entbalpies of ordering in the plagioclase feldspar solid...

W16-7037/8.5/53.00 + .oo

Entbalpies of ordering in the plagioclase feldspar solid solution

M. A. CARPENTER and J. D. C. MCCONNELL

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England

A. NAVROTSKY

Department of Chemistry, Arizona State University, Tempe, AZ 85287, U.S.A.

(Received July 17, 1984; accepted in revised form January 14, 1%)

Abstrnet-Enthalpies of solution in lead borate at -7WC have been measured for 36 natural and heat treated plagioclase feidspats. The samples made up two series, as character&d by TEM and XRD. A “low” series contained the natural ordered material and a “high” series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions:

low albite - high albite: -3 kcal/mole “e” structure -4 Ci high albite structure: - 1.4-2.8 kcal/mole Ii structure - Ci high albite structure: -0.7-1.9 kcal/mole Ii stmcture equilibrated at low temperature - Ii structure equilibrated at high temperature: - 1.8-0.8

kcal/mole.

AI& data for the high series overlap with the data of NEWTON et nl. (1980) for synthetic high structural state plagioclases except in the composition range -AnwAniao. They are consistent with an in~~~~tion of the solid solution as being composed, at high temperatures, of two ideal (zero heat of rn~n~ segments, one with CI s~rne~ and one with Ii some, and having a non-first order (~ntinuous) order/c&order tmnsfo~ation between them. The low fries can also be separated into two distinct trends, for I1 and “e” structures.

Values of the enthalpy change due to disordering (A&,.& also show a number of systematic trends. Firstly, the values for e - Ci are larger than for Ii - Ci in the composition range where both e and Ii structures are ol~rved (--A%$-Anrr). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in AH,,,+ which occurs at -An=, may be important for the origin of the Baggild miscibility gap. Thirdly, the large entbalpy change of the e structure, due to ordering, may be sufficient to stab&se it relative even to a mixture of low albite plus anorthite. VaIues for the enthalpy change on disordering Ii anorthites and bytownite-s to a Ci structure have been estimated by assuming that the Cf solid solution is ideal (non- enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure

anorthite.

P~_AGIOCLASE EELDSPARS are among the most abun- dant minerals of the earth’s crust. They are also among the most extensively studied but, due to their remarkably diverse subsolidus ordering and unmixing behaviour, our understanding of their thermodynamic properties is rather limited. A ~omp~hensive model of the solid solution which took full account of both ordering and mixing ef%ct.s would undoubtedly be of great value to petrologists, in geothermometric and geobarometric calculations for example, because of the numerous heterogeneous equilibria in which plagioclases are involved during the evolution of igneous and metamorphic rocks. An important step towards setting up such a mode1 was the solution calorimetric study of mixing in high structural state plagioclases by NEWF~N et al. (19801, which supplanted the earlier results of KBACEK and NEUVONEN (I 952). Newton ef al. found small positive deviations from ideal mixing in synthetic samples prepamd at 1200°C, ZOkb, from glasses. The high temperature plagioclase

solid solution is unusuaf, however, in that the end members have different translational symmetry, with an Al/Si order/disorder transformation occurring at an intermediate composition. This change in order between ordered anorthite and disordered albite may be responsible for the observed non-ideality (CAR- PENTER and MCCONNELL, 1984). At low temperatures the solid solution contains three different ordered structuresz low albite, the intermediate or e structure and anotthite, and to treat the overah mixing properly it will be necessary to define the ordering of each of these both as a function of temperature and of commotion.

A thorough inv~i~tion of ail aspects of the ordering and mixing of plagioclase feldspars, sufficient to generate quantitative thermodynamic properties for all temperatures and compositions, would obviously be a daunting task. However, a judicious choice of thermochemical measurements on selected samples might prove instructive at least as to the relative stabilities of tbe separate su~~~u~s and the relationships between them. With this more lim-

947

M. A. Carpenter, J. D. C. McConnell and A. Navrotsk)

Teblr 1. Provenance. srructurrl stat(~Q Qnd aicrosrructurrs of Mtur81 plrgioclrrcs wed ia this study. All crc~pt 8197% vese usad for solutim calorisst?y. References are given to prcvioua dQQcriptions of the same 8-i~~ or

similar uterial from the *em locrlitier. Ct * calcite, Di - diopQide, Pm - FPQQaite, Sp - apinel, Aa = morthite. Phi = PhlOgOpitc. Pr = prQhnitc, Mu - SUICOVite, Gt - 8QmQt. U.S.N.X. * UnitQd States #ation& Husaum.

SMQJi.2 source Locality Ueecription structure Mieroistructure RQfcrQnccr

hr8.3 Cl,+$tah in vu** of tharmlly m?tQmorphoQed linrntone: Di- Ct-An-Phf-SP-

(Prf- eklt

Pl (#harp b,c, d rcflcctionr

Volcanic ejecta, rberlM11y meteaorphoscd limeorone: Pa:- An-Phl-Ct-Sp

Py (rbarp b.c, d rcflQctionQ)

Cranulitt (cc- Pl (&a+~ b-c. kn intergrOWth) d raf leerions)

No b domains, c dauina rcvcral urn in diumrtcr (MUlIar h Uenk. 1973)

Palmeda

nDnta sarv

115G82a

217Ma

101377r

cryrta1

B*Y

42771b

i&e co

67799

1104401*

91413b

T-12-22*

91315c

9749G

Hawker mineral coIl9ction no. 3776

Pmmda Alp, Pama VLlley, Auntrio

Gay (1953, 1951) UQinwright 6 StQrkcy (1971) ?mllQr 6 w*nk (1973) MUllc1 et Ql. (1973) Frey Qt al. (1977) Adlhart Qt al. (19SOa.b)

Nc b dmina. c dooll‘nr SGD- 3a3$ in diwr.Qr (Czmk at al. 1973)

liarker tiin4?r2ll collection

VrQuviur , ItQly

Gay (1953) Elmw ct al. (1962) K&r.r et .i. (lb621 Cuak et al. (1973) GruadF & Brown 0974) Bruno Qt al. (1976)

xi (harp b rQflactionQ, diffum aad Qtrakad c r*fl*ctionr)

Ii (*hrp b rrf lw!.tionQ, diff+mQ ad 3trmkQ.d c rQflictfonr)

IT (rlurp b rQfl*ctionQf

lkrker coll*ctim no. 115082

s. of Elultyra NymQlmd

1QolQt.d b antiptuQQ bound*risr

This iQ Q-As tlo. 902 of ua8lxQr 1192r>.

Harka collection no. 21704

Vlakfmtixin, Bwhtnld Cn*rlQX, TrmrvaQl

AoorthoQitt

Anorchorite frm large inclu*ion in Brwer River &abbro

Aaarthoaite

Iwilated b QntipiwQ beusdQriQ8

Similar **la fra St. LouiD co., lnri.*att, deQeribQd by cly (19§3) Ilest at Ql. (1966)

Yracek 6 NQuvar,en (1952) Gap (1953, 19%) Cerp~nrer & I(eCom~ll 11984) Sa@Qr with diffartnt eqorition fron Qme locality dmeribrd by &iaVriBht (1%9), M‘%QtQn &

EhrQbell (1974). Gruady h Brown (19741, Wary 6 Weak (1978). NiQQm (19th)

SqlQ with aimilat cosositioa from BuQhwld darcribed by WeLarca (1970)

Harkar collectioa ilo. 1013T7

Silver Bay, L&r Superior, XilLmrotr

P. Gay

Hoslyk, BushvQld CaplQX, E. TrmrvQQl

Norite “a” (aharp Q, f r.flQCtiOM)

Narkmr CollQctim no. 42771

Ii (WY rliCtly QlmQtQ b rQflQCtiOW)

StewQrt Qt 41. (1966) ncL.arm k nQZQbQl1 a9741 It&my I UQ& (1978) WQnk l t r1. (19BG) WeaL 8 Naluji6~ (19%) TQgQi 6 lhrckrvr 0981)

P. Gay, U.S.N.M. collcctioa no. 115900

LILsga

phQl%oGrysts in basalt

NW 11976) Siril&r QQI@Q &tc+ikd W&II* L Nakajiss (19SG)

SluarSutd intrusion, E. GrcNnland, drill COrQ ‘I, 968’ (Bidden ZOflQ)

Gabbro “Q” faarp L,

f raflQCtiOW. *oa VQry diffuQQ f raflrctionr)

Earkar cotlQction no. 118724

Harker collection no. 67796

Ilaker CollQtticd no. 11046

Barker coll*otion no. 91413

Anartboeitic grmulitc

Anorthoritc

“C” (Dh.Kp Qr f r.flQCtiOnQ)

“C” (*bmp . . f rQf1QCtiWbQ)

lbmptmaur piaD, , mei- phwa dariss

w rmeiphQaQ daninr

ncconu*lf (197&l Capa~tar 6 IMon~Qlf (1984)

Bioac fX96b. 1965)

Ouluth, ximlQ*ctr

StirlinS Pill, SraLm Bill. Ha South UQlQ!.

Aaphibolitc “C” fDtwp l t f rQflQCtiWQ>

x&By gmin* coI)‘cIiIp low r*1ituds

Philpotts (1966) BelleAW DQ*WlUi8rl .t.‘. Qmba!

Rscrystalli8ed

(sync*ctomic norite

t*cry*calli*rtioa at hi&h prQQ.ure)

lbrkrr collection no. 9bb60

"Qw (wQk and diffum Q

raf 1QctioaQ)

Biarl (1964. 1965) SlidaB (1976) (.rplQ P2)

RQrkor coll~etioa no. 91315

Worth Nine, Broken Hill. New South li*1**

p. WY, NQad of LivLlQ clear cry*tlls “a*‘ (wry

U.S.N.X. Rock Creak, fro5 pQ_titc diffueQ . no. 97090 Mtcb*ll Co. * reflection*)

Krwak 4 Nsuvonen (1952) Gsy $1956) Phillipa Qt al. (1971)

N. Carolina

Ordering enthalpy in plagioclase 949

SUplC Source Locality structure niero#tructure Kef*rmce*

Uaokb Earvard University dIl*rnl collactiori no. 97606

Aulia Ab

Barker miraral collection

8797% Urkcr collection no. 07975

Nikaj iu Prof. R. c. NeVt0n

buk Nine. B&*rsville. l4itcbe11 co., 1. Urolitu

hlia Courthoure, Arlia Co., Virginia

Sittcmpundi, Plldras

Clear crystal8 fro0 pegmntite

Clear crymtals from pegvtite

!3ytmmitc- edcnitc gnciss

abbroic nodule*, ejected from VOlCUlO

Extramlg veti md diffrue c reflections, not alii~p~ detectable

Ci low albite strWzture

Ii (sharp b reflections. diffuse and- Itretied c rcfl*ctions)

11 (ghrrp b reflections. streaked c reflections)

No obviow sisns of luolution

Kracek b Ncuvvmn (1952) CW (1956) c&l& 1 Llm (1974) McLarm (1976) (described faint adulation)

Wny ref*renc*a, c.*. see: PWguMD at al. (1958) Ribbe at al. (1%9) Smith (1974) liarlow 6 nrmm (1960) Kibbc (1983) Kroll 6 Kibbc (1963)

Subr-iu (1956)

char1u et al. (1978) Newton et al. (1980)

ited objective in mind we have undertaken a prelim- inary study of the order/disorder properties. In this paper we present the results of heat of solution measurements made at -970K in Pb&O5 on natural “low” and heat treated “high” plagioclase samples.

It is not possible to reproduce the lowest structural states seen in natural plagioclases on a laboratory time scale, and a study of ordering properties must depend on ordered crystals separated from rocks. For solution calorimetry, particularly pure and homogeneous material is required so that considerable em- phasis has had to be placed on selection and purification; the final suite of plagioclases used for the solution measurements came from a very wide range of geological environments. In addition, conventional powder X-ray diffraction methods of characterisation would not, alone, have been adequate either to ascertain the homogeneity of the crystals or to divide them into more specific categories than simply “high” or “low” structural states. Every sample has therefore also been examined by transmission electron microscopy (TEM).

Enthalpy differences associated with changes in cation order between the natural and heat treated samples have turned out to be significant at every composition examined. The implications and rami- fications are numerous but we have restricted our analysis only to the most immediate issues. Readers are referred to Reviews in Mineralogy, Volume 2, second edition (1983, RIBBE, ed.) and to SMITH

(1974) for detailed descriptions of the plagioclase feldspars, their structures, superstructures and microstructures.

SAMPLE DESCRIPTION AND METHODS

Natural plagioclase feldspan were selected for this dori- metric study on the basis of their composition, structund state, homogeneity and lack of alteration or inclusions. A very large number of ~ampks from a wide range of geological environments was examined. From these, a final set of nineteen was sekted, consisting of seventeen prepared specifically for heat of solution measurements, one which

was used only for cell parameter determination (87975a) and a final sample, anorthite from Mikajima volcano, Japan, which was generously provided by Professor R. C. Newton to allow some interlaboratory comparison of calorimetric data. Table 1 gives the provenance and structural states of the natural samples. Many have been described in the literature and, as far as possible, references are given to the principal papers in which the same or very similar material has been used for structural, TEM or experimental studies.

Separation and purification

We have attempted tc remove all impurities from the plagioclase powders. First, the rocks or pegmatite crystals were crushed and passed through a 76 rm sieve and the dust fractions separated by flotation in water. A feldspar concentrate was then obtained using standard heavy liquid methods and this was further split into nanow density fractions with tetmbromoethane (TBE) diluted as necemary with acetone. At this stage the most promising density fraction, i.e. the fraction with the cleanest looking grains and the narrowest composition range (as determined by electron microprobe analysis of gram mounts), was selected for the final purification. In each case about I g of powder was wrapped in a small envelope of thin Pt foil and heated for up to 12 hours in air at -800°C. The purpose of this treatment was to decrepitate any fluid inclusions, dehydrate any aheration products in the feldspar grams and oxidise any Fe-bearing impurities. Rather small heavy and light fractions were then removed in diluted TBE, generally leaving -0.8 g of plagioclase grains spanning a very narrow range of density. As the last step, any remaining grams which were obviously not plagioclase were removed by handpicking under a high powemd, incident-light micromope. By the end of this procedure the powders contained much less than 1% of impurity grains and consisted of equidimen- sional or slightly acicuhtr plagioclase crystals in the size range -SO-X@ pm (none, of course, being wider than 76 pm, the sieve size).

The density of quartz is very close to that of plagioclase in the composition range AnM-Ah, making it virtually impossible to obtain a complete separation of the two. This restricts the range of sodic-plagioclase samples available for calorimetry to those taken from quartz free rocks, unless the grain size is large enough to allow an initial separation by hand picking. 8imiiatiy, the density of natural calcites spans that of anorthites and the Pasmeda anorthite could only be obtained as a pure powder because the 8OO’C heat treatment caused decarbonation of the calcite and, therefore, a change in the relative densities of the two phases. It WBS

950 M. A. Carpenter, J. D. C. McConnell and A. Navrotsk!

found that significant amounts of impurity grains were separated even from pegmatite or phenocryst crystals which appeared to be translucent and clean (e.g. Amelia albite. oligoclase from North Carolina, Lake County labradorite and Mikajima anorthite). A final difficulty was that in a few cases (91315c, 91413b, llO44pl*) the 800°C anneal caused the grains to develop a pale brownish tinge. On inspection under an optical microscope this colouration appeared to be due to a red surface coating, presumably of iron oxide, on many of the crystals. The worst grains could be removed by hand picking but the 9 I3 15~ powder still looked brownish. The I 1044pl* and 91413b powders used for calorimetry had only a very pale brown colour while the other powders were not visibly affected and remained white.

Heat treatments

Each of the purified plagioclase powders was split into two batches, one for calorimetry without any further treatment, and the second for disordering at high temperatures prior to a second set of calorimetric measurements. The disordering was accomplished by wrapping the powders in Pt foil envelopes and suspending them in vertical Pt. MO or kanthal wound furnaces, at temperatures of between 107O’C and 1350°C and for times of up to 48 days (Fig. 1). Annealing times were selected on the basis of previously published experimental data. For example, Amelia albite is generally consiieted to reach an equilibrium state of disorder after about one month at 1050°C (MCKIE and MCCONNELL, 1963; HOLM and KLEPPA, 1968) and labradorite of composition -Anlo after about l-2 weeks at 1300°C (CARPEN- TER and MCCONNELL, 1984). Kinetic experiments to ensure that an equilibrium state of disorder was achieved at each composition, however, were not undertaken.

After the high temperature heat treatments. each sample was held in air at 800°C for a few hours in order to try to return any Fe-bearing impurities to the same oxidation state as in the starting material. All the samples emerged looking pure white in colour, including those which had started with a brownish tinge. This suggests that some irreversible change in oxidation state of the small amount of iron in these samples (91315~ 91413h 11044pl*) may have occurred.

Some of the powders also became slightly sintered dunng the high temperature annealing, but no glass was observed in them by transmission electron microscopy and they were easily disaggregated by very light pressure in an agate mortar and pestle. In view of the extremely slow rates of .41/Si order/disorder at relatively low temperatures and under dry conditions it is unlikely that any of the 800°C heat treatments caused a modification of the structural states of the crystals.

Characterisation (!I structural .\raw

Grain mounts of the powders used for solution calonmetry were prepared for electron microprobe analysis. Approxi- mately 10 grains per sample were analysed for Na. Mg. Al. Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, using an energy dispersive system following the procedures described by SWEATMAN and LONG (1969) and STATHAM (1976). Only Na, Al. Si. K, Ca and Fe were detected. Proportions of Ab, Or and An molecules were calculated from the Na. Ca and K contents for most of the plagioclase range. The Amelia albite crystals. however, appeared to suffer Na loss in the electron beam and it was necessary to assume that Na = I.0 - (Ca + K) atoms per formula unit. In addition, compositions more An-rich than --AQ,~ consistently had Na + K + Ca < I.0 per eight oxygen& a feature which has been ascribed to incomplete stripping of the Na peak in the energy spectrum at low Na contents (CARPENTER and MCCONNELL, 1984). For these samples, An = Ca. Or = K and Ab = I.0 - (Ca + K) have been assumed, giving satisfactory agreement with the measured Al and Si contents, and analyses obtained by other means (CARPENTER and MCCONNELL, 1984). Mean molecular proportions of Ab, Or. An and the range of compositions in each sample are given in Table 2. Iron was only detected in the more calcic plagioclases but was present at rather low levels (given as Fe0 in Table 2). Two samples were reanalysed after the high temperature heat treatments (T-l2-22a/l, 1 190°C, 110377a/l. 1370°C and 1300°C): no changes in composition due, for example. to evaporation could be detected.

Crushed grains from all the calorimetric samples were examined in an AEI EM6G transmission electron microscope operating at IOOkV. Of particular interest was the nature of

I I I I I I 1 I I

1600 -

1500-

800-

1 I I I I I I I I 60 70 80 90

An

FIG. 1. Heat treatments of “high” structural state samples in relation to the melting curves (solidus and liquidus from MURPHY, 1977, in HENRY et al., 1982) and to the Ci/Ii transformation line (from CARPENTER and MCCONNELL, 1984). Horizontal spread of each point represents range of composition. vertical spread represents range of annealing temperature.

Ordering enthalpy in piagioclase 951

PMU& lbmt. sou nikajir 115092* S7975a 21704a 101377s c4ste1 Ilay b277tb Laka co. SKSHW 677%b 11044e1* 91413b I-12-22a 91315c 97490 B.wkb Amelia Ab

cioj (10) (10) (10) (10)

I:; (10) (20) (10) iioi

(9) (8)

(1.0) 110) (8)

(11)

n.d. 0.14 0.42 0.22 s.d. 0.29 0.3k 0.28 0.20 0.36 0.31 0.d. 0.34 xl.*. n.d. l&d. Ld. Il.*. “.d.

reflections at h _+ k = odd, I = odd Positions which were either absent (Cl structure), diffuse (Cl structure with short range cation ordering), single (type b reflections of the Ii struchm) or paired (type e iefkctions of the incommensurate, intermediate plagioclese ~tmCtaue). In addition, the presence or absence of type f satelfi re5ections round the type a (fi + k = even, I = even) retlections, and of type c (h + k = even, I = odd) reflections was noted. Selected samples were also mounted on titanium grids, ion beam thinned and examined in more detail under bright field and dark

field conditions in the electron microscope. The only piagioclases with obvious signs of exsolution were 4277 1 b, in which some grains had multiple fine scale exsoiution lamellac. and 9 1413b, which had rather weakly developed Boggild lamellae. One grain of I 1044~1. showed inconclusive evidence for a faint composition modulation. Summaries of the TEM observations on the natural and heat treated samples are included in Tables 1 and 4.

Unit cell parameter data were also collected for the calorimetric samples, and are given in Table 3. Up to fifty re5ections in X-my powder ditfmction traces were indexed with the assistance of the calculated powder patterns given

by BORG and SMITH (1969) and the tables of BAMBAUER et al. (1967a). The X-ray traces were collected at a scan rate of 118’ 28 per minute over the range 20-60’ 28 (CuK, radiation), and pure silicon was used as an internal standard. The least squares computer programme used for the ceil parameter refinements was written by Profffsor C. T. Pmwitt and is based on the method of BURNHAM (1962). Conven- tional plots of 7 and A20 13 I ,, 13 1 against mole ok, An (see

SMITH, 1974 and KROLL, 1983, for complete references) show that the natural samples, with the exception of Lake County labradorhe, have “low” structural states and the heat treated samples have “high” structural states (Fig. 2). There was insufficient material to measure the heat of solution of anneald Pasmeda anorthite (Pasmeda/l) but cell parameters of this and of two extra samples (87975a. 87975a/l) are included in Table 3.

The calvet type twin calorimeter and the techniques used for measuring the heats of solution have been described

Table 3. Unit call parlortam of natural and heat treated naplee. A unit cell with c - 14! is wed for the 21 aad Pi

ettuctur.s. and . unit cell with c - 7ii for Ci md . .tructur.,. Hontc Soul2 w.. prepared in the mm way .a kbnte Somall (me Table 4). Corrected voluac - volraa adjurted La allow for the effect of ame mbatitution of orthoclaaa (Or) component.

v (9% IrMOt Y (XV v (II’) content correction corrected

mIlta sou/2 115oB2~ 115062a/l 67975a fJ7975a 21704a

6.imMilj 8.179(l) 8.185(l)

21704all 101377a 101377*/l Cryeta nay Cnatal Bay/l 42771b 4277lb12 L&t Couaty Lake alnt~/l BKllElP Sxaiwll b2779b 62779btl 11044@1* 11044g1*/1 91413b 91413b/l T-12-22a T-12-22a/l 9131% 91315c/l 9749Q 9749011

E $1 Amlia Ab _lia Ab/2

S.lSlilj 8.181(l) 8.184(l) 6.181(l) 8.179(l) 8.1BO(l) 8.175(l) 8.1790) 8.181(l) &180(l) 8.173(l) 8.176(l) B. 180(l) 8.177(l) 8.176(2) &172(l) 6.162(l) 6.176(l) 6.167(l) 6.169(l) 6.163(2) 6.167(l) 6.162(l) 8.164(l) 6.165(l) 6.169(2) 6.168(l) 6.166(2) 6.141(l) 6.162(2)

12.676(l) 12.8730) 12.674(2) 12.678(t) 12.876(l) 12.878(2) 12.876(l) 12.673(l) 12.678(2) 12.87siij 12.679(2) 12.676(l) 12.673(Z) 12.874(2) 12.871(l) 12.880(2) 12.874(l) 12.876(l) 12.873[2) 12.677(l) 12.656(2) 12.875(2) 12.870(l) 12.876(l) 12.854(2) 12.675(2) 12.857(2) 12.877(2) 12.853(2) 12.875(2) 12.849(l) 12.880(2) 12.8390) 12.681(l) 12.791(Z) 12.876(l)

8.176(l) 8*100(l) B.183M

14.%75(2) 14.176f2) 14.176(2f 14.18Ot2) 14.1&I(2) 14*lBO(3) 14.166(2) 14.167(2) 14.197(2) 14.193(2) 14.201(2) 14.201(2) 14.205(2) 7.100(l) 7.104(2) 7.102(l)

14.205(2) 7*102(l)

7.104w 7.101(l) 7.105(I) 7.102(l) 7.106(l) 7.103(l) 7.111(l) 7*106(l) 7.116(l) 7.106(l) 7.116(l) 7.109(l) 7.127(l) 7.111(l) 7.136(l) 7.113(l) 7.160(l) 7*111(l)

93*11(l) 93.16(l) 93.19(l) 93.15(l) 93.16(l) 93.23(l) 93.32(l) 93*35(l) 93.37(l) 93.38(l) 93.45(l) 93.46(l) 93.45(l) 93.45(2) 93.43(l) 93.45(l) 93.460) 93.460) 93.460) 93*45(l) 93*50(l) 93.46(l) 93*47(l) 93.44(l) 93.61(l) 93.50(l) 93.57(l) 93.42(l) 93.65(2) 93.47(l) 93.71(l) 93.37(l) 93.63(l) 93.37(l) 94.25(l) 93.46(l)

115.69(l) 115.65(l) 115.650) 115.69(l) 115.YZW 115.64w 116,01(l) 115.6S(l) 116.050) 115.92(l) 116.05(l) 115.95(l) 116.09(l) iis.oiiij 116.150) 116.01(l) 116.06ilj 116.03(l) 116.13(l) 116.01(1) 116.20(l) 126.06(l) 116.16(11 llS.O6(ll 116.2411)

91*28(l)

x:*:;::; 91:23(l) 91,17(l) 91*16(l) 90.94(l) 90*99(l) 90.63(l) 90*65(l) 90.67(l) 90.67(l) 90.56(l) 90.55(2) 90.43(l) 90.560) 9oo.sl(l) 90.51fl) 90.40(l) 90*55(i) 90.07(l) 90.47(l) W.lOW 90.45(l) 89.83(l) 90.37(l) 89.790) 90.31(l) 69.67(l) 90.27(l) 69.25(l) 90.26(l) 66.91(l) 90.24(l) 67.710) 90.26(l)

.~. 116.15(l) 116.26(l) 116.23(l) 116.36(l) 116.24(l) 116.36(l) 116.26ilj 116.45(l) 116.33(l) 116.56(l) 116.42(l)

1336.7(4) 1339.50) 1340*3(S) 1340.2(4) 1339.5(4) 1341.4(5) 1339.5(4) 1340.60) 1340.5(5) 1341.2(4) 1340.3(S) 1341.2(S) 1339.0(5) 670.2(2) 669.6(2) 670.7(3)

1339.2(4) 670.1(2) 669.9(2)

~~~~~~

669:5(2) 670.3(2) 670.2(2) 666.0(2) 669.1(2) 66&O(3) 669.0(2) 667,5(2) 666.6(2) 666.4(2) 669.4(3) 668.6(2) 669.2(2) 664.9(2) 667.7(2)

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1

0.5 0.5

1 1 1 1

2.5 2.5 0 0 1 1 1 1

2.5 2.5

3 3 1 1

-0.5 -0.5 -0.5t -0.3 -0.6 -0.6 -0.6 -0.6 -1.4 -1.4

_

669.3(2) 670.2(3)

1336.7(4) 669.8(2) 669.3(2) 669.7 (2) 667.863) 66S.9(2) 668.9(2) 666.6(2)

-0.7 667.3(3) -0.7 666.3(2) -0.7 666.8(Z) -0.7 667.9(2) -1.8 666.6(2) -1.6 667.6(3) -2.2 666.4(2) -2.2 667.0(2) -0.6 664.1(2) -0.6 666.9(Z)

952 M. A. Carpenter, J. D. C. McConnell and A. Navrotsk!

91

X”

90

89

88

a Ab

IO 20 30 40 50 60 m so 90 mol % An An b

10 20 30 LO 50 60 70 60 90 I L A HO mot Ye An Hn

FIG. 2. y (a) and 28 131,ljl (b) plots for natural and heat treated plagioclase samples used in this study. Lines marking trends of “high” and “low” series are from KRoLL ( 1983). Lengths of arrows indicate corrections to 28 13 l,, 131 to allow for Or content (using correction factors given by KROLL,

1983). Crosses = heat treated samples, filled circles = natural samples. Values of 28 13 1,151 for 62779b and 11044pl* (both -An& are almost identical and only one pair of values is shown. With the exceptions of Crystal Bay (An& and Lake Co. (An&, all the natural samples fall close to the “low” trend of KroU(l983) and all the heat treated samples plot along the “high” trend.

elsewhere (NAVROTSKY, 1977). Thirty milligram batches of the p&&&se powders were dissolved in R&OS at -7tiC. T&y. 3-5 batches (i.e. 90-150 w in all) wm dissolved in each 30 a batch of flux. The plagiociase powdm dissolved rapidly, giving reaction times df l&s than 30 minutes and actual heats of solution of - 1.5-2.0 calories. A correction lo allow for the effect of stirring was applied to each measurement This was determined in dummy runs with no sample and was frequently rechecked; it amounted to a few % of the total heat of solution signal. Runs which did not return to a steady background were t+%ed, leaving 4-6 rcceptlbk heat of soh&on values per sample. Calibfation was by the Pt drop method, allowing for 1% heap pick up by the Pt nugget during its drop. No special prrautions were taken to dry the powders before loading them into the calorimeter. They each spent between 5 and I5 hours at -700’C in the calorimeter while it equilibrated, however, which should have been sufficient 10 drive off any adsorbed water. It is unlikely that any change in StNaural state occurred during this equilibration period.

Although the heat of solution measurements were spread over three main sessions spanning several months. a sin& master stock of i%&O, was used thro@out. There was some variation in the calorimeter temperature between sessions, however, from 708’C in the first, 704°C in the second, to 694°C in the third. This variation of up to 14’C probably did not contribule significant drift to the Ai&+ values but the calorimeter temperature is specifrd for each sample in Table 4. The complete calorimetric dala are given in Table 4 and Fig. 3.

DlSCU!SSlON OF THE DATA

Enthalpies of solution

There is clearly a fair degree of scatter in the AH&. data plotted in Fig. 3, but in view of the fact that the samples are not cogenetic and have been subjected

to very different P-T histories this is hardly surprising.

Much of the variation must be due to differences in structural state and cumposition though it is necessary to consider the effects of the minor components (principally potassium and iron) which also varied between samples.

The mean Or content of each sample is between 0 and 3 mok%. Unfortunately a heat of solution value for K feldspar in lead borate at 700-C does not appear lo be avaibbk, but there m data for albite and K-fel&ar glasses. which show a di5erence of -6 k&/mole between the two (HERVIG and NAVROTSKY, 1984). This would imply a 60 &mote increase in AIf, for each mde % Or addal and thcrrfore. a total effect Of O-180 Cal/mole variation between sampks. Even though the estimation is very approximate and is based on glasses rather than crystalline felbpan it serves to show the order of magnitude of the likely contribution. In the present context such a small tire can be ignored.

Potenlially more serious is the e%cl of changing the ox&ion s!ate of imn. SMmr (1983a). in nGewi% pub&ed data for iron in pla@oclam!s. has concluded that in terrestrial samples it is pnsmt as a mixture of Fti+ and Fe’+. Half the calorimetric smpks had no &ectabk iron and only 5 had more than 0.30 wts (given as FeO). The amount of iron available to be reduad or oxidised during diodution runs was therefore sma& and it rtms likeiy that the principal elTect of variations in Fe content would only have ban to introduce a degree of scatter. One exception may be 9 I3 15c which lies off the main “high” aru~rural state trend and which was distinct from the other samples by virtue of the brownish colour of its grains.

Although the u and Arizona State high temperature sdution calorimdar are very similar in design and 0puistion there mry be variations between W&s of flux A?icicnt lo give slightly different AH* results. Nevertheless, some useful comparison may be made between the data for


synthetic “high” piagiociases given by NEWIQN ef al. ( 1980) and the heat treated natural samples presented here. The data of Newton PI al. are plotted in P&t. 4 along with the lines drawn from the data in Fig. 3. For the range from nure albite to -Ana them is a remarkable degree of overlap but for Aqe-An,ea&!re is a distinct divergence. Newton et al. svnthesised their olaaioclases from glass at 1200°C. 20 kb for three hours. ‘T6e difference in heats of solution between the natural anorthites (this study) and the synthetic anorthite (NEWTON et al., 1980) could be explained either by some effect of calorimeter conditions or by a degree of metastabie Al/Si disorder in the synthetic samples. The heat of solution values for two natural anorthites reported by &HARLU et al. (1978) (a volcanic sample horn Mikajima volcano, Japan, and a metamorphic sample from the Sittam- pundi complex, Madms) agree well with Newton et al.% synthetic anorthite. We have found a distinct difference between volcanic and me~mo~hic anorthites, however, and would have expected to see the same difference between the A&,, values for Mikajima and Sittampundi anorthites irrespective of differences in the calorimeter conditions. Our value of LV&, for Mikajima anorthite, prepared in the same way as all our other samples, plots exactly on our trend for Ii structures annealed at high temperatures. We believe that the anorthite data of CHARLU ef al. (1978) and NEWTON et a!. (1980) am slightly low due to metastable Al/ Si disorder in their synthetic sample, and perhaps to impurities in their natural samples (the purification procedures were not described). Metastable Al/Si disorder has also been found in cot&rite cqstalhsed from glass (CANWTEI~ ef al., 1983). Our results for the natural samples are very different from those of KRACEK and NEUVONEN f 1952). They used acid calorimetry, however, and reported the presence of precipitates in their acid alter solution runs on anotthite-rich compositions.

Systematic trends shown by the A&,, data in Fig. 3 must be explicable in terms of the mixing and order/disorder behaviour of the system. In the most general terms, the high structural state (CT) samples plot along a fairly well defined line. The Ii structums annealed at - 13OO’C define a second trend and the natural Ii structures a third. The natural e structures are the most scattered and the straight line drawn in for them is rather arbitrary. An important difference between the “high” and “low” series is that the former represents samples which have been subjected to known heat treatments and tberefore forms a more internally eons&tent set. The natural “low” samples come from metamorphic, vokanic and plutonic rocks, and pegmatites. They have very variable states of order and this may be reflected in some of the scatter of the results for e plagieclases. In the “low” Ii series, the Anloo, An% samples are metamorphic, the An%, Ann and Anw samples an: from slowly cooled igneous rocks and the A&,* sample (Lake County) is volcanic in origin. The metamorphic and plutonic igneous samples should perhaps lie on the same trend for highly ordered Ii structures but the Lake County sample need not, because it is representative of a relatively high ~u~ibmtion tempemtum. A straight tine is therefore an alternative to the curved trend shown in Pig. 3. It should also be noted that the calorimeter temperature of -7OO_“C is well above the temperature of the displacive Ii S PI transformation in anorthite (see, for exampk, FREY er al., 1977; ADLHART et a/., 198Oa,b). ~11 the heats-of solution of ano~ite-~ch ctystals are therefore for the I1 structure; low temperature atomic displacement effects associated with the Pi structure can be ignored.

A marked change in the trend of A&,,. values for “high” Ph@CfascS occurs between -A&, and -AnM, (Fig. 3). CARPENTER and MCCONNELL ( 1984) have argued that this feature, which also appears at about the same composition in the data of NEWTON et al. (1980), is due to a discrete order/disorder fci * ii) t~nsfo~ation. The new data are consistent with such an in~~~~tion in that the anneal& At+A% samples have diffuse b reflections, indicative of

short range ordering in the Cf stability field, just outside the li field, and the Anrr-Anroo samples have sharp b relkrctions, indicative of long range ordering. CARWml$ and MCCONNELL ( 1984) also suggested that the “high” C I solid solution can be described as having zero heat of mixing, i.e. that the AH,, values lie on a straight line. If this assumption is accepted, a simple, linear e$rapolation of the Ci solid solution to a fictive disordered (Cl) anorthite end-member can be made. Deviations from the straight line are then due to the enthalpy change on ordering from Cl to the Ii structure. In Fig. 3 the Cl solid solution line is a Ieast squares fit for the A&, values of heat treated samples with compositions between An, and Anrr. Sample no. 9 131 SC (An& was excluded from the fit because of possible problems due to an iron oxide coating on the grains (see above), and only one of the values for annealed An,, (4277 1 b/2, 1300°C) was used.

Enthalpies of ordering

With the exception of Monte Somma anorthite, a change in Ai&, was detected between every natural sample and its heat treated equivalent. This difference will be referred to as the enthaipy of ordering (A.&& and is simply the enthalpy change at 7OPC on transforming from a relatively ordered state to a relatively disordered state (see Table 5 and Fig. 5). While there may be some scatter in the absolute A&+, values introduced by variations in Or and iron contents, AH& should be much less dependent on impurity contents because it is given by the difference between two samples with identical compositions. For albite this change in order corresponds to the difference between low and high albite, both of which have Ci symmetry. Between Anrs and Ant,, with the exception of Lake County labradorite, AE& is for the e piagioclase structure transforming to the Ci, high albite structure, For Lake County (An& and Crystal Bay (An& samples the symmetry change is from Ii to Ci. The anorthite-rich samples (An,,- An& were annealed below the Ci * Ii transformation Iine (Fig. I), however, and AE& therefore does not include the effects of an actual change in symmetry; in these cases it describes only a change of structural state within the stability field of Ii ordering.

The present value of AEiH,, for low * high albite (3.08 + 0.30 kc&/mole) is in good agreement with previous dete~inatjons at 700°C (3.4 * 0.25 kc&/mole, HOLM and KLEPPA, 1968; 2.86 & 0.23 k&/mole, NEWON et al., 1980; 2.80 2 0.29 kcal/mole, BL~NOVA and KJSELEVA, I982) and at 5VC (2.60 C 0.30 k&/mole, WALDBAU~~~ and ROBE, 1971; 2.63 f 0.40 kc&/mole, THOMPSON et al.. 1974). The only published value known to the present authors for di~rde~~ a plagioeiase of int~~iate ambition is given by BLINOVA and KEELEVA (1982). They obtained 2.66 + 0.59 k&/mole as the enthalpy difference at -700°C between andesine of composition An&b,&& and the same sample annealed at 1250°C for 22 hours. Their sample was reported to show a blue iridescence indicating the presence of exsolution lamellae within the crystals. This compares with 2.14 f 0.2 I kcalfmole for the An,,* specimen described hen and a complete range of PJi, values for “e‘ plagioclams of l-3 kcal/mole (Table 5, Pii. 5) for the intermediate compositions.

954 M. A. Carpenter. J. D. C. McConnell and A. Navrotsk!

Table 4. Heats of solution for natural and heat treated plagioclase feldspers

Sample (solution T, ‘Cl

Origin and structural state at

700°ct

Physical Mean An Molecular Mean AH content appe.¶riXlce veight (kcal/d?tP

(9) (No. of lXX?asUre!ZlelltS)

Pesmeda (704)

Paameda/l

Metamrphic, Ii

Monte SOnma (704)

nonte S-/l (704)

Hikaj ima (694)

115082a (704)

115082a/l (704)

20 days 1302 * 4'C. Ii (sharp b reflections, elongate c reflections)

Volcanic, Ii

21 daya 1360 t 4%. Ii (sharp b reflections, streaked c reflections)

Volcanic. Ii

Metamorphic, Ii

21 days 1306 + 4’C, Ii (sharp b reflections, streaked c reflections)

87975a Metamorphic. Ii

87975all 21 days 1300 f 10°C, Ii (sharp b reflection, very diffuse and streaked c reflections)

21704a (704)

21704aJl (704)

Igneous, plutonic, Ii

101377a (704)

1013771/l (704)

23 days A366 s 8’C + 21 days 1306 i 4 C, 11 (*harp weak b reflections, very diffuse and streaked c reflections)

Igneous, plutonic, Ii

23 days A366 ; B’C + 21 days 1306 t 4 C, I1 (*harp weak b reflections, extremly diffuse c reflections)

Crystal Bay (704)

Crystal Bay/l (704)

42771b (704)

42771bll (704)

Igneous, plutonic. Ii

4277Lbl2 (704)

4277lbl6 (694)

Lake co. (704)

Lake Co.11 (704)

23 days 1346 f 7’C, Ci (SOW faint diffuse intensity at positions of b reflections)

Igneous, plutonic. “e” (amoe exsclved grains)

14 days &373 $ 3’C + 23 days 1346 ? 7 C, Cl (possibly some very diffuse intensity at positions of b reflections)

14 days 1273 *_3’C + 21 days 1300 2 10 C, Cl (some faint diffuse intensity at positions of b reflections)

21 days 1312 f 3’C. Ci (diffuse intensity at positions of b reflections)

Volcanic, Ii

SKHlW (708)

SKHHw/l (708)

21 days 1300 f 1oOc. ci (ame diffuse intmmity at positions of b reflections)

Igneous, plutonic, “e”

67796b (704)

67796bf 1 (694)

10 days 1334 * 5%. Ci (SO= diffuse intensity at poaitiOIm of b reflections)

Hetemorphic, “e”

21 days 1300 i 1O’C. Cl

White powder

White powder

White powder

White powder

White powder

White powder

White powder

White powder

White povder

White powder

White powder

White powder

Slightly riatered.

white

White pOWd*t

Slightly wintered, vhite

White powder

Slightly ointered, white

Slightly sincered, white

Uhite pOVder

Uhite

pOWder

Slightly riatered, white

Off-white powder

Slightly sistered. white

White powder

Slightly sincered. white

loo

90

98

97

96

96

89

89

86

86

78

78

72

72

71

71

71

71

68

68

67

67

60

60

278.211

277.891

277.891

277.731

277.572

277.572

275.973

275.973

274.964

274.964

273.735

273.735

273.736

273.736

273.736

273.736

273.257

273.257

273.096

273.096

271.978

271.970

17.90 * 0.24 (5) ii

17.28 t 0.17 (6)

17.05 * 0.23 (5)

17.03 t 0.33 (4)

17.81 t 0.14 (6)

16.98 ? 0.17 (6)

tt

-t 1

17.53 * 0.14 (5)

16.13 f 0.19 (6)

17.21 + 0.14 (5)

15.46 + 0.23 (5)

16.66 ? 0.24 (6)

14.77 + 0.10 (6)

17.12 t 0.15 (6)

14.54 * 0.24 (6)

14.70 f 0.11 (5)

14.73 ?: 0.31 (4)

16.01 f 0.17 (5)

15.33 * 0.18 (5)

17.36 ? 0.13 (6)

15.57 ? 0.23 (6)

17.77 i 0.16 (6)

14.97 t 0.23 (6)

Ordering enthalpy in plagioclase

Sample Origin and structural state (solution at

T. ‘0 700°ct

Physical Mean An no1ecu1llr weight

Mean AH1 1 content (kcal/mo!eP appearance

(g) (No. of messurem?nts)

110440 1* (708)

11044P1*11 (708)

91413b (704)

91413b/l (704)

T-12-22a (694)

T-12-228/1 (694)

91315c (704)

91315c/l (704)

97490 (694)

97490/l (694)

Hawk b (694)

Hawk b/l (694)

Amelia Ab (694)

Amelia Ab/2 (694)

Igneous, plutonic, “e”

lo-14 days 1278 f 6’C. Ci (some diffuse intcnsitv .% msitions of b reflectionsj

Metamorphic, “e” (with some B&gild exsolution)

25 days 1250 * 10°C, Ci

Metamorphic, “e”

35 days 1192 * 2Oc, cl

Metamorphic. “e”

37 days 1172 f l°C, Ci

Pegmatite, “e”

38 days 1148 f 2Oc; ci

Pegmatite, “e”

48 days 1110 * 20°C. ci

Pegmtite, ci (low albite)

42 days 1067 + 4’C, Ci (high albite)

Pale brown powder

Slightly aintered, white

very pale brown powder

Slightly sintered, white

White p0Wder

White powder

Brownish powder

White povder

White povder

Uhite powder

White powder

White powder

White p0Wder

White powder

60 272.140

60 272.140

49 270.058

49 270.058

40 268.780

40 268.700

35 267.981

35 267.981

27 266. a63

266. a63

265.905

265.905

262.546

262.546

17.15 f 0.27 (5)

15.42 f 0.18 (6)

17.76 f. 0.16 (5)

15.62 f 0.14 (5)

17.25 f 0.19 (6)

15.85 f 0.19 (5)

18.09 + 0.23 (5)

16.53 f 0.10 (6)

17.59 t 0.20 (6)

15.95 t 0.09 (5)

18.61 f 0.17 (6)

16.31 f 0.23 (5)

20.26 f 0.22 (5)

17.18 f 0.20 (4)

955

t Space groups given for structural state It 700 C; diffrmtion information in brackets refers to room temperature observations. + .t Samples used for cell determinations only; heats of solution not measured.

It would be useful, for a comparison of the relative stabilities of the different-ordered plagioclase structures to have Mf,,,,, values for the I1 * C I transformation at anorthite rich compositions. At high temperatures anorthites and bytownites melt before they disorder to the Ci structure and so, at these compositions, it is not possible to produce crystals with equilibrium_ Cl states. Values for the enthalpy change on going from I I to C I can. however, be extracted using the extrapolated Ci line shown in Fig. 3. This line gives an estimate of what the heats of solution of C? structures between An,, and Anloo would be if they could be prepared. The difference between this extrapolated value and the value measured for the natuml, ordered sample is then Hord, the enthalpy change for II G= Ci, as would be measured at 700°C (see Table 5 and Fig. 5). For pure anorthite AH& = 3.7 & 0.6 k ca moe, where the error in- I/ I cludes an estimate of the uncertainty in the extrapolation.

The ovemll trends shown by the data should be more reliable than any conclusions based on a single composition and in this respect a number of distinct features are evident from Fig. 5. Firstly, A&, and AH&, for the Ii - Ci transformation show a rapid decrease in magnitude from Anloo to AQ~. (This trend is apparent even if the volcanic sample, Lake County, Ass, for which AH& = 0.7 kcal/ mole, is excluded). Secondly, in_ the composition range Anb,-An,2 AH, values for e - Cl are signilicantly greater than for Ii - Ci. There is also some variation in AIf_, for e - CT at these compositions (A-An&, where more than one sample has been measured. Thirdly, the most ordered e plagioclases show a difference in LM&, according to whether they have An > -50 mole% (-2.8 kcal/mole) or An < 50 mole% (- I .5 kcal/mole). The sample at An,9 gave an intermediate value of -2.1 kcal/mole. Finally, at

the albite end of the solid solution, AHd increases from - I.5 kcal/mole at An2, to -3 k&/mole at A%.

Volumes of mixing

Small but distinct volume differences were detected between the natural ordered samples and the same samples after they had been heat treated. These volume changes (AL’,) are given in Fig. 6a and Table 5 and, for comparative purposes, all refer to a unit cell with c = 7A. An important difference between the enthalpy and volume measurements is that the former were obtained at 700°C and the latter at room temperature, -25°C. While it is possible to ignore the Pi G Ii dispiacive transformation in the interpretation of the enthalpy data, the volume data may include effects of the lower symmetry (Pi) of the most calcic samples below -250°C.

The largest volume change associated with a change in order was found for albite: 2.8 + 0.4A” (-0.45%). This compares with the value of 2.63A’ given by KROLL and RtBllE (1983). For e plagioehrses the volume change, with the exception of I 1044~1. and SKHHM* samples, is relatively eonstant at -I + OSA’ (-0.15%). 11044pl* and SKHHM* are both igneous in origin so their smaller volume change on disordering may indicate either that they did not achieve as high a degree of order as the metamorphic samples or that their experimental heat treatments were insufficient to produce equilibrium disordered states. Of the

M. A. Carpenter. J. D. C. McConneif and A. h\iavrotsk!

AH,,,lg'O 1843

kcallmole

17.0

160

15.0

110

13.0 1

Ab 10 20 30 LO mo;e;aAn60 70 80 90 An

FIG. 3. EntMpy of solution data for natural (“low”) and heat treated (“high”) piagioclases. C = Ci structure, I = Ii structure. e = “e” structure. Open circles = sampks from pegmatites and their hear treated equivalents. filled circles = metamorphic samples and their heat treated equivalents, open squares = igneous (plutonic) samples and heat treated equivalents, filled squares = igneous (volcanic) samples and heat treated equivalents. Horizontal bars show the range of compositions present in each sample, vertical bars are 2 one standard deviation of 4-6 individual AH aln measurements. The dashed line represents a linear extrapolation of the A&,” trend for Ci “high” st~ctures. Trmds for 11 “low”, and ii “high” structures are also drawn in: the line for e samples is rather arbitrarily included to show the most general trend.

FIG. 4. A?& data of NEWTON et al. (1980) for synthetic hi structural state pla&clases in relation to trends from Fig. 3. Trian@es = Mikajima volcanic anorthite, open triangle = from CHARLU er al. (1978), filled triange = from this study. lnwzted triangk = rne~~c anorthite, from CHARLU n al. (1978).


Table 5. ) and unit cell volm differences (AV unit call) @eat natural md heat treated plaSiocl8iP'

Saqle

Pasocda Monte sm 115082a S7975a 21706a 101377a Crystal Say 62771bll 62771bl2 LakC Camty SKHRW 67796b 11066Pl* 91413b T-12-22a 91315c 97690 Hawk b Anlia Ab

Structural change

Ii + Ii w4dc~ 11 * Ij mOO%) 11 * 11 wOo"c) Ii + 11 wJO~c) Ii * 1-1 (13000C) 11 * 11 mOOoc) 11 l cl (13500C) e *cl (13500C) p aci (13000C) 11 l Cl (13000C) c *CL (1335,C) I? + Cl (13000C) c *cl (12SOoC) e “Cl (12500C) e *cl U1900C) e z Cl (11700C) e + Cl (11500C) S *cl (1110 C) Cl *cl (1070°C)

100 98 96

:z 78 72 71 71 60 67 A0 60 69 60 35 27 20 1

AV (at 25' ) (93, li c - 7)

0.6 f 0.6 0.05 + 0.65 0.95 + 0.65 0.55 t 0.65 0.35 f 0.65 0.65 * 0.5 0.65 f 0.5

0.9 f 0.5 0.65 2 0.6 0.6 f 0.6 1.1 f 0.5

-0.1 f 0.6 1.1 f 0.6 1.0 * 0.5 1.1 f 0.6 1.0 + 0.5 0.6 + 0.6 2.8 f 0.6

A&d (at 700%) (kcalfmlc)

0.23 + 0.29 0.83 + 0.22

1.60 + 0.26 1.75 + 0.27 1.89 f 0.26 2.58 f 0.28 2.62 t 0.19 0.68 ? 0.25 1.79 f 0.26 2.80 * 0.28 1.73 f 0.32 2.16 ? 0.21 1.60 t 0.27 1.56 i 0.25 1.66 * 0.22 2.30 * 0.30 3.06 r 0.30

2580% (kcallmole)

3.7 * 0.6 3.1 * 0.5 3.5 t 0.6

3.0 t 0.6 2.6 f 0.6

two samples which were transformed from Ii to Ci states, In Fig. 6b the volume data are plotted against composition. Lake County labradorite had AVd barely greater than zero A small correction has been made to allow for the effect of (0.5 + 0.4A’) and Crystal Ray bytownite with AL’, = 0.7 Or content because potassium feldspars have substantially f 0.5A3, had a marginally smaller volume change than larger cell volumes than either albite or anorthite. The most of the e samples. A small volume change was also found for some of the calcic samples annealed within the Ii

correction factors were obtained by taking the slope of the volume versus composition curves at small Or contents

field. 115082a, a metamorphic sample, gave the largest volume change of 1 .O + OSA” and Monte Somma. a vokxutic

from the At&r data given by KROLL and RIBBE ( 1983) and the ion-exchanged An-Or data given by KROLL and MULLER

anorthite, the smallest, of effectively zero. Pasmeda anorthite (1980). It seems to make little difference whether the slope showed a volume change of 0.4 f 0.4A3. Almost all of the is taken for high or low series (KROLL and RIBBE, 1983, igneous samples have AV, smaller than the metamorphic Fig 2; and see SMITH, 1974, Fig. 7.56). These slopes give samples, though there is overlap of the error bars (Fig. 6a). volume changes of 0.8A3 and 0.3A3 per mole W Or added

L.5

LO

35

3.0

AHO, (gAH&) 2.5

kcol lmde

2.0

b

FIG. 5. Values of AH0@ and Mb, the enthalpy differences between ordered and disordered samples. Symbols as in Fig. 3. Dotted error bars = Ci (low albite) dashed error bars = Ii

- Ci (high albite), solid error bars = e - Ci, - Ci. Note that for An,-Ann AHti is the difference in AH,,. between the

natural and heat treated samples. and for An,.s-An,oo the values given are for AHod, the enthalpy difference between the natural sample and an extrapolated Ci state. Trends for the Ii - Ci and metamorphic e - Ci transfonnations are drawn in.

958 McConnell and A. Navrotskb M. A. C‘arpenter. J. D. C

25

20

A&xi

cR1 t5

10 20 30 10 50 60 70 60 90 Ab mol % An An

b

v

Ab mot% An An

FIG. 5. Unit cell volume data (using a c = 7A c&l for purposes of ~orn~n) for natural and heat treated pIagio&ses. (a) Changes in volume faP’& due to heat treatments. Symbols as in Fig. 3. A trend fine is drawn in for the metamorphic sampIes. (b) Unit cell volumes (7A cell) corrected for Or content (see Table 3). Approximate trend lines are drawn in for “high” and “low” series.

to albite or anorthite, respectively (for a c = 7A unit cell). To get the volume change at intermediate compositions, a linear ~mbination of the two end member values was adopted, giving corrections to the measured cell volumes of up to 2.2A3. The corrections are clearly arbitrary but give volume-composition curves which are considerably more systematic than the raw data alone (Fig. 6b).

In Fii. 6b two lines are drawn in, representing, approximately, the trends for the low temperature, ordered samples, and the high temperature, annealed samples. The unit cell volumes of crystals at A% lie off these trends but this may or may not be a sigoificant detail. Broadly speaking, the plagiociases with compositions between -AnzO and -AnW lie on two parallel straight lines. Low albite fatls well below the trend for ordered structures, probably because of its different (Ci) ordering scheme, and there is also a distinct change in slope at calcic compositions. The reduction in volume from a maximum at -An@ towards pure anonhite coincides with the increasing intensity and sharpness of e reelections in electron diffraction patterns from the crystals. and could. therefore, be due to structural changes in the If s Pi transformation. It also corresponds with an increasing degree of Al/S order. however. and this is a more likely cause of the volume fusion. There is no evidence for a marked break in the volume~om~itioR relations for the “hi so&l solution so the Ci/Ii tmnsfo~ation could be continuous in V.

Volume data for natural plagioc~ given by BAMBAUER et al. (1967b) have a greater scatter than shown in Fig. 6b but show essentially the same trend. The data for high structural state (synthetic) plagiocIases of KROLL and MOLLER (1980) and NEWTON et al. ( 1980) give comparabie variations with composition. The synthetic sampIea pnpared at 12OO”C, 20 kb by NEWTON tl al. ( 1980) plot along the “low” trend of Fig. 6b for An,-Anta, and atong the “‘high” trend for Ah--An%. NEWTON and WOOD (1980) have accounted for

the common sigmoidaI vdume of mixing behaviour observed for many solid solutions in terms of the size effects of the cations substituting for each other in each crystal structure. In the case of the ptaleioclases the &ect of tetmhedrrd cation order may be more important, because changes in slope of the volume-composition curves coincide approximately with changes in ordering scheme. The actual volume changes tiated with disordering are small in comparison with the total effect of #motion across the soIid solution, however.

IMPLICATIONS

It might seem at first sight that the new calorimetric data should be sufficient to establish quantitative enthalpy models for both ordering and mixing in the plagioclase solid solution at a range of temperatures. However, because of the highly variable or~r/~~~r behaviour the most straight-forward mixing models, i.e. regular sotutions, sub-regutar solutions etc., are not strictly appropriate. Moreover, estimating configurational entropies in a system which has three, possibly contin~us, order/di~rder tmRsfo~ations would be a major task in itself even if the usual problems of determining Al/Si distributions in feldspars did not exist, The implications of this study relate, therefore, more to identifying the stability relations of the different structures, with refenmce to the overall solid state behaviour, than to producing free energy models of immediate petro~ogicai use. Our calorimetric measurements provide the first sys-


tematic enthalpy dataset for “high” = “HOW” tmnS- formations in plagioclases and are used here to identify some of the compositional and temperature constraints on the different ordering processes.

AI/Si disorder in anorthite

We have found that volcanic anorthites give different heats of solution from metamorphic anorthites and that the values for volcanic samples correspond closely to the values obtained for a metamorphic sample annealed at 1300°C for three weeks (115082a/ 1). Both the high temperature and low temperature crystals have sharp b reflections and should have Ii symmetry at the calorimeter temperature. We suggest that the change in AHml, is due to a continuous variation in the degree of Al/Si order with temperature within the stability field of long range Ii ordering.

There is indirect evidence of some Al/B disorder in anorthite quenched from high temperatures. This takes the form of small variations in unit cell parameters and spectroscopic properties (SMITH, 1972, 1974; BRUNO and FACCHINELLI, 1974) and of variations in the character of c reflections. The type c reflections can be diffuse or absent, depending on the temperature from which anorthite crystals are quenched, and it has been proposed that the low temperature displacive transformation, which gives rise to them, is sensitive to the degree of Al/Si order (LAVES and GOLDSMITH, 1954, 1955; GAY, 1954; GOLDSMITH and LAVES, 1956; MEGAW, 1962; SMITH, 1974; BRUNO and FACCHINELLI, 1974; BRUNO et al., 1976). Ii symmetry is maintained up to the melting point, however, (LAVES and GOLDSMITH, 1955; LAVES et al., 1970; BRUNO et al., 1976) and the degree of disorder even in samples quenched from 1530°C is probably small (BRUNO et al., 1976). Thus the enthalpy difference of -800 Cal/mole observed between high and low temperature anorthites can arise only from a limited number of Al/Si exchanges,

Published values for the enthalpy of formation of anorthite from oxides (AH&,), as determined by solution calorimetry, are not all in agreement. Some of the scatter could be due to differences in Al/Si order between different synthetic and natural samples, though the most recent value of NEWTON et al. (1980) on a synthetic sample (24.06 + 0.31 kcal/ mole) is consistent with the value obtained by CHARLU et al. (1978) for two natural samples (23.93 f 0.48 kcal/mole) and also with a value based on acid calorimetry of a synthetic sample by ROBIE et al. (1978) (23.89 + 0.82 kcal/mole, interpolated to 970 K by NEWTON et al., 1980). We have not measured heats of solution for the oxides but if our anorthite AH,, values can be compared directly with those of Newton et al. our data suggest that AH&,0 for high temperature anorthites should be more negative than their value by 820 + 330 Cal/mole and for low temperature anorthites by 1670 + 330 Cal/mole. If the Newton et al. synthetic anorthite had equilibrium

AIJSi order for the temperature at which it was annealed ( 12OO’C) and our different LW~, values are. caused by the difference between their calorimeter conditions and ours, then AH&c should be -850 f 330 cal/mole more negative for low temperature anorthite than the value they quote, to allow for the effect of ordering. It is interesting to note that the value for AH: derived by HELGESON et al. (1978) from phase equilibrium studies is more negative than the calorimetric values of CHARLU et al. (1978, natural samples), ROBIE et al. (1978) and NEWTON et af. (1980) by -900 Cal/mole.

These small adjustments to the enthalpy of anorthite can be associated very approximately with an entropy change if the order/disorder transformation (ci = Ii) in anorthite is treated as being first order. An estimate of the equilibrium order/disorder (Ci = Ii) transformation temperature for pure anorthite (T0,.J is made by extrapolating the experimental results of CARPENTER and MCCONNELL ( 1984) for the transformation at intermediate compositions to Anlo,. Using this approach gives TOd = 2000-2250 K. An 800 calorie correction due to ordering would then correspond approximately with an entropy correction of -800/2000 = 0.4 cal/mole* K while a 1700 calorie adjustment would give -1700/ 2000 = 0.8 cd/mole - K for the related entropy change. In this context it is interesting to note that the third law entropy of anorthite given by ROBIE et al. (1979) is in good agreement with phase equilibrium data if a configurational contribution of - 1 &/mole - K is added to it (HELGESON et al., 1978; GOLDSMITH, 1980, 1981; PERKINS et al., 1980; WOOD and HOL- LOWAY, 1982, 1984).

The value of AHbti = 3.7 + 0.6 kcal/mole, extracted for the enthalpy of disordering of pure anorthite from an Ii (ordered) state to an extrapolated Ci (disordered) state, as measured at 700°C seems “reasonable” when compared with a total of -3 kcal/mole for disordering in albite (HOLM and KLEPPA, 1968; NEW- TON et al., 1980; BLINOVA and KEELEVA, 1982; THOMPSON et al., 1974). If again, as a first approxi- mation, the transformation is treated as being first order, the total entropy of disordering is given by AHaT,, = 1.4-2.2 cal/mole - K. This is rather less than the maximum possible value (for complete order to complete disorder) of 5.5 &/mole - K, probably because the Ci solid solution at high temperatures has significant short range ordering of Al and Si (KROLL, 1978; KROLL and RIBBE, 1980).

Solid solution at high temperatures

The suite of synthetic high structural state plagioclams prepared by NEWTON ef al. ( 1980) was subjected to a uniform heat treatment. In the present study the ‘*high” samples were not all annealed at the same temperature and the natural crystals also had different origins and impurity contents. AH&, values for the tw0 serks do overlap, however, from An,, to Anso

960 M. 4. Carpenter. J. D. c‘. McConnell and A. ‘u~rotsk~

and the following general conclusions regarding mix-

ing at high temperatures are consistent with both. The data are somewhat scattered, so that small

deviations from linear composition-U,,, relations

cannot be ruled out, but they otherwise conform to the interpretation of two ideal (zero excess enthalpy of nixing) segments related by a non-first order transformation (CARPENTER and MCCONNELL. 1984). Mmeral solid solutions commonly show positive deviations from ideality (NEWTON TV al.. 198 1) and

these can be understood in terms of strain as a structure adapts to the substitution of cations with different sizes (NAVROTSKY, 1971: NEWTON et al.. 1980). Where cation ordering occurs at intermediate compositions the net enthalpy of mixing at low temperature tends to zero. as in the jadeite-diopside solid solution (WOOD et al.. 1980). or might even become negative, as in enstatite-ferrosilite (CHATIL- LON-COLINET et al.. 1983). Both the Ci and Ii

segments of the plagioclase solid solution at high temperatures have a degree of Al/Si order which varies with composition. Thus the apparently zero excess enthalpies of mixing for the CT and Ii segments may be the net result of both mixing and ordering contributions.

A change from short range ordering to long range ordering at -Am4 (where the break in slope has

been placed in Fig. 3) is exactly as would be predicted for a non-first order transformation, and analogous diffraction effects have been observed in natural

sodium-rich pyroxenes (CARPENTER and SMITH, 198 1). AH,,, values in the composition range of the crossover between Ci and 17 structures are not sufficiently precise to pin down the mixing curve unambiguously. For a “mixed or X” transformation,

as defined by THOMPSON and PERKINS ( 198 I ) a step would occur in the enthalpy of mixing curve at the crossover point (Fig. 3) but for a classical second order transformation the curve would simply have a cusp. If the enthalpy of mixing is continuous then the entropy of mixing probably will be also.

NEWTON et al. (1980) used their enthalpy data to set up a convenient mixing model for petrological applications. The new data suggest that a slightly larger excess heat of mixing should be used, because

of the different results for anorthite. Some of the overall constraints on the mixing behaviour. to take

account of the Ci/li transformation. have been

outlined by CARPENTER and FERRY ( 1984).

Stability qf‘the “e” plagioclase structwc

In treatments of the phase transformations and stability relations of plagioclase feldspars it is generally assumed that the e structure represents a metastable state. For example, SMITH ( 1983b) states that “only low albite and P-anorthite are stable at low temperatures”, and that “the ‘e’ structure type is merely a coherent small-scale intergrowth of domains which locally have structures like those of low albite and

anorthite”. MCCONNELL ( 1974) suggested that the incommensurate superlattice develops metastably under conditions of substantial undercooling when. for kinetic reasons. the stable commensurate superlattice fails to develop. and this argument has been

supported, on the basis of microstructural evidence. by WENK and hbiK.AJIhlA (1980) and WFN~ cv ui

( 1980). GROVE et N/. ( 1983) chose to treat the e structure “as a thermodynamically stable phase which has ordered with the IT structure” and suggested that it “may be produced by at least two mechanisms’*. The present calorimetric data are the first directly measured thermodynamic properties available for c plagioclases and they appear to point to a rather different set of conclusions from these views. Without

the appropriate entropy and heat capacity (Cp) data absolute statements on the relative stabilities are not

possible. but the trends are clear. The enthalpy stabilisation due to ordering on the

basis of the e structure is significant for all intermediate compositions and the maximum observed value (-2.8 kcal/mole at An,& is comparable to the cnthalpy change associated with commensurate ordering in albite and anorthite. Thus. whatever the precise cation configurations of e plagioclases might actually be,

they are energetically favoured (in terms of enthalpy) to a remarkable degree. It is also evident. from Fig. 5, that Ii ordering is viable only for a limited composition range from An,,x, towards albite. LHoti

and AH&,, for Ci = 11 ordering decrease rapidly

with increasing albite content and would extrapolate to zero at -An6” (Fig. 5). For the composition range

-An,,-An,o both e and 11 structures arc possible but the enthalpy of e ordering is significantly greater than for 11 ordering. For compositions more albite rich than -An,. the 11 scheme is simply not available as an alternative to the e structure.

What are then the sequences of ordering at each composition within the plagioclase solid solution? A range of possibilities is shown in Fig. 7. In pure

albite, of course. the sequence with falling temperature is monalbite (C2/m) - high albite (CT) - low albite (CT) with some kinds of transition between them (Fig. 7a). At -Anur the only ordering scheme possible at low temperatures appears to be of type e. so the

sequence must be Cl - e (Fig. 7b). At Anh. ii and e ordering are alternative possibilities and three different sequences (Fig. 7c-e) must be considered. It is known from experiments that -Anhs crystals with the e structure will transform to the Ii structure at - 1000°C (MCCONNELL, 1974: CARPENTER and

MCCONNELL, 1984). The relatively high temperature part of the sequence must therefore be CT -+ 11. The low temperature part of the sequence could have e as a metastable alternative to 11 (Fig. 7c) or as a stable ordered state replucing 17 (Fig. 7d). The cnthalpy advantage for e ordering is greater than for Ii ordering so it seems inevitable that at some low temperature the former must become the stable state. The sequence must therefore be CT ---- 1: -- e (Fig. 7dL though


there may be a small temperature range over which e is just metastable relative to Ii (Fig. 7e). At Anloo the sequence would be Ci - Ii - Pi if the melting relations were ignored (Fig. 7f).

A very approximate estimate of the entropies involved may be made by treating the ordering reactions as being first order in character and ignoring AC,, effects. The enthalpy of ordering (A&,) for each transformation can be taken from Fig. 5 and values for the transformation temperatures (Toti) from the experimental results of CARPENTER and MCCONNELL (1984) though it is necessary to guess a value _for T$$f’. Taking Ar+,r as an example, AHz&$=” z - 1500 &/mole and 7$,$=li = 12OO’C. This gives AS~~F=li ~ -1500/1473 = - 1 Cal/mole - K. A maximum value for AH:;=’ is - -2600 Cal/mole, so that AH?:’ is -(-2600 + 1500) = - 1100 Cal/mole. If T$zc is assumed to be -800°C (and the precise temperature selected makes little difference to the general argument), A$,$’ = - 1 lOO/ 1073 = - 1 Cal/ mole-K. Thus e ordering might result in a significantly lower configurational entropy than Ii ordering at this composition. We may use the total entropy change for e ordering from a Ci structure, AS$,Ge ET 1 + 1 = 2 Cal/mole * K, and the total enthalpy change, AH:&=’ = -2600 Cal/mole, to estimate a value for Z$Aze (see Fig. 7e) of w-2600/2 = 1300 K; i.e., the Ci structure would transform to the “e” structure at - 1000°C if Ii ordering failed to occur.

The entropy values in these rough approximations should not be taken too seriously except insofar as they point to the reason why the “e” structure takes over from Ii at low temperatures when the composition deviates from anorthite towards albite. Ii ordering is most appropriate for an Al:5 ratio of 1: 1. Solid solution involves a change in this ratio and the Ii scheme becomes increasingly ineffective as a means of distributing Al and Si in an ordered manner. The free energy. enthalpy and entropy changes associated with the ordering progressively decrease, as does the equilibrium order/disorder temperature. On the other hand, the incommensurate structure involves an in- teraction between AI/Si ordering and Na/Ca ordering, according to MCCONNELL (1978), and is therefore

C21m CT

T T CT - Cl - Ii

--___ ci 00~) e IS (e)

*nll - An,, -A%

(a1 (b) (C1

impossible for pure anorthite but most viable in the intermediate composition range. The effectiveness of type e ordering as a means of lowering the free energy increases as the albite component is added to anorthite. while the effectiveness of the Ii structure is greatly reduced. As borne out by our enthalpies of ordering, one structure simply becomes more stable than the other. with a range of intermediate compositions (-An~-An,5) at which both are possible. At the pure albite end, of course, low albite becomes the stable ordered state. The stability of the e structure with respect to two phase mixtures is considered in a later section.

An important consequence of these arguments is that natural samples with compositions of --Atim- Anr0 would, on cooling from solidus temperatures, pass through the stability fields first of the Ci structure, then of the Ii structure and, finally, of the e structure. Serious kinetic problems with regard to ordering at temperatures above - 1000°C do not appear to exist (CARPENTER and MCCONNELL, 1984) and natural samples quenched from high temperatures can, in- deed, have Ii order (STEWART et al., 1966; MCLAREN and MARSHALL, 1974; WENK et al. 1980). There is, therefore, little reason to doubt that slowly cooled igneous plagioclase crystals in this composition range order first on the basis of the Ii structure before developing the e structure at lower temperatures. The evidence of microstructures in plagioclases is not necessarily inconsistent with this.

In the context of the structural states of slowly cooled plagioclases with compositions of -AnTo, the Crystal Bay sample is highly unusual. On the basis of other TEM studies (HEUER et al., 1972: NISSEN, 1974; NORD et al., 1974; MCLAREN, 1974; MCCON- NELL, 1974; MCLAREN and MARSHALL, 1974: CLIFF el al., 1976; GROVE, 1976, 1977a; WENK and NA- KAJIMA, 1980) exsolution lamellae would have been expected but, instead, it is homogeneous with the Ii structure. The thermal histories of the anorthosites from which the sample came, however, are highly unusual. They had a complex crystallisation and metamorphic history before being incorporated as rafts in a gabbroic intrusion (MORRISON ef al., 1983).

ci

IT

e

CT

Ii

PI

- *“ss .. *“I3 (4 (4

FIG. 7. Possible sequences of structu@ states with falling temperature (schematic). The displacive transformations C2/m = Ci and Ii = PI are shown only for completeness. (a) Pure albite. (b) -A&,. (c) _A%. At low temperatures (below the dashed line) the e structure becomes a metastable alternative to 1 I. (d) -An65. e structure becomes_stable relative to Ii at low temperatures. (e) -An,,, as in (d). but @h a temperature range (between T”=’ ord and TI$z”) over which the e structure is metastable relative to I I. (f) Sequence for pure anorthite, if melting is ignored.

962 M. 4. Carpenter. J. D. c‘. McConnell and A. Navrotsk\

Presumably the unexpected structural state of the plagioclase reflects the unusual heat treatment to which it was subjected.

Temperature dependence q/‘ ordering

Values for the enthalpy of ordering (AHO, and AH&) described in this paper refer only to the difference in enthalpy between two rather arbitrary states, one substantially ordered and the other equilibrated at some high temperature. The order/disorder behaviour of each structure appears to involve at least some component of a continuous variation with temperature. The Ci Z Ii transformation seems to be continuous with respect to composition and, therefore, also with respect to temperature (see CAR-

PENTER and MCCONNELL, 1984). We have argued that the AH=,. values obtained with different anorthites are consistent with a continuous change in Al/ Si order. For e plagioclases we might expect the same kind of result. Igneous and metamorphic e samples in the composition range - Anm-AnTO gave different heats of solution which could be explained in this way but, because they were not all subjected to identical heat treatments in the laboratory, the con- clusion is not as certain. End member albite also shows signs of having continuous order/disorder properties (summarised by SMITH, 1983b).

While the transformations between the different structures may or may not involve a first order break actually at their equilibrium transformation temperatures, it is clear that physically realistic models of the ordering will need to account for a temperature dependent ordering contribution at every composition in the solid solution.

e and Ii solid solutions at low temperatures

AH=,” values for the “low” Ii series show a fairly well defined trend in Fig. 3. If the curvature of the trend is real it probably reflects a reduction in the degree of order at compositions away from pure anorthite. As with the “high” series, separating out the contributions of straight mixing from those of ordering will not be a simple matter.

Similarly, compositional changes in the low temperature intermediate solid solution are accompanied by changes in order, as evidenced by variations in the orientation and spacing of the type e antiphase domains (GAY, 1956; SMITH, 1974; GROVE, 1977b; WENK, 1979a; RIBBE, 1983). It is evident from Fig. 5 that the enthalpy associated with ordering of the most ordered (metamorphic) An-rich e-plagioclases is greater than for ordering of Ab-rich samples. The apparent change through - Anw coincides with breaks in other structural parameters (SMITH and GAY, 1958; DOMAN et al., 1965; BAMBAUER et al., 1967a,,b; SLIMMING, 1976) and also with the approximate position of the experimental Cl/Ii transformation line when extrapolated to low temperatures (CARPEN- TER and MCCONNELL, 1984). This difference in the

enthalpies must reflect some difference between c structures which form within the Ii field and those which form in the Cl field. perhaps due to the different driving forces for AI/Si order.

MCCONNELL (1974) has argued that a structural break at - AnSo relating to the “e” ordering behaviour could be responsible for the Beggild miscibility gap. We can rule out exsolution driven simply by non- ideal mixing in this range. because there is no evtdence for a large positive excess heat of mixing for the e solid solution. The model proposed by MCCONNELL (1974) is therefore not inconsistent with our results and the correct answer to the problem must surely involve differences in ordering behaviour with composition.

If, as has been commonly proposed (see review by SMITH, 1983b). the e plagioclase structure is simply a fine scale intergrowth of albite-like and anorthite- like slabs, the latter having an antiphase relationship from one slab to the next. a simple linear variation in AH,, with composition might be expected, reflect- ing merely the changing proportions of the Ab and An units. Extrapolation to the pure end members should then give AHwd for pure albite and pure anorthite, less a small contribution for interfacial effects. Clearly the calorimetric data suggest a more complicated picture than this and. for thermodynamic purposes, the e structure is rather more than a simple intergrowth of two phases.

Electron diffraction patterns from the ordered sample at -AnM (Hawk Mine, Bakersville, N. Carolina) had e reflections which were extremely weak and diffuse, indicating only limited short range ordering on the basis of the e structure. Its AH,,, value, however, shows an increase relative to the value at Ant, (Fig. 4), as if tending towards the value for low - high albite. It is possible that the ordering too is tending towards that of low albite. Curves linking the e and low albite data have not been drawn in on Fig. 3 and Fig. 5 because the thermodynamic relations between the Ci (low Ab) and e structures are not well understood. For the same reason no attempt has been made to show a definite relationship between the e and low Ii curves.

Subsolidus phase relations

Having decided that the e structure might be stable relative to an Ii structure at the same temperature and composition, the next question to ask is whether it can also be stable relative to two phase mixtures and therefore have a true equilibrium field of stability. In the past the general view has been that at intermediate compositions and low temperatures the equilibrium assemblage is low albite plus anorthite (SMITH, 1974, 1983b; WENK, 1979& GROVE et al., 1983). Again, the calorimetric data are not quite so unequiv- ocal.

Treating the low temperature end-members, low albite and anorthite, as having complete Al/Si order


(i.e. zero configurational entropy) and excluding possible AC’, contributions provide a very approximate means of assessing stable equilibrium assemblages at intermediate compositions. If a line is drawn between

the A&l, values of Amelia albite and Pasmeda anorthite in Fig. 3, most of the e samples plot within - 1500 Cal/mole below it. For the e structures to be stable as homogeneous phases in place of Ab + An this apparent positive excess enthalpy of mixing must be balanced by a configurational (and/or vibrational) entropy contribution to the free energy. Thus at 25’C the excess entropy of the e structures relative to the pure ordered end members must be at least 1500/ 298 = 5 Cal/mole - K. Corresponding values at 300 and 500°C are -2.6 cal/mole- K and -1.9 cal/ mole - K respectively. The configurational entropy of e plagioclase is unknown, but if it is zero, i.e. if the structure has complete cation order, albite plus anorthite will almost certainly be the stable assemblage at all three temperatures. A configurational entropy of 5 caj/mole . K is unreasonably large given that the maximum possible configurational entropy at Anso is 6.7 cal/mole - K; Ab + An is probably the stable assemblage at 298 K. Values of 2.6 and 1.9 Cal/ mole - I( (1.7 and 1.3 Cal/mole - K for 67796b, Anm, which gave a larger AH,,,,,, value), however, are not impossible and, therefore, e plagioclases might be truly stable at these temperatures.

Estimates arrived at in this way are sufficient only to demonstrate that, because of the large enthalpies of ordering, true equilibrium stability for the e structures is a real possibility. Some means of assessing the entropies properly is needed to resolve the issue but, because of the general problems of distinguishing Al and Si atoms with X-rays and of obtaining only average structures (see WENK et al., 1980, and review by RIBBE, 1983). conventional structure refinements may not be very helpful in this respect. Clearly, however, e ordering is likely to be highly influential in the BBggild and Huttenlocher exsolution reactions and, if the e structures do have a significant stability range in temperature and composition space, then every equilibrium phase diagram which does not specifically include them is wrong. The volume effects, both of ordering and mixing, are too small for pressure to be an important factor.

CONCLUSIONS

Although a great deal of data has been generated in this study, the ability to calculate exact thermodynamic properties is, as yet, rather limited. The problems are firstly to separate out mixing and ordering contributions, secondly to distinguish between the thermodynamic behaviour of the different possible ordered structures and, finally, to produce physically realistic, continuous ordering models for every composition in the solid solution. The data do, however, provide the first systematic set relating to the ordering and provide a basis for testing hypotheses of equilib-

rium and stability. We have deliberately tried to assess the implications of the enthalpy and experimental results independently of previous accounts of the plagioclase subsolidus phase relations. Interpre- tation of the many and complex microstructures observed by transmission electron microscopy, for example, involves a different set of subjective judge- ments. Perhaps the questions raised here, particularly relating to the sequences of ordering, will lead to some reconsideration of these observations, but in the future it will obviously be necessary to reconcile all strands of the microstructural. petrographic, experimental and calorimetric evidence.

Our principal conclusions are as follows:

1. The enthalpy differences between ordered and disordered plagioclase feldspars vary in the range l- 4 kcal/mole.

2. Enthalpies of mixing for the “high” structural stale series are consistent with the interpretation of two ideal segments, Ci and Ii, related by a non-first order transformation.

3. Pure anorthite has a temperature dependent variation in its degree of Al/Si order which contributes to an enthalpy difference (as measured at 7OO’C) of -800 Cal/mole between metamorphic anorthite and anorthites equilibrated at - 1300°C.

4. The enthalpy of ordering for a symmetry change ri s Ci decreases in magnitude from a value estimated at -3.7 T 0.6 kcal/mole at Anloo and extrap elates to zero at -Anm.

5. In the composition range AGS-Anlo the enthalpy change associated with ordering on the basis of the e structure is greater than for Ii ordering. It has been argued that this implies a field of true stability for the e structure relative to the Ii structure at low temperatures.

6. Type e ordering at anorthite-rich compositions gives a larger enthalpy effect than e ordering in more albite rich compositions. This change in the enthalpy of ordering, at -AnSO, may be important for the origin of the B0ggild miscibility gap.

7. The assemblage Ab + An is probably stable in place of an intermediate structure at room temperature, but it is not inconceivable that e plagioclase could become the stable state at higher temperatures. Only moderate ( - l-2 Cal/mole - K) configurational entropies for the e structures may be required.

Acknowledgements-We are most grateful to P. Gay, C. Francis and R. C. Newton for generously providing us with important samples for this study, and to J. V. Smith and G. L. Hovis for their critical comments on the manuscript. We also thank A. R. Abraham for assistance with collecting the X-ray data. Financial support from the Natural Envi- ronment Research Council of Great Britain (grant no. GR3/ 4404 to JDCMcC) and from the National Science Foundation (NSF grant no. DMR 8 106027 to AN) is gratefully acknowl- edged. This is Cambridge Earth Sciences Contribution no. Es 517.

Editorial handling: P. C. Hess

964 M. 4. Carpenter, J. D. c‘. McConnell and :\. \javrotsh\

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