black, not labeled). The om-phacite crystal in the lower left grows across the grain boundarybetween zoisite and clinozoisite and also across the core (Fepoor, 37 molo/o AlrFe) and the rim (Fe rich, 49 molo/o AlrFe).

centimeter-sized zoisite crystals of the segregation are re-placed along their rims by clinozoisite, paragonite, albite,and calcite, and adjacent to these newly formed minerals,zoisite changes its composition from 6-10 molo/o AlrFeat the rim (cores are I l- l5 molo/o) to 16-21 mol0/0. Thesecondary clinozoisite ranges from 39 ro 47 molo/o AlrFe.

Samples FT-l and DT-8 (Figs. 5, 9) are from the samearea as sample St l. They are both from banded eclogites(localities Frosnitztal, southern margin of the eclogite zone,and Dorfer Tal, western end of the Gastacher Wiinde).The eclogites experienced pressures near 2.0 GPa, at tem-peratures near 590 "C (Holland, 1979; Frank et al., 1987;Spear and Franz, 1986) with some subsequent reequili-bration to lower greenschist facies conditions. The FTsample shows no signs of this reequilibration, whereasthe DT sample is slightly retrograded (as indicated by thebeginning of symplectite formation of some omphaciteand the scattering of Ko values for garnet clinopyroxene).The mineral assemblage in both samples is epidote +garnet + omphacite * amphibole + phengite + dolomite+ magnesite + rutile + quartz. Clinozoisite is in mostcases strongly zoned with increasing Fe contents fromcore to rim; in coexisting zoisite the trend is similar but

Fig. 10. (A) Backscattered electron image of zoisite and cli-nozoisite, together with albite (black, unlabeled), (B) 20-1rm in-clusions ofclinozoisite in zoisite (close-up ofthe left central partof the zoisite crystal in A). Matrix and inclusion clinozoisiteshow the same type of zoning (sample RMTl).

less pronounced. The zoisite composition ranges from l0to 15 molo/o AlrFe (one exception with 18 molo/o), theclinozoisite from 34 to 52 mol0/0, with core compositionsbetween 29 and 38 mol0/0. There are two average com-positions of clinozoisite, one near 34, the other near 45molo/0. Figure 9 shows that omphacite inclusions are intextural equilibrium with both of these compositions. Weuse this observation to argue that the clinozoisite grainswith the high AlrFe component are not late-stage over-growths but rather formed during prograde metamor-phism.

Sample RM 7l (Figs. 5, l0) is a garnet amphibolitewith strong indications that it is a retrograded eclogite(symplectite of amphibole and plagioclase, corroded gar-net; the main mineral assemblage is amphibole + plagio-clase * chlorite + epidote + titanite). It comes from thebasement complex of the Tauern Window, Maurer Tal.Eclogites within this basement record lower pressures (1.2

639

GPa) and temperatures (500 + 50 "C; Zimmermann andFranz, 1989) than those within the eclogite zone. Thecoexisting zoisite-clinozoisite minerals occur in a smalllayer in the sample, coexisting with albite and garnet (+paragonite), with good indications for textural equilibri-um. The zoisite crystals are embedded in clinozoisite andalbite but also have inclusions of clinozoisite. These in-clusions show the same type of zoning as the matrix cli-nozoisite, with very narrow rims of Fe-rich composition(42-54 mol0/o AlrFe), central parts with 3 I -34 molo/o (Fig.l0b). This is again taken as an argument for progradezoning of the minerals not modified by retrogression. Thezoisite ranges from 8 to 16 molo/o AlrFe, with core to rimzoning toward Fe-rich compositions.

Sample RW 103 (Fig. 5) is a calc-silicate nodule in anamphibolite. It comes from the Nubian Desert basement,approximately 30 km west of Delgo (20"N, Nile River)in the northern province of Sudan. This basement is char-acteized by a main stage of mineral formation in theupper amphibolite facies (P near 0.5 GPa, I near 650'C), but contains relicts of granulite facies metamorphism(P : 0.8 GPa, temperatures near 800 "C; Bernau et al.,1987; Huth et a1., 1984). The calc-silicate nodule consistsofgrossular + zoisite + diopside + anorthite + qtarlz,separated by a small reaction zone ofclinozoisite + zois-ite + diopside + anorthite from the amphibolite (mainlyplagioclase + amphibole). Zoisite is strongly zoned from2 to 12 molo/o AlrFe, with Fe-rich rims, but zones of allcompositions coexist with clinozoisite. The variations inclinozoisite composition (36-42 molo/o AlrFe) occur insharp zones parallel to the b axis. It is assumed that thezoisite formed during granulite facies conditions abovethe upper stability limit of clinozoisite, which is near 650'C (Holdaway,1972). During the amphibolite facies event,epidote component was formed, changing the zoisitecomposition and also forming separate clinozoisite crys-tals. Although both minerals are in grain contact andpresent in equal grain sizes, the large range in zoisiterim compositions coexisting with clinozoisite indicatesdisequilibrium.

DrscussroN

A primary observation in nearly all of the samples thatwe have studied is that both zoisite and clinozoisite com-monly show disequilibrium features, such as complexzoning patterns (especially in rocks of high-pressure ori-gin), formation of texturally and chemically different types,and modifications ofearlier minerals. Within a single grainthe composition can vary widely and often irregularly,i.e., not parallel to the outer crystal faces. These varia-tions in composition are controlled by mineral reactionsproducing epidote component in the rock and by crystalgrowth phenomena such as nucleation and growth rates.Intra- and intercrystalline exchange reactions seem to beof minor importance, and we conclude that volume dif-fusion in the epidote mineral group is slow. Another im-portant factor influencing the compositional variationsfor coexisting minerals shown in Figure 5 may be the fact

Fig. rr. )-* ur^*')0"..0 ":

-trr,"", .",",";r;";and text. The average values for several analyses are shown bythe symbol; the horizontal line through the symbol gives therange of composition (compare with Fig. 5 for number of anal-ysis pairs). Symbols with double lines refer to eclogites, and thedouble line labeled "high P" indicates the two-phase loop forhigh pressure. The solid symbols refer to the core compositionsofthe high pressure clinozoisite.

that minor elements can influence the width of the gap.This is evident for the REE (Fig. 5) and has also beenshown for Sr by Maaskant (1985).

In Figure I I we have arranged the data on coexistingclinozoisite-zoisite pairs (outer rim analyses) in a T-Xdiagram, using average values (mean of the analyses), butalso showing the range of analyses, together with an es-timate of the temperature of formation of the epidoteminerals. The rocks were split into two groups, one withlow to intermediate pressure of formation (< I GPa), theother with high pressure (> I GPa). These estimates arerather uncertain: even when rigorous thermobarometriccalculations are made, the timing of equilibration of theepidote minerals is still unknown. Error boxes in the T-Xdiagram would be large, but several key features are ev-ident despite these uncertainties: (l) The overall chemicalvariation in zoisite is much smaller than in clinozoisite,but both show an increasing AlrFe component with in-creasing temperature. This supports the previous findingsof Enami and Banno (1980), Maaskant (1985), and Pru-nier and Hewitt (1985). (2) The largest compositionalvariations, in both zoisite and clinozoisite, were consis-tently observed in rocks of high-pressure origin. High-pressure rocks necessarily have a complex metamorphichistory with more possible stages of mineral growth than,for example, during simple contact metamorphism. (3)Clinozoisite samples from the high-pressure terranes aresignificantly higher (by approximately l0 molo/o) in AlrFecomponent than those from low-pressure terranes, al-though their estimated formation temperatures are sim-ilar. The same trend can be observed in Enami and Ban-no's (1980) data: Their FU and OM samples (their Fig.4) come from low-pressure terranes, whereas their TO,NI, and IR samples are from eclogites; the compositionalgap for the high-pressure rocks is significantly wider.

These observations provide evidence that pressure ex-erts an important influence on the position of the clino-

Toc

FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION

e /tw-interm

P

T O C

640

Al2A l 2s so 7 5 A l 2 F e

Fig. 12. Influence of a possible miscibility gap at low 7, highP, as indicated by data ofSelverstone and Spear (1985). A mis-cibility gap at low temperatures demands a second monoclinicvariety labeled clz$.

zoisite limb. This is supported by observations of Spenceret al. (1988), who described pairs ofzoisite and clinozois-ite with l3- l4 and 40-51 molo/o AlrFe, respectively, fromeclogites of the Western Gneiss Region (Norway), andMaaskant (1985), who reported a zoisite-clinozoisite pair(16.5 and 45 molo/o AlrFe) from the high-pressure rocksof Galicia (northwestern Spain). We cannot distinguishunambiguously between the two possibilities discussed inFigure 2 because of the large variation in the composi-tions, but a configuration similar to Figure 28 seems quitelikely.

At temperatures below approximately 400 .C the two-phase region is not well constrained. Our sample 1266(see Fig. 5) shows that intermediate compositions formonoclinic crystals lying within the two-phase region athigher temperatures do exist. Therefore at low P-Z con-ditions, the limb restricting the monoclinic phase areaextends toward very Al-rich compositions, but its exactposition is still uncertain. An equilibrium temperature forReaction I near 200 oC at low pressures is likely, in agree-ment with calculations using the Berman (1988) data setand with the experimental determination of Jenkins et al.( l e85).

The secondary epidote minerals formed in samples 85-l, 85-2, and St I show a large variation of compositionsand textures within a single thin section, reflecting pro-longed cooling of the rocks. The primary zoisite crystalswith 10-12 molo/o AlrFe were transformed during a hy-drothermal event into grains with rtp to 26 molo/0, typi-cally without changing the morphology and the straightextinction of the crystal. The increasing Fe content is vis-ible in narrow zones with increasing birefringence. Pre-liminary TEM data suggest that these zones consist of

FRANZ AND SELVERSTONE: CLINOZOISITE-ZOISITE TRANSFORMATION

intergrowths of monoclinic and orthorhombic domains(Smelik and Franz, in preparation). Secondary clinozois-ite crystals precipitated in cracks range from 30 to 54molo/0. This indicates that at low P-T"conditions (approx-imately 0.3 GPa, 350 'C) no miscibility gap is apparent.The existence of almost Fe-free epidote minerals (sym-metry not determined) in altered plagioclase, only a fewmicrometers away from the primary Fe-bearing zoisiteand the secondary high-Fe clinozoisite, is typical of thedisequilibrium preserved among neighboring grains of theepidote minerals. An orthorhombic symmetry of theseFe-free epidotes would be consistent with the transfor-mation temperature for Reaction I near 200 "C, as men-tioned above.

There is also evidence to suggest that miscibility gapsmight occur in the monoclinic series at low temperaturesor high pressures. Apparent miscibility gaps have beenreported by several authors (Strens, 1965; Holdaway,L972;Hietznen, 197 4; Raith, I 976; Selverstone and Spear,1985) on the basis of coexisting compositions of mono-clinic epidotes in natural rocks. Our own analyses andbackscattered electron images ofepidotes show that dis-equilibrium textures are commonly preserved in singlegrains and even in coexisting monoclinic and orthorhom-bic forms; it is thus possible that published reports ofmiscibility gaps in the monoclinic series are merely anartifact of complex growth histories. On the other hand,such effects as ordering of Al and Fe3* in Ml and M3sites may create structural variations leading to miscibil-ity gaps. We hope that detailed TEM studies will resolvethis problem. Because most of the reported miscibilitygaps are located at high AlrFe contents (between 35 and75 molo/o) and close at temperatures near 550 "C (Raith,I 976) or even lower (450'C; Selverstone and Spear, I 985),they probably have no influence on the steep orthorhom-bic to monoclinic transformation loop reported here.However, if the clinozoisite limb is significantly shiftedtoward Fe-rich composition with increasing pressure, asindicated in Figure I l, the suggested monoclinic misci-bility gaps might intersect the transformation loop. Sucha configuration is shown in Figure 12 for the miscibilitygap proposed by Selverstone and Spear (1985), with acritical temperature of 450 "C.

CoNcr-usroxs

This study supports the previous findings of Enami andBanno (1980), Maaskant (1985), and Prunier and Hewitt(1985) that the transformation loop between clinozoisiteand zoisite depends strongly on temperature and that Fe3*(as well as the minor elements Ti, Mq and Mg) is pref-erentially distributed into clinozoisite. In addition, weshow that pressure may also be a significant variable andthat in high-pressure rocks clinozoisite is enriched in Fe3*compared to low-pressure rocks (at the same tempera-ture, coexisting with zoisite) at intermediate temperaturesbetween 400 and 600'C; zoisite-clinozoisite relations re-main uncertain at lower temperatures, although a nar-rowing of the compositional gap is likely, and at higher

FRANZ AND SELVERSTONE: CLINOZOISITE.ZOISITE TRANSFORMATION 641

temperatures the upper stability limit of clinozoisite dis-turbs the simple binary phase relations.

The phase diagram that we present for the orthorhom-bic to monoclinic transformation can be used as an aidin the calculation of reactions involving epidote minerals.For example, if the pressure-sensitive reaction anorthite+ HrO : zoisite-clinozoisite + kyanite + qvartz is con-sidered, its position in a P-T diagram depends on thestructural state, as well as on the composition of the ep-idotes. Calculations involving such a reaction at high P-7"conditions (eclogite facies) should consider zoisite at bulkAlrFe compositions from 0 to l5 molo/0, coexisting zoisiteand clinozoisite in the range l5-45 molo/o AlrFe, and cli-nozoisite only at AlrFe contents greater than 45 molo/0.At low P-Z conditions (greenschist to amphibolite facies)the compositional gap is small and of only minor effect.Hence, at these conditions clinozoisite should be used inthe calculations for all bulk rock compositions that mightyield more than l0-15 molo/o AlrFe; zoisite would be theappropriate phase only for very Fe3*-poor compositions.

For a more temperature-sensitive reaction such as law-sonite : zoisite + kyanite + quafiz + H2O, the pressureinfluence is more pronounced: at low P and 7t, 5-10 molo/oAlrFe can be incorporated in zoisite, and the coexistingclinozoisite has around 20 molo/o, whereas at low ?" andhigh P the clinozoisite has around 40 molo/o Al,Fe. Thelikelihood of forming zoisite in addition to clinozoisiteincreases with rising pressure because of the wider com-positional gap. This provides an explanation for the rel-ative abundance of zoisite in high-pressure rocks, al-though zoisite itselfis also stable at low pressure.

Epidote minerals, owing to their stability over a widerange of P-?" conditions, can be produced in many dif-ferent stages during the metamorphism of a single rock;the apparently slow diffi.rsion rates of Al e Fe3* exchangeresult in preservation of complex zoning patterns in in-dividual grains. The epidote minerals thus have the po-tential to provide a wealth of petrologic information rel-evant, for example, to the reconstruction of P-I paths,provided that quantitative data on P-T-X relations in theepidote series are available. Though this is not yet thecase, the empirical phase diagram presented in Figure I Ishould serve as a starting point for future work. Moreanalyses of coexisting mineral pairs from areas with well-known metamorphic histories are necessary to refine thisdiagram further. For fine-grained rocks, this will requiresimultaneous determination of crystal structure andchemical composition by transmission electron micros-copy.

Ultimately, single epidote grains might be useful can-didates for radiometric age determinations with a micro-beam technique, both because they can contain high con-centrations of trace elements such as REE, Sr, Pb, U, andTh and because they can record different stages ofgrowthduring complex metamorphic events; in this regard, theymay eventually serve the same purpose in geochronologicstudies of intermediate-temperature metamorphic rocksthat zircon does in higher temperature (e.g., magmatic)

rocks. Such studies have the possibility to yield timinginformation on the prograde paths ofhigh-pressure rocksand thereby provide useful data for geodynamic studiesof paleosubduction environments.

AcxNowr,nncMENTS

The authors acknowledge a DFG grant Fr 557/!,4 (G.F.), a NATOtravel grant (G.F. and J.S.), and an NSF grant EAR-8658145 (J.S.). Someofthe sample material was collected on field trips supported by SFB 69.TEM work was carried out at Johns Hopkins University (TransmissionElectron Microscopy Facility, supported by NSF grant EAR-8903630).G.F. gratefully acknowledges G. Smelik's help and introduction into TEMwork, as well as the kind permission of D. Veblen to use his laboratory.We thank W. Heinrich, D. Hewitt, D. Jenkins, D. lattard, G. Smelik andJ.B. Thompson for discussions and critical reviews ofthe manuscript.

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