phase transitions and decomposition relations in calcic plagioclase · phase transitions and...

American Mineralogist, Volume 68, pages 4l-59, 1983

Phase transitions and decomposition relations in calcic plagioclase

Ttl,loruv L. Gnovn,

Department of Earth and Plunetary SciencesMassachusetts Institute of Technology

Cambridge, Massachusetts 02139

JoHN M. FBnnv

Department of GeologyArizona State University

Tempe, Arizona 85287

eNo FnaNr S. Spr.en

Department of Earth and Planetary SciencesMassachusetts Institute of Technology

Cambridge, Massachusetts 02139

Abstract

A subsolidus temperature-composition diagram for the calcic portion of the systemNaAlSilOrCaAl2Si2Os is derived from the following data: (a) compositions of plagioclase

crystallized under a range of metamorphic conditions; (b) TEM observations of coexistingandesine (An3e) and bytownite (Anes); and (c) TEM and crystallographic studies of calcicplagioclase from the literature. The most significant feature of the diagram is a proposed

existence of two tricritical points and their associated conditional spinodals. One tricriticalpoint is a consequence of the Cl e n transition, which is inferred, from microstructures ofplutonic plagioclase, to occur near An73-An75 at -900"C. The other tricritical point is aconsequence of the /l = Pl transition and is inferred to be located near An75-An7s at-675"C. The Cl P /l conditional spinodal is responsible for submicroscopic tw-o-phaseintergrowths in labradorites and for Bdggild intergrowths (Anqs-Anes). The 1l a Plconditional spinodal is responsible for Huttenlocher intergrowths (Anoo-Ans7). At tempera-tures between -400" and -575'C, plagioclase in the range of bulk composition Ana6Ane6should produce an equilibrium oligoclase/andesine f bytownite assemblage by nucleationand growth. Intermediate compositions are interpreted to be the result of metastableplagioclase growth and/or decomposition to submicroscopic intergrowths within either ofthe two conditional spinodals. Even the long equilibration times associated with regionalmetamorphism fail to produce equilibrium plagioclase feldspar compositions.

Introduction

The topology of the subsolidus temperature-com-position diagram for the system albite-anorthite hasstimulated continued discussion among mineral-ogists and petrologists which, to date, has notproduced an adequate graphical or even conceptualtreatment of the phase relations. Previously pro-posed phase diagrams treated the Huttenlocher(An66-Aq7) and B/ggild (Ana5-An65) miscibilitygaps as conventional solvi (McConnell, 1974a, Fig.5; Wenk, 1979, Fig. 3). As an alternative, Smith

0003-004x83/0102-0041$02.00 4l

(1972, Fig. 9; 1974, Fig. S-2a) suggested that theBdggild gap was the result of a first-order Ci a Iitransition while retaining the Huttenlocher gap as asolvus.

We propose an alternative to these ?-X diagramswhich integrates the miscibility gaps and the order-ing transitio_ns in c-alcic p_lagioclase. Assuming thatthe Cl <z 11 and Il e Pl transitions in plagioclaseare higher than first order, tricritical points mayoccur in analogy to other well-studied binary sys-tems (Allen and Cahn, 1976a,1976b). The proposedT-X diagram contains two such tricritical points, in

GROVE ET AL.: PHASE TRANSITIONS IN CALCIC PLAGIOCLASE

addition to the tricritical point proposed by Carpen-ter (1981) for the peristerite region. The tricriticalpoints each generate a conditional spinodal (condi-tional on the appearance of a lower-symmetry or-dered phase) as well as a region of stable two-phasecoexistence for the ordered and disordered phases.

The paper begins with a summary of the composi-tions of coexisting plagioclase feldspars in rocksmetamorphosed under a range of temperature con-ditions. Next, we present observations of micro-structures in calcic plagioclase made with the trans-mission electron microscope (TEM). Two possiblephase diagrams for the calcic portion of the albite-anorthite system are derived from the informationon the compositions and microstructures of plagio-clases. Finally, the proposed phase diagrams areused to interpret the variety of microstructuresreported in the literature for intermediate and calcicplagioclase feldspars.

Many of the microstructures in plagioclase areinterpreted in terms of metastable behavior. This isnot unreasonable, because subsolidus reactions inthe plagioclase system require the coupled solidstate exchange NaSi a: CaAl. Interdiffusion coeffi-cients for the exchange couple may be estimatedfrom the heating experiments of Nord et at. (1974)who homogenized the lamellae of Huttenlocherintergrowths by annealing them at high tempera-tupq for short periods of time. Assuming an x :VDt law for homogeni2ation of lamellae, a diffusioncoeftcient of D - 10-17 cm2lsec at 1240" and 1300"Cis calculated. Under conditions of upper amphibo-lite facies metamorphism (Z - 650"C), Huttenlocherlamellae grow to a thickness of >10004 (Grove,1977) and an estimated D would be on the order of10-26 cm2lsec. These two data allow estimation ofthe kinetic rate constant and yield an activationenergy for coarsening in excess of 420 kJ/mol.Thus, plagioclase reactions that occur by diffusion-controlled NaSi e CaAl exchange are extremelyslow, and the opportunity for the formation ofmetastable plagioclase compositions in nature isgreat. Some reaction mechanisms (heterogeneousnucleation) may indeed produce equilibriumphases, while other mechanisms (e.g., spinodaldecomposition) may commonly produce metastablefeldspars. An important aspect of our proposed ?-Xdiagram is that it allows prediction of possiblemetastable ordering and decomposition mecha-nisms and hence permits identification of equilibri-um vJ. metastable feldspars in natural occurrencesof plagioclase.

BeoverBrook, NH

Augusto, METhetford, VT

Sidney, ME

Woterv i l le ,

ME

02 0_4 06

XAn02 0 .4 06 08

XAn

Fig. l. Electron microprobe analyses of plagioclase frommetamorphic rocks. The dark rectangles represent preferredcompositions of plagioclase obtained by compiling analyses onhistograms and choosing composit ions with maximumfrequency. Temperatures were estimated using geothermometnctechniques. Beaver Brook locality temperature estimate is fromRumble et al. (1982). The Thetford, Vermont temperarure lsfound in Spear (1980, 1981). The Augusta (Femy, 1976), Sidney(Ferry, 1979) and Waterville (Ferry, 1980a) localities arerepresentative of Buchantype metamorphism in the Waterville-Vassalboro region, South-Central Maine. This technique mayshow equilibrium compositions, if grains represent plagioclaseproduced by heterogeneous nucleation. The peristerite gap isdrawn as a conditional spinodal (Carpenter, lggl), and theunivariant equilibrium at 390"C would involve pi, Cl disorderedandesine and Cl ordered albite. (a) This variant treats theBlggild gap as stable at T > 575'C. (b) This topology shows nostable Blggild gap at high temperarures.

Compositions of plagioclase feldspar inmetamorphic rocks

Compositions of coexisting plagioclase feldsparsin metamorphic rocks constrain the topology of anyproposed T-X diagram for the albite-anorthite sys-tem. Many feldspars in metamorphic rocks proba-bly formed by heterogeneous nucleation and there-fore may represent stable equilibrium phases.Figure I summarizes temperature and compositiondata for coexisting feldspars from metamorphicrocks crystallized under a variety of temperatureconditions. Coexisting albite and oligoclase arefound in phyllites metamorphosed at P - 3500 bars,T : 400"+ 10oC near Waterville, Maine (Ferry,1976, unpublished data). Coexisting albite and oli-goclase have long been recognized in low-grademetamorphic rocks (Crawford, 1966; Evans, 1964).Coexisting andesine and bytownite have been re-ported in carbonate rocks near Sidney, Maine,

(-)o

o

Eo

F

o-o

o=

Io

GROVE ET AL.: PHASE TRANSITIONS

metamorphosed at P - 3500 bars, ? : 450'-r10'C(Ferry, 1979) and in mafic volcanic rocks nearThetford, Vermont, metamorphosed at P - 5500bars, T: 535ot20oC (Spear, 1980, 1981).

The coexisting andesine * bytownite from Thet-ford, Vermont, is particularly important becauseSpear demonstrated, on the basis of plagioclase-amphibole cation exchange equilibria, that it indeedrepresents a stable equilibrium pair. An exchangereaction exists among the albite (NaSi) and anor-thite (CaAl) components of plagioclase and theglaucophane lNaM4SiIV; and tschermakite (CaMaAIIV) components of calcic amphibole. At Thetford,coexisting andesine (Anrs) and bytownite (Anrr)occur with amphibole of a unique composition interms of glaucophane and tschermakite components(Spear, 1980, Fig. 1). Feldspars more sodic thanAn3e coexist with more sodic (glaucophane-rich)amphiboles. Feldspars more calcic than Atus coex-ist with more calcic (tschermakite-rich) amphiboles.The systematic NaSi ? CaAl partitioning amongphases at Thetford, Vermont, is strong evidencethat coexisting An3e and Ans represent a stableequilibrium assemblage. In fact, any supposed two-plagioclase pair must be consistent with mineralequilibria involving coexisting phases, if the pair isto be treated seriously as an equilibrium assem-blage.

The plagioclase feldspars in samples from Thet-ford, however, do exhibit some complications. Inthin section the coexisting andesine and bytowniteare usually present as patchy mutual intergrowthsor as bytownite overgrowths on andesine cores.The overgrowth relation is consistent with the pro-duction of CaAl2Si2Os by prograde mineral reac-tions. The sodic plagioclases acted as nucleii onwhich heterogeneous nucleation of an equilibriumcalcic plagioclase occurred. Detailed examinationof the coexisting andesine and bytownite revealsthat the andesine cores are optically homogeneous,but are continuously zoned from An3s to An3e.Occasionally there is an abrupt transition from An3eto an Aqs bytownite rim. Commonly, there areregions of optically visible two-phase intergrowthsbetween the andesine core and bytownite rim whichrange in bulk composition from An6e to Aq6 (Fig.2). The two-phase lamellar intergrowths grade out-ward into optically homogeneous Aqs bytownite.As discussed further below, we interpret the coex-isting An3e * Ans as a stable equilibrium pair andthe lamellar intergrowths as a metastable phenome-non.

IN CALCIC PLAGIOCLASE

THETFORD. VERMONT

Fig. 2. Histograms of electron microprobe analyses ofplagioclase from metamorphic rocks. (a) Thetford, Vermont.Closed circles are analyses of areas which appear to be ahomogeneous single phase under the optical microscope.Triangles are bulk analyses of optically visible two phaseintergrowths. (b) Augusta, Maine. A compilation of analysesfrom outcrop 987 (Ferry, 1980a).

Coexisting andesine and bytownite are also in-ferred to occur at temperatures -530"C (P - 3500bars) based on the study ofa large single outcrop ofmetamorphosed carbonate rock near Augusta,Maine (location 987 of Ferry, 1980a). Figure 2summarizes compositions of plagioclase found in22 samples from the outcrop. Although no rockcontained coexisting andesine and bytownite, anal-yses demonstrate that compositions of -Anan and-Anso are strongly favored. The activity of anor-thite was calculated for plagioclase in each sampleby the following procedure. First, using samplesthat contain plagioclase of composition Xa' = 0.9,the temperature of metamorphism at the outcropwas calculated from the assemblages plagioclase *calcite * zoisite + calcic amphibole + quartz +diopside and plagioclase + calcite * zoisite *qaartz * garnet. Calculations followed a schemeoutlined earlier (Ferry, 1976), modified by usingthermochemical data for the zoisite-calcite-anor-thite equilibrium extracted from recent experimentsby Allen and Fawcett (1982). Conditions of P66, *PHro : Ptotut:3500 bars were assumed. Only thosesamples containing plagioclase of Xa. > 0.9 wereused in calculations of temperatures because forthese sampl€s 4sn - Xen(Orville, 1972; Goldsmith,

43

8roo

1!

o 5

oo=o

. optically homogeneous

optical ly two- phase intergrowths

^ . 1^ aal

a l O ^ -

o t a ^ ^ ^ ^ ^ b ( t x r o


Table l . Calculated activi ty of anorthite componentplagioclase for samples collected from a single outcrop

amphibolite facies metamorphosed carbonate rock

Sample o*'

1 . 0 4 - 1 . 3 4

0.42-0 .97

0 . 8 6 - 0 . 9 6

0 . 9 5 - 1 . 4 1

1 , 1 0 - 1 . 2 9

r . 1 6 - 1 . 3 0

0 . 9 2 - 1 . 0 1 *

r . 0 r - 1 . 2 0

0 . 8 1 - 0 . 9 4

r .06-7 .22

o . 9 7 - 7 . 2 5

0 . 9 2 - 1 . 0 6

0 . 9 2 - 1 . 0 1

0 . 9 0 - 1 . 0 5

0 . 8 7 - 0 . 9 9

0. 86-0 . 89

0 . 8 9 - 1 . 0 2

0 " 8 1 - 0 . 9 2 , 0 . 9 2 - 1 . 2 2 *

0 . 8 0 - 0 . 9 r *

o .95- t .27

o , 8 7 - r . 1 2

o.84- t .24

lcalculated from act iv i ty coeff ic ients of orvi l le (1972).

_Calculated by neEhod described in text using diopside-calci te_

amphibole-quartz-f lu id equi l ibr ium except where noted by *) .

act iv l ty calculated fron plagloclase-gatnet-quat lz-zoisi teequl l lbr im.

1982). Temperature so calculated from five samplesis consistent with 530'-f 10'C. The 530'C tempera-ture is confirmed by results of a regional study ofmetamorphic temperatures in an area immediatelyadjacent to the outcrop (Ferry, 1980b). Second, forall 22 samples the equilibrium composition of coex-isting H2O-CO2 fluid was calculated at P : 3500bars, T : 530"C using either the amphibole-calcite-quartz-diopside-fl uid or th e zoisite-calc ite4uartz-garnet-fluid equilibrium. Third, the activity of anor-thite in each sample was determined from thezoisite-plagioclase-calcite-fl uid equilibrium at con-ditions of P : 3500 bars, Z : 530'C, and Xss, aspreviously determined. Results are presented inTable 1. The range in calculated activities is thatpermitted by the range in measured mineral compo-sitions in the assemblages zoisite-quartz-calcite-garnet and zoisite-quartz-calcite-amphibole-diop-side. For comparison, the composition of plagio-clase in each sample is listed as well as the activityof anorthite that would be calculated from theactivity coefficients of Orville (1972). For twelvesamples in which Xen:0.35-0.51, the mean a4n :0.97 (o : 0.10). Mineral equilibria indicate that

plagioclase of composition Xa. - 0.4-0.5 behavedchemically as if it were equivalent to plagioclase ofcomposition Xen - 0.9-1.0 during the metamorphicevent. Calculated activities (Table l), combinedwith the clustering of observed compositions (Fig.2), are interpreted in terms of a miscibility gapbetween -An+o and -Ane,6 at the physical condi-tions of metamorphism (530'C).

Figure 2, however, reveals complications similarto those observed in the sample from Thetford,Vermont. Although a stable two-phase region isinferred between -An+o and -Anes, compositionshave been observed that lie within the proposedmiscibility gap. We believe these intermediate com-positions represent metastability and will discussthem fully in a later section.

Coexisting labradorite and bytownite were ob-served in carbonate rocks metamorphosed at P -3500 bars, I : 600"-r l0'C from Beaver Brook, NewHampshire. Coexisting -Atur and -An35 havebeen reported in metamorphic rocks from a numberof other localities (e.9., Wenk and Wenk, 1977).

Two interpretations of the observed plagioclasecompositions in metamorphic rocks are presentedin Figure l. Above 575"C either there may exist asingle two-phase region representing coexisting lab-radorite and bytownite or there may exist two two-phase regions, one for coexisting labradorite andbytownite and the other for coexisting andesine andlabradorite. Existing data force us to adopt both aspossible valid phase diagrams. Figure la obviouslywould be preferred if a stable equilibrium coexis-tence between andesine and labradorite could everbe demonstrated.

TEM observations of metamorphic plagioclase

Thetford, Vermont

Samples of the two plagioclase assemblage fromThetford, Vermont were chosen for TEM observa-tion. Thin sections polished on one side were pre-pared using Crystalbond and areas were selectedfor TEM examination. The thin sections weremounted between 3 mm diameter Cu grids withepoxy, and an ion micro-milling device was used tothin the sample. Observations were made with aPhillips EM300 TEM operating at 100 kV. In orderto relate the observed microstructures and diffrac-tion geometry to bulk chemical composition, thinregions of interest were semi-quantitatively ana-lyzed on an electron microprobe using energy dis-persive techniques.

l n

of

^ 1

A 0" 88-0 . 94

B 0 . 90-0 .93

c 0 . 4 3 - 0 . 6 5

D O . 4 2 - O . 4 4

E 0 . 8 5 - 0 . 8 8

F 0 . 8 3 - 0 . 8 5

c 0 . 3 5 - 0 . 3 7

H 0 . 4 4 - 0 . 4 6

I 0 . 4 2 - 0 . 4 5

K 0 . 6 8 - 0 . 8 6

I 0 . 4 9 - 0 . 5 1

M 0 . 4 0 - 0 . 4 3

N 0 . 4 r - 0 . 4 7

P 0 . 8 4 - 0 . 8 7

q 0 . 4 8 - o . s o

R 0 . 4 0 - 0 . 4 5

s 0 . 4 1 - 0 . 4 7

T 0 . 9 2 - 0 . 9 5

w 0 . 4 0 - 0 . 4 2

x 0 . 9 6 - 0 . 9 8

9a7t 0 . 83-0 .86

987r r 0 . 94-0 . 95

0 , 8 9 - 0 . 9 4

0 . 9 0 - 0 . 9 3

0 . 5 5 - 0 . 7 6

0. s4-0 . s5

0 . 8 7 - 0 " 8 9

0 . 8 5 - 0 . 8 7

0 . 4 5 - 0 , 4 7

0 . 5 6 - 0 . 5 9

0 . 5 4 - 0 . 5 7

0 . 7 7 - O . 8 7

0 . 6 3 - 0 . 6 5

0 . 5 1 - 0 . 5 5

0 . 5 2 - 0 . 6 0

0 . 8 6 - 0 . 8 8

0 . 6 1 - 0 . 6 4

0 . 5 1 - 0 . 5 7

0 . 5 2 - 0 . 6 0

0 . 9 2 - O . 9 5

0 . 5 1 - 0 . 5 4

0 . 9 7 - 0 . 9 8

0 . 8 5 - 0 . 8 7

0 . 9 4 - 0 . 9 5


Bytownite (Anaz{s) appears to be a single phasewhen viewed with the optical microscope. Selectedarea diffraction patterns show that this compositionhas the ordered anorthite structure. Weak-diffuse cand. d reflections reveal the presence of domainswith Pl symmetry (Fig. 3a). Dark field observationsmade using the b reflections (/z * k : odd, / : odd)characteristic ofthe 11 ordered structure show (Fig.4a) zig-zag b antiphase boundaries (APBs) thatseparate ordered domains.

TEM observation of regions containing opticallyvisible two-feldspar intergrowths show characteris-tics identical to those of Huttenlocher intergrowths(Nissen, 1968, 1974; Mclaren, 1974; McConnell,1974a; Grove, 1976). Selected area diffraction pat-terns (Fig. 3b) show that the two-phase inter-growths consist of coexisting Pl ordered bytowniteand labradorite with the periodic antiphase struc-ture of intermediate plagioclase. In calcic bulkcompositions (Fig. 4b) the bytownite phase pre-dominates and lamellae of intermediate plagioclaseshow a continuous variation in orientation andspacing of the periodic APBs. The variation inorientation and spacing is consistent with composi-tional variation in the labradorite phase from An66to An75 (Bown and Gay, 1958). Dark field imagesmade with Pl diffractions reveal Pl antiphase do-mains (APDs) elongate parallel to (010) and 30 to60,{ in width (Fig. 4c). In two-phase intergrowths ofsodic bulk compositions the labradorite (An66)phase predominates (Fig. 5), and it contains bytow-nite lamellae. The bytownite lamellae, in turn,contain lamellae of intermediate plagioclase whichhave periodic APBs that show continuous varia-tions in periodicity and orientation. A similar varia-tion was observed in other Huttenlocher inter-growths (Cliff et al., 1976:. Grove 1976, 1977). Thebulk composition of the two-phase intergrowths(Fig. 2), their diffraction geometry (Fig. 3b), and theperiodic APB morphology all indicate that the la-mellae are members of the Huttenlocher miscibilitygap (An66-Aq7).

Regions of Huttenlocher intergrowths are in di-rect contact with andesine (Fig. 4d), which has theperiodic antiphase structure. Sometimes a plagio-clase grain contains patches of single-phase andes-ine surrounded by regions of Huttenlocher inter-growths, and such textural relations are also visiblein the optical microscope. Selected area diffractionpatterns of andesine lack f diffractions, and showdiffuse, weak e superlattice spots characteristic ofthe Anas periodic antiphase structure (Fig. 3c) with

Fig. 3. Selected area diffraction patterns of the [100] pole ofThetford plagioclases. c* is horizontal and b* is vertical. SeeRibbe (1975, Fig. R-40) for plagioclase diffraction patternnomenclature. (a) Bytownite (An37) shows Pl diffractionsymmetry. Streaks on b reflections arise from e plagioclaseprecipitates that have nucleated on b APBs Specimen showsalbite and pericline twinning. (b) Huttenlocher intergrowths withdiffraction geometry characteristic of coexisting Pl (Ans7) andintermediate plagioclase (Anoe). The Pl phase has sharp b anddiffuse c and d diffractions and the intermediate plagioclasephase is identified by e satellites. The maximum intensity of the esatellites occurs at a reciprocal lattice position characteristic ofAn66. The curved streak connecting e-b is caused by variableperiodicity and orientation of intermediate plagioclase APBswithin the intergrowth. (c) Andesine difraction geometry showsweak e satellites with orientation and spacing indicative of Ana6.

orientation and spacing identical to those of andes-ine measured by Bown and Gay (1958). In the (100)projection the periodic APB orientation (Slimming'1976) is nearly parallel to c* and the splitting

GROVE ET AL.: PHASE TRANSITIONS IN CALCIC PLAGIOCLASE 47

magnitude is 0.036A-t or 284. All features arecharacterisitc of andesine intermediate plagioclase.Conversely, the periodicity and orientation of esatellites in the Huttenlocher intergrowths (Fig. 3b)are identical to those rneasured in An66 to An75labradorite (Bown and Gay, 1958; Grove, 1977).

Dark field micrographs taken in the andesineregions using e reflections show a mottled contrast(Fig. 4d), which indicates that the periodic APBstructure is coherent over small areas of <10004and may be separated by regions of Cl. The diffuse-ness of e spots in selected area difraction patternsis further evidence of the presence of e domainswith short-range order and, as a consequence, it isdifficult to obtain superlattice images of the periodicAPBs.

Augusta, MaineSamples of An5e labradorite from 987-L appeared

to be a single phase when viewed with the opticalmicroscope, and were chosen for TEM study. Se-lected area diffraction patterns (Fig. 5a) contain, inaddition to a diffractions, sharp e and/superlatticediffractions with orientation and spacing character-istic of those measured in An5s labradorite by Bownand Gay (1958).

Dark field images using e diffractions (Fig. 6b)show a domain texture with a weak, mottled con-trast. Mclaren and Marshall (1974) observed simi-lar domains in a Lake County labradorite, andinterpreted the boundaries between domains to beregions of Cl structure. The texture in the Augustalabradorite consists of domains of coherent periodicantiphase structure separated by boundaries of Clstructure. Similar periodic APB morphologies wereobserved by Wenk and Nakajima (1980, Fig. 2a) inan igneous labradorite.

Tricritical Points: theory and application to calcicplagioclase feldspars

The phase transitions that occur in the calcicportion of the plagioclase binary (Ci = /i and /i e

Pl) were inferred by X-ray crystallographers(Laves and Goldsmith, 1951; Gay, 1953) throughobservation of variable Bragg diffraction morphofo-gy. Some plagioclases were found to contain weak,diffuse b reflections (/r + t : odd, / : odd),suggestive of a transition from a Ci, c : 7Adisordered form to an ordered Ii, c : 14A modifica-tion. Further ordering led to the Pi, c : AAstructure, and Bragg reflections associated with thisspace group (c and d diffractions, with h + k :even, / : odd or h + k : odd, / : even) had amorphology distinct from other diffractions, indi-cating the presence of short-range ordered domainswith this lowest symmetry. The existence of twotypes of APDs and the presence of two order/dis-order transitions were further substantiated by di-rect observation of both types of domains in theTEM (Mclaren and Marshall, 1974; Heuer andNord, 1976).

Thus, in the calcic portion of the plagioclasesystem there exist three structural modifications,and the transformation sequence for a phase offixed qomposition, from high to low temperature,is:

Cl + Il ---> Pl

The lower symmetry structures are derived fromtheir higher symmetry precursors through the lossof a translational operation, which reduces thenumber of inversion centers in the triclinic cell byhalf. The consequence of the transformation fromCl to Il is a doubling of the c axis or the loss of thell2 UlDl translation. Similarly, the 11 to Pl transi-tion results in the loss of the ll2 llll) translation.

Transitions involving ordering and atomic posi-tion displacements may qualify as higher order(Landau and Lifshitz, 1958). In the case of the Cl ?11 transition, a continuously varying set of Al-Sitetrahedral site occupancies probably exists be-tween the ordered 1l and disordered Cl states. Forthe 11 ? Pl transitipn, there appear to be both

Fig. 4. TEM observations of Thetford plagioclase. (a) Dark field (DF) electron micrograph of bytownite (Ans7) shows zigzag bAPBs; g (reciprocal lattice vector) = 031. The APB orientations parallel the Huttenlocher exsolution interfacial orientation (031) andthe orientation of the An75 periodic antiphase structure (01l). (b) Huttenlocher intergrowths in bytownite (-Anrr). A b diffraction andits sunounding e satellites were used to fprm this dark field image. Labradorite plagioclase ordered on the periodic antiphasestructure is intergrown with a Pl calcic phase. The periodic APBs extend into the calcic regions as isolated boundaries and as singleperiodic antiphaSe domains. Note variation in orientation and periodicity of periodic APBs in labradorite. This variation is equivalentto that observed over the compositional range An66 !o An75. DF, I = 015 near il001. (c) Elongate fine scale c APDs in the Plbytownite phase of a Huttenlocher intergrowth. Some contrast from the labradorite lamellae shows the interphase boundaries to benearly vertical in this photo. DF, I : 005 (d) contrast from an e reflection in a region ofAna6 andesine. The mottled texture resultsfrgm the growth of domains ordered on the periodic aptiphase structure, and is typical of textures in which decomposition tometastable Cl and 11 (periodic antiphase) has occurred within the B/ggild conditional spinodal DF, g = 931.

48 GROVE ET AL.: PHASE TRANSITIONS IN CALCIC PLAGIOCLASE

Fig. 5. Huttenlocher intergrowths in Thetford labradorite (-Anz,). DF, g : 031. This region was adjacent to bytownite in Fig. 3a.

atomic position (displacive) and Al-Si site occupan-cy contributions to the transition. Thus, both Ci =Ii and tt e pl may be higher order transitions. Forsecond order or higher order transitions there is acontinuous change in the physical properties of thephase up to the transition, where it acquires a newsymmetry property. State functions vary continu-ously through the transition. The free energy at thephase transition point has a singularity; the deriva-tives with respect to intensive parameters (e.g.AGIAn are continuous, but the second derivativesoffree energy with respect to intensive variables arediscontinuous. In one direction along the composi-tion coordinates from the transition point on a G-Xdiagram there are two free energy us. compositioncurves: one for the ordered and another for thedisordered phase (Fig. 7a). In the other directionalong the composition coordinate from the transi-tion point the G-curve for the ordered phase does

not exist. For transitions of this type the degree oforder in the ordered phase changes continuouslywith composition up to the transition and an order-ing parameter (4) may be used to describe thedifferences. As the transition point is approached 4changes continuously. At the transition tempera-ture 4 : 0, the symmetry increases and a freeenergy curve for the lower symmetry phase nolonger exists.

When phase transitions are second order or high-er, tricritical points and conditional spinodals mayexist under some conditions (Allen and Cahn,l976a,b; Allen, 1977). At a tricritical point a2GlaP: 0 for the lower symmetry phase (Fig. 7b), and attemperatures below the tricritical point a2Gla* < 0for a range of composition (Fig. 7c). Thus, a misci-bility gap develops between the ordered and disor-dered phases (Fig. 7d) which is conditional on thepresence of the ordered phase. Within the miscibil-


Fig. 6. (a) Selected area diffraction pattern of the [00] pole ofAugusta An5s plagioclase. cx is horizontal and b* is vertical. Thediffraction geometry is characteristic of Anro intermediateplagioclase. (b) Dark field electron micrograph of Augustalabradorite (An56) taken with an e reflection. The domain-likecontrast results from the following transition sequence. Cllabradorite orders to Il and APBs are formed. Spinodaldecomposition to Cl + 11 within the Blggild conditionalspinodal follows with the Cl phase forming at the antiphaseboundaries.

ity gap a spinodal is defined on one side by themetastable extension of the order-disorder transi-tion (where the slope of the G-X curve changesfrom positive to negative) and on the ordered side

by the loci of points for which a2GlA* : 0 (Fig.7d). Within the spinodal region a phase will beunstable with respect to any small change in compo-sition and spinodal decomposition may occur.

In a system with a tricritical point, phase separa-tion within the conditional spinodal requires thepresence of the ordered phase. Consider a composi-tion in Figure 7d which lies in the disordered (a)phase region and that will cross the metastable

Fig. 7. Free energy (G)-composition (c) diagrams are used toconstruct a temperature-composition diagram of a tricriticalpoint. Free energy surfaces are constructed for 3 temperatures:T1 is above the tricritical point; ?2 is at the tricritical point; and?3 is below the tricritical point within the miscibility gap. FromAllen and Cahn (1976a).

+G

+G

Disordered


extension of the line of the higher order transitionon cooling. For this composition ordering mustprecede phase separation, and domain size andmorphology will influence the resultant decomposi-tion texture. A composition initially in the 82 fieldwill undergo decomposition within the conditionalspinodal to a * B2 (Fie. 7d). As discussed by Allenand Cahn (1976a) the relative energies of APBs andthe interface between the ordered and disorderedphases become equivalent and vanish at the tricriti-cal point. As an order-disorder transition is ap-proached from the lower symmetry phase, theAPBs, which are present as a consequence of thelower symmetry, will disappear, and the interfacialenergy approaches zero. The phase boundary ener-gy for the interface between ordered and disorderedphases in the vicinity of the tricritical point will alsotend to zero. As a consequence, the higher symme-try phase will wet APBs in the ordered phase. Ifsmall ordered domains with dimensions of the spi-nodal wavelength are present, the disordered phasewill form at the APBs, where 4 approaches zero. Ifdomains are large, spinodal decomposition will oc-cur within the ordered regions and will have itscharacteristic wavelength and orientation.

In their study of Fe-Al alloys, Allen and Cahn(1976a) provided a theoretical framework for therepresentation of phase changes near tricriticalpoints and applied it to ordering and decompositionphenomena in the system Fe-Al. There are twoordering reactions in Fe-Al: one involves the transi-tion from a (Im3m, disordered FeAl) to 82 (Pm3m,ordered FeAl). A second ordering transition in-volves B2 a DO3 (Fm3m, ordered FerAl). There isa symmetrical tricritical point associated with the o? 82 transition (Fig. 7d) and a metastable tricriticalpoint associated with the 82 C DO3 transition. Thethree structural modifications are related by higherorder transitions which proceed through the loss ofa translational symmetry operation. The transitionsequence a---> 82 ---> DOr thus is directly analogousto the Cl + Il -+ Pl transition sequence in calcicplagioclase because both involve the loss of half theinversion centers in the lower symmetry form.

Carpenter (1981) has suggested the existence ofatricritical point associated with the peristerite gap inthe system NaAlSi3Os-CaAl2Si2O3. Because of theanalogy between the orderdisorder transition se-quence a--> B2-+ DOr in the system Fe-Al and thetransition sequence Cl + Il + Pl in the systemNaAlSi3Os-CaAl2Si2Os, we propose that two addi-tional tricritical points occur in the calcic portion of

the albite-anorthite system. One tricritical point isstable and associated with the Cl e Il transition.The other tricritical point is associated with the 1l? Pl transition and may be either metastable (as inFe-Al) or stable. In the remainder of the paper wewill explore the application of tricritical phenomenato the calcic portion of the system NaAlSi3O3-CaAl2Si2Os. We will derive a model subsolidus Z-Xphase diagram for the plagioclase system and devel-op predictions of phase transition/exsolution se-quences and limits of metastability. Finally, we willdiscuss the voluminous TEM observations of pla-gioclase microstructures in light of our model phasediagram.

A phase diagram for the calcic, subsolidus portionof the system NaAlSi3Os-CaAl2Si2O3

Location of Cl e Il and ? PI transitions inT-X space

Although there is some uncertainty as to theprecise temperatures of the C7 ---> Il and 1i - Pitransitions in anorthite, single crystal X-ray struc-ture refinements performed at elevated tempera-tures and heating stage observations of APBs madewith the TEM may be used to infer the transitiontemperatures. Similarly, the temperature-composi-tion path traced by these transitions into the binarysystem and the positions of the tricritical points canbe inferred from TEM observations of natural sam-ples.

The Cl ? Il transition. McLaren and Marshall(1974) synthesized anorthite near its melting tem-perature (1530'C), cooled the sample to 1400'C andannealed it for 8 days. The sample was examined inthe TEM and found to contain b APDs - 0.2microns wide arising from the Cl to Il orderingtransition. Single-crystal X-ray structure refine-ments of anorthite performed at elevated tempera-tures have been carried out at 400'and 830"C byFoit and Peacor (1973), at 250'C and 1430"C byCzank (1973), and at room temperature on ananorthite annealed at 1530'C for 30 minutes (Brunoet al., 1976).In all cases the diffraction symmetrywas 11 and the results of structure refinements werecompatible with the presence of 11 order. Theseobservations indicate that the Cl to Il transitionoccurs above 1430"C for anorthite. The TEM obser-vations of Nord et al. (1973) provide a point incomposition-temperature space for the Cl to 11transition. In compositionally zoned plagioclase(-Anso) from the groundmass of lunar breccia

GROVE ET AL,: PHASE TRANSITIONS IN CALCIC PLAGIOCLASE 5 l

60335, Nord et a/. found the transition from a regionof single-domain Ii to multi-domain Ii, indicatingthat this plagioclase passed through the Ci a Iitransition as it crystallized from the melt. Theexperimental results of Walker et al. (1973) deter-mined that the groundmass crystallization tempera-ture and hence the Cl P 11 transition temperaturefor 60335 was -1275' to 1225"C (Figs. 8 and 9a).

The I1 e Pl fiansition. Several investigators(summarized by Heuer et al., 1976) have usedheating stage experiments in the TEM to determinethe temperature of the 11 ? Pl transition in anor-thite. They found that the diffractions characteristicof the Pl space group and the domain contrastbecame weak at 200'to 250"C, but that the domainmorphology reappeared unchanged after cooling.This behavior persisted up to the highest tempera-ture studied (575"C) by Mtiller and Wenk (1973) andto a temperature between 400o and 623"C in ananorthite studied by Lally et al. (1972). Only afterprolonged heating at higher temperatures were thePl domains destroyed. Mtiller and Wenk found thatirreversible changes in domain morphology oc-curred between 575' and 1200"C. Moreover. the

Fig. 8. Proposed temperature--composition diagram for thecalcic portion ofthe system anorthite-albite. Temperature oftheCl e n transition for Ane6 is inferred from TEM observations ofNord er al. 1973. The temperature of the Il c Pl transition isuncertain and may lie within the range of temperatures (1000'-700'C). Dashed lines are metastable extensions of the higher-order transitions within the miscibility gaps. Dash-dot lines arethe traces of the spinodals on the free energy curve of the lowsymmetry phases. The positions and temperatures of thetricritical points are inferred from TEM observations.

P t p

o.5^ o n

P'

9 P r4

Temperoture "C

Fig. 9. (a) A possible topology of the temperature-compositiondiagram for the calcic portion of the system albite-anorthite.This version contains a region of stability for the ll-Plconditional spinodal. Compare with Fig. 8. (b) Pressure-temperature relations for the univariant equilibrium Pl + Il +Cl (3@-I-P) and the two tricritical points; Cl-I| (C-I t.p.) andIl-Pl (l-P t.p.). The T-X diagram above corresponds to anisobaric section at P1. The T-X diagram in Fig. 8 wouldcorrespond to an isobaric section_ at P2, a pressure above theinvariant point terminal to the Cl-Il-Pl univariant reaction. TheIl-Pl tricritical point is metastable under these conditions.Temperatures are approximate and pressure increases from P1 toP2.

high-temperature, single-crystal X-ray structure re-finements of Foit and Peacor (1973) indicated thatAnes plagioclase has the primitive anorthite struc-ture at 830'C, despite the weak intensities of the Pldiffractions. Heating experiments on Anes by Foitand Peacor (1967) determined the temperature ofthe Il to Pl transition at 1000'C. Differences in thedegree of order in the anorthites studied, as well ascompositional variability have affected the transi-tion temperature. It is inferred that the transition in

4 ' , t

,"/,


Anlso lies between 1000'C and 700'C (Figs. 8 and9a).

Modulated structure of intermediate plagioclase

The periodic antiphase structure characteristic ofAn33 to An75 plagioclase will be treated as a thermo-dynamically stable phase which has ordered withthe 1l structure. A minimization of free energy isattained by transforming from a Cl disordered formto the 1l ordered structure, and an accompanyingminimization of lattice strains is produced by order-ing on a periodic antiphase structure. Structuraldistortions are associated with Al-Si ordering andthe periodic antiphase structure compensates forthe overall strain by reversing the distortions atevery APB. Fleet (1981) has rationalized the com-positional dependence of the orientation of theperiodic antiphase boundary by calculating thecrystallographic orientations for which dimensionalmisfit and coherent elastic strain energy of orderedanorthite and disordered albite structures are mini-mized. The periodic APB represents the best fitbetween antiordered 11 domains for a particularcomposition. McConnell (1978) envisions periodicantiphasing as a structural resonance in which theinteractions between ordering and displacements ofthe appropriate symmetry (the displacement on theAPB is the translational operation ll2 [110]) interactto minimize free energy. Although these two viewsof the intermediate plagioclase structure differ inseveral important respects, both involve the genera-tion of 11 domains. We wish to emphasize the 11-like characteristics of this structural form and treatit as an antiphased variant. Whether or not it is aspinodally unmixed modulation (Fleet, 1981), or acompositionally homogeneous ordered structure(Korekawa et al., 1978) is a question that is notresolved in this study.

The periodic antiphase structure may be pro-duced through at least two mechanisms. (l) Plagio-clase with Cl symmetry orders to 11. Continuedordering in the 11 domains occur and the thermody-namically stable form is the modulated intermediateplagioclase structure. TEM microstructures charac-teristic of this transition would be the existence ofmany small domains (<10004 in size) which con-tain the periodic antiphase structure. (2) Spinodaldecomposition of preexisting 1l plagioclase into twocompositionally distinct phases may result in theformation of an /l-ordered phase with periodicantiphase structure. Upon decomposition the reor-ganization of bonds in the Na-rich phase requires

solid state diffusion over large distances, and an 11structure with periodic APBs forms as the stablephase. TEM microstructures characteristic of thistransition variant would be oriented intergrowths ofan Na-rich periodic antiphase structure phase and aCa-rich ordered phase.

Location of the tricritical points in T-X space

Compositionally zoned plagioclases grow duringfractional crystallization of basaltic liquids and pro-vide a continuously varying medium which recordssubsequent ordering and decomposition sequences.Thus, textural variations in a single zoned feldsparcrystal can be used to infer transition and decompo-sition reactions. Microstructures observed in pla-gioclase from the Ardnamurchan and Nain intru-sives (Grove , 1977) the Stillwater complex (Nord etal.,1974) and from Lake County, Oregon (Mclarenand Marshall. 1974: Wenk et al.. 1980) will be usedin the following discussion to locate the tricriticalpoints in temperature-composition space.

The tricritical point associated with the CI e I1transition. Examples of decomposition which waspreceeded by ordering below the metastable exten-sion of the Cl -+ 11 transition are found in labrador-ites from volcanic and plutonic environments.Mclaren and Marshall (1974, Fig. 2) and Wenk etal. (1980) noted a domain texture in Lake Countylabradorite of /1 and Cl. Similar domain morpholo-gy is found in Ardnamurchan plagioclase (An66-7a,Grove, 1977, Fig. 5). These morphologies are pro-duced when plagioclase that is initially Cl is cooledinto the conditional spinodal. Ordering to 11 occursbelow the metastable extension of the Cl -+ 11 lineand fine scale APDs (-100 to 2004 in width) areformed. Subsequent decomposition of the disor-dered phase occurs on the APBs. In AnTs Ardna-murchan plagioclase the 1l phase has ordered fur-ther upon cooling to the periodic antiphasestructure. Plagioclase with composition An75-An73from Ardnamurchan contains large micron-sized 11domains separated by APBs (Grove, 1977, FiS. 6).The AnTs first ordered to 11 , then decomposedwithin the conditional spinodal and produced ori-ented lamellae. In regions of bytownite (Anao-sr)large APBs are decorated with an Na-rich Cl phase(Grove, 1977,Fig.7). The bytownite lies outside thespinodal in a region of metastability, and the disor-dered phase nucleated heterogeneously on theAPBs. Subsequently the Cl phase ordered to theperiodic antiphase structure. These observationsplace the tricritical point for Cl ---> Il near An75.


The temperature of the tricritical point must bebetween the temperatures at which the plutonicplagioclase crystallized (- 1100'C) and the tempera-tures at which the metamorphic plagioclase crystal-lized (-650"C, discussed in a later section). Experi-mental evidence (Orville, 1972; Gol{smith, 1982)places the point above 700'C. The results of anneal-ing experiments performed on a volcanic labrador-ite (Wenk, 1978) set further limits on the tempera-ture of the tricritical point. Wenk found thathydrothermal annealling at 850"C and 2000 bars for50 days caused the labradorite to unmix into 1i andintermediate plagioclase. A temperature of -900"Cfor the Cl e n fiicritical point is consistent with allthe above constraints. Figures 8 and 9a show ourestimated position of the tricritical point on a pro-posed ?-X diagram for the calcic portion of thesystem NaAfSi3Os-CaAl2Si2Os. By analogy withFigure 7, Figures 8 and 9a contain appropriatecurves which must radiate from the tricritical point.

The tricritical point associated with the tI e pitansition. A tricritical point associated with the 11=r Pl transition in plagioclase must occur at atemperature below that of the Cl ? 11 tricriticalpoint because the Il e Pl ffansition curve alwayslies at a lower temperature than the CI = titransition curve for a given composition. BecauseHuttenlocher intergrowths involve coexisting Pland 11 feldspars, we associate Huttenlocher inter-growths with phase decomposition at femperaturesbelow the Pl ? 1l tricritical point. Data on theHuttenlocher intergrowths therefore permit approx-imate location of the Pl = n tricritical point in T-Xspace. Huttenlocher intergrowths are composed ofplagioclase of composition -An66 coexisting withplagioclase of composition -Aq7. We thereforelocate the 11 :: Pl tricritical point midway betweenthese compositions at -An77. Huttenlocher inter-growths are observed in metamorphic plagioclasecrystallized al T - 650"C (Grove, 1977). We there-fore locate the 11 A Pl tricritical point above thistemperature but below 900oC, the temperature ofthe Ci ?2 /i tricritical point. Figures 8 and 9a showour estimated position of the tricritical point onproposed T-X diagrams for the calcic portion of thesystem NaAlSi3OpCaAl2Si2Os. By analogy withFigure 7, appropriate curves are shown radiatingfrom the tricritical point.

Figures 8 and 9a illustrate two possible interpre-tations of the ti e pi tricritical point which areboth consistent with existing data for plagioclasefeldspars. In Figure 8, in analogy to phase relations

in the system Fe-Al, the 1l € Pl tricritical point isshown as metastable. The experiments of Gold-smith (1982) show no evidence for stable B/ggild orHuttenlocher gaps at P :7000-9000 bars; I = 700-1000'C but are consistent with a wide Cl-Pl two-phase region at temperatures as low as 700'C.Goldsmith's experiments therefore support Figure 8in which the 11 € Pl tricritical point is metastable.Data of Wenk and Wenk (1977), on the other hand,suggest a stable Bdggild miscibility gap and there-fore support Figure 9a in which tne fi ? Pitricritical point is stable. Compositional data oncoexisting calcic plagioclase feldspars (Fig. l) areconsistent with the topology of either Figure 8 orFigure 9a. As mentioned earlier, a stable ti e pitricritical point could be confidently endorsed if theexistence of stable coexisting Ana5 * An65 feldsparcould ever be demonstrated.

Model phase diagrams. Figures 8 and 9 summa-rize our best estimates of subsolidus phase relationsin the calcic portion of the albite-anorthite system.Existing data are consistent with either of thesephase diagrams. It may turn out that one or theother is correct for P-T conditions that are presentin planetary crusts. Alternatively, Figure 8 maycorrectly illustrate phase relations at high pressureswhile Figure 9a correctly illustrates phase relationsat low pressures. If the latter interpretation iscorrect, then the Il e Pl tricritical point, the Cl ?1l tricritical point, and the univariant three-phaseequilibrium among Cl,Il, and Pl plagioclase musttrace out curves in P-T space as shown in Figure9b. T-X diagrams with a topology like Figure 8would be correct at pressures such as P2 in Figure9b; T-X diagrams with a topology like Figure 9awould be correct at lower pressures such as P1 .

Microstructures in calcic plagioclase: TEMobservations and interpretation using model

phase diagrams

In this section voluminous data on microstruc-tures in calcic plagioclase are reviewed. A varietyof TEM observations can be explained readily bystable and metastable ordering and decompositionsequences predicted by Figures 8 and 9a.

Calcic plagioclase in igneous rocks

The slow cooling rates experienced by plagio-clases from large igneous intrusives produce dis-tinctive TEM microstructures. Grove (1977) foundsparsely distributed lamellae of calcic 11 plagioclasein An56-7e Nain plagioclase (Fig. 10a). This compo-

54 GROVE ET AL.: PHASE TRANS/TIONS IN CALCIC PLAGIOCLASE

Fig. 10. TEM observations of plutonic plagioclase. (a) Nain plagioclase An66 70 contains isolated lamellae of the calcic phase. DF, g: 013. (b) Nain labradorite An72, Two phase intergrowth of Cl and 1l formed by cooling into the conditional spinodal from thestability field of Cl. Ordering on 11 produces APBs, which on decomposition are wetted by the Cl phase. Because the 1l periodicantiphase domains overlap, it is not possible to see the regions ofCi. Light regions are inferred to be Il, dark regions ar-e inferred tobe Cl . DF, C : 022. (c) Stillwater bytownite Ans2. Zigzag b APBs have served as sites for heterogeneous nucleation of Cl phase. TheCl phase has transformed to the periodic antiphase structure as seen by the periodicity on the APB (upper center). Within 1l ordereddomains metastable Pl and 1l decomposition has occurred. Regions adjacent to the APB are devoid of these precipitates. DF, e :

031. (d) [00] selected area diffraction pattern of Stillwater bytownite. The diffuse c reflections indicate Pl symmetry. b+ is horizontaland c* is vertical.


sition lies in the metastable portion of the Cl ---> Iiconditional spinodal. The ordered calcic phaseformed by heterogeneous nucleation, which oc-curred randomly on defects in the disordered phaseand produced the observed texture. In slightly morecalcic bulk compositions An72_75 (Fig. lOb) themicrostructure wds produced by ordering below themetastable extension of the Ci -- Ii transition. Thesmall APD size was on the order of the spinodalwavelength, and the disordered phase wetted theAPBs, producing a connected intergrowth of Ciwith isolated 11 cores. Subsequent ordering of 1idomain cores on the periodic antiphase structureproduced the observed superlattice periodicity.

Some_slowly cooled plutonic plagioclase undergoCl ---> 17, grow with large Il ordered domains andthen enter the Cl-Il two phase region as coolingcontinues. The compositions of these plagioclases(An7s to An3s) lie outside the Ci-li conditionalspinodal, but inside the two phase region, so thatheterogeneous nucleation of the Ci phase on the IiAPB can occur. Textures consistent with this tran-sition sequence are found in bytownite from Nain(Grove, 1977, Fig. 9) and the Stillwater igneouscomplex (Nord et al., 1974). The Ii ApBs arewetted by Cl (Fig. 10c) which later transformed onthe 11 periodic APB structure. Further decomposi-tion occurs on cooling and a spinodal texture formswithin the 1l domains, which consists of intergrownPi and Ii with the periodic antiphase structure (Fig.10d). The transition sequence described above forbytownite (Anao+s) would be predicted by Fig. 8.Ordering from Ci to 1i occurs and ApDs form.Subsequent cooling places the Ii domains withinthe conditional spinodal for Pi and /i, and decom-position occurs within the 1l domains. Orderingwithin the conditional spinodal forms pi domainsand decomposition of an Ii Na-rich phase. Thetopology suggested in Fig. 8 interprets the /i + pitwo-phase intergrowth as metastable with respectto an equilibrium assemblage of Ci + pl, whichcould form by nucleation and growth. The lamellarorientation of the spinodal texture parallels thatpredicted for the minimization of interfacial strainenergy (Williame and Brown, 1974). The pi do-mains are less than l00A in width and are elonsateparallel to (010.1. This Pi APB orientation does-notcorrespond to one in which the strain energy isminimized and is not an orientation for coarseningof lamellae.

B/ggild intergrowths (McConnell, 1974b) are

found in labradorites (Anso-;s) and contain coexist-ing Ana5 and An65 plagioclase. These intergrowthssuggest that the C1-11 conditional spinodal extendsinto the sodium-rich portion of the diagram (Fig. 8,9a). The B/ggild intergrowths form by orderingwithin the conditional spinodal below the metasta-ble extension of the Cl ---> Il transition. Ordering ofCl creates small 1l domains and abundant APBs.Spinodal decomposition to Cl + n folows. Inlabradorites that contain small APDs the Na-richdisordered phase wets the 11 domain boundariesforming an interconnected region of Cl and isolatedvolumes of the 11 phase. In labradorites with large11 APDs, decomposition results in oriented lamellarintergrowths. The calcic phase or both phases ree-quilibrate at low temperatures and order on theperiodic antiphase structure. TEM observations ofBlggild intergrowths (Hashimoto et al. 1976,Fig. l;Wenk and Nakajima, 1980, Fie.2c) show the mot-tled texture characteristic of 11 APB ordering fol-lowed by decomposition below the metastable ex-tension of the Cl ---> 11 line in the conditionalspinodal. Many intermediate plagioclases show evi-dence of this transition sequence. For example, thetextures observed by Grove (1976, Fig. lc,d) inmetamorphic An66, the TEM microstructures ob-served by Mclaren and Marshall (1974, Fig. 7) inAn32, the textures in Thetford An:g (Fig. 4d) and theAugusta An5s (Fig. 6b) can be produced by orderingto 11 within the Bdggild conditional spinodal fol-lowed by unmixing of a Cl phase that wets thepreexisting 11 APBs.

Calcic plagioclase in metamorphic rocks

The Thetford two-plagioclase intergrowths areevidence that the An3e-Aqs assemblage is stableand that the Huttenlocher intergrowths formedmetastably. As discussed in the previous sectionmicrostructures observed in the An3e phase may bemetastably precipitated Cl + n. The G-Xdiagrams(Fig. 11) show many possibilities for metastablebehavior, and suggest a transition sequence thatproduced the TEM microstructures observed inThetford plagioclase. Equilibrium Cl (An3e) and Pl(Anas) plagioclase nucleated and grew during pro-grade reaction. Continued plagioclase growth sur-rounded the An3e grains, removing them from reac-tion and the calcic plagioclase grew into the Na-richregion of instability. Spinodal decomposition oc-curred within the Il-Pl conditional spinodal andproduced the metastable Huttenlocher inter-


o

oL

oF

-To

- T b

-Tc

C o m P o s i t i o n +

Fig. l l . Free energt (G)-composit ion diagrams showpossibilities for metastability. For selected isotherms on theschematic T-X diagram G-X diagrams are constructed below. T.shows G-X curves at a temperature below the CI-II tricriticalpoint. Tb shows G-X curves above the P1-11 tricritical point, butbelow the intersection of the Pl stability neld with rhe two-phaseregion. T" shows G-X curves at a temperature below bothtricritical points. Tangent planes show the equilibriumcompositions of two-phase assemblages that define themiscibility gap. Vertical marks below the tangent surfaceemphasize the compositions. Vertical marks above the G-curvesshow the stable and metastable transition points and spinodals.

growths. Finally, decomposition of the An3e oc-curred on cooling below the metastable extension ofthe Cl ? 1l ordering transition and producedmetastable B@ggild precipitation.

Discussion and conclusions

Through consideration of the G-X curves inFigure I I it is evident that metastable phenomenashould be common in the plagioclase system. TheZ. isotherm is constructed to pass through the Cl <=

Ii conditional spinodal. After ordering to fl, com-positions lying within the spinodal decompose todisordered Cl and ordered 11 plagioclases. The ?6isotherm intersects the two-phase field below the 1l? Pl transition. The conditional spinodal moves toprogressively more sodic bulk compositions withdecreasing temperature, and a large region existsover which the 11 phase is metastable with respectto Pl * C|. A bulk composition on the Ca-rich sideof the spinodal can transform to Pl + Cl only byheterogeneous nucleation. At ?" the metastableconditional spinodal for 1i-pi is intersected.Bdggild intergrowths are produced by ordering on11 followed by decomposition to a metastable as-semblage of Ci and Ii, where the Ci phase haswetted the 1l domain boundaries. The textures ofthese labradorite intergrowths (see Hashimoto,1976,Fig.1, Wenk and Nakajima, 1980, Fig. 4) andAugusta labradorite (Fig. 6b) are strikingly similarto those produced by the same transition mecha-nism in more calcic bulk compositions (Fig. 10b).The inability of some investigators to find 2e plagio-clases in B/ggild intergrowths (Hashimoto et al.,1976 vs McConnell, 1974b) is consistent with theformation of a periodic antiphase structure in thecalcic 11 phase and wetting of APBs by the disor-dered Cl phase. The Cl phase is the member of theB/ggild gap that lacks the e reflections.

As proposed in the preceding section the topolo-gy for the phase diagram in Figure 8 also showsHuttertlocher intergrowths as a metastable assem-blage of coexisting Pl and 11 periodic antiphaseplagioclase which should unmix to Cl * Pl. Thepersistence of Huttenlocher intergrowths is a con-sequence of the extremely slow interdiffusion of theNaSi and CaAl couple, and the existence of largeactivation energy barriers for heterogeneous nucle-ation. The metastable assemblage forms by spino-dal decomposition, and further equilibration canoccur only by nucleation and growth, as in the caseof the Thetford plagioclase assemblage.

Our proposed phase diagrams are consistent withthe 700'C, 2 kbar chloride exchange equilibria ofOrville (1974). He found that plagioclase coexistingwith aqueous chloride solutions could be modeledas disordered albite over the range An6 to An55 andas ordered plagioclase over the range Aq5 toAnroo. Intermediate compositions implied an equi-librium of two phases. Similarly, Goldsmith (1982)found evidence for an even wider gap at 700'C andP : 7 to 9 kbar in his experiments on the plagio-clase-zoisite-kyanite-quartz-H2O equilibrium. Our

+I

(,)oUoon.o-oo

GROVE ET AL,: PHASE TRANSITIONS IN CALCIC PLAGIOCLASE 57

results and the temperature-composition informa-tion for peristerites (Carperiter, 1981) may be com-bined to infer low temperature equilibria. At tem-peratures below 400"C, there exists a univariantequilibrium among Pi anorthite, Ci disordered an-desine and Cl ordered albite. Below this univariantline Ci ordered albite and Pi anorthite are theequilibrium phases. This assemblage may be diffi-cult to find in metamorphic rocks and would beproduced by reaction mechanisms which involvednucleation.

An important consequence of the existence of thetwo-plagioclase region under metamorphic condi-tions is that in many assemblages there is an addedconstraint on the chemical potential of the plagio-clase components. Feny (1979) exploited this con-straint in his analysis of solid-fluid equilibria in theBlue Rock Quarry, and Spear (1980,1981,1982) hasexplored the consequences in the graphical analysisof amphibolite assemblages. Consideration of suchan equilibriurn constraint on plagioclase assem-blages may assist in determining the temperature,pressure and fluid composition during metamor-phism.

To conclude, plagioclase microstructures ob-served by TEM have presented mineralogists with aconfusing variety of textural relations. Using theproposed phase diagram, our interpretation impliesthat much of the submicroscopic observational datamerely outlines the limits for metastable behavior,and shows the existence of two metastable condi-tional spinodals. Submicroscopic textures delineatethe positions of conditional spinodals and recordmetastable equilibria. We note that the topology ofany phase diagram may vary according to T, p, and4 (the degree of ordering in the plagioclase) and thephase diagrams suggested here are only one setamong a range of possible topologies. Carpenter(1981) shows a range of possibilities for the perister-ite spinodal, and a similar line of reasoning appliesto conditional spinodals in the calcic portion of thesystem. Metamorphic plagioclase assemblages, onthe other hand, may contain information on theequilibrium phase coexistences, when evidence forheterogeneous nucleation and exchange equilibriumwith coexisting silicates can be shown. This evi-dence requires careful petrographic and analyticalwork as these plagioclase assemblages are oftendifficult to identify and may be complicated bycontinued reaction that departs from equilibrium.The results of this study serve to re-emphasizeseveral aspects of equilibria in the plagioclase sys-

tem. Plagioclase is extremely susceptible to decom-position to metastable intergrowths which may thenpersist for long periods (>10e years). The submicro-scopic textural information recorded by plagio-clase, while useful in recording the time-tempera-ture path followed during the sample's formationhistory, may often be a record of metastable ratherthan stable equilibrium. Traditionally, mineralogistshave thought that equilibration reactions within theplagioclase system involve ordering. It is ironic thatunder equilibrium conditions of low temperatureregional metamorphism a region of stability fordisordered andesine and Pl bytownite exists. Atlower temperatures, however, the proposed or-dered albite * Pl anorthite assemblage presumablyexists.

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stability relations in H2O-CO2 fluids at 5000 bars: an experi-mental and SEM study. Geochimica et Cosmochimica Acta. inpress.

Allen, S. M. (1977) Phase separation of Fe-Al alloys with Fe3Alorder. Philosophical Magazine, 36, 181-192.

Allen, S. M. and Cahn, J. W. (1976a) Mechanisms of phasetransformations within the miscibility gap of Fe-rich Fe-Alalloys. Acta Metallurgica, 24, 425-437.

Allen, S. M. and Cahn, J. W. (1976b) On rricritical pointsresulting from the intersection of lines of higher-order transi-tions with spinodals. Scripta Metallurgica, 10, 451-454.

Bown, M. G. and Gay, P. (1958) The reciprocal lattice geometryof the plagioclase feldspar structures. Zeitschrift fiir Kristallo-g raph ie , l l l , l -14 .

Bruno, E., Chiari, G. and Facchinelli, A. (1976't Anorthitequenched from 1530'C I. Structure refinement. Acta Crystallo-graphica, 832, 3270-3280.

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Manuscript received, February 24, 1982;acceptedfor publication, September 6, 1982.

phase transitions and decomposition relations in calcic plagioclase · phase transitions and...

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