Entbalpies of Ordering in the Plagioclase Feldspar Solid Solution

W16-7037/8.5/53.00 + .oo

Entbalpies of ordering in the plagioclase feldspar solid solution

M. A. CARPENTERand 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 revisedform 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 NEWTONet 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.

solid solution is unusuaf, however, in that the end members have different translational symmetry, with P~_AGIOCLASEEELDSPARS are among the most abun- an Al/Si order/disorder transformation occurring at dant minerals of the earth’s crust. They are also an intermediate composition. This change in order among the most extensively studied but, due to their between ordered anorthite and disordered albite may remarkably diverse subsolidus ordering and unmixing be responsible for the observed non-ideality (CAR- behaviour, our understanding of their thermodynamic PENTERand MCCONNELL,1984). At low temperatures properties is rather limited. A ~omp~hensive model the solid solution contains three different ordered of the solid solution which took full account of both structuresz low albite, the intermediate or e structure ordering and mixing ef%ct.s would undoubtedly be of and anotthite, and to treat the overah mixing properly great value to petrologists, in geothermometric and it will be necessary to define the ordering of each of geobarometric calculations for example, because of these both as a function of temperature and of the numerous heterogeneous equilibria in which pla- commotion. gioclases are involved during the evolution of igneous A thorough inv~i~tion of ail aspects of the and metamorphic rocks. An important step towards ordering and mixing of plagioclase feldspars, sufficient setting up such a mode1 was the solution calorimetric to generate quantitative thermodynamic properties study of mixing in high structural state plagioclases for all temperatures and compositions, would ob- by NEWF~N et al. (19801, which supplanted the viously be a daunting task. However, a judicious earlier results of KBACEK and NEUVONEN (I 952). choice of thermochemical measurements on selected Newton ef al. found small positive deviations from samples might prove instructive at least as to the ideal mixing in synthetic samples prepamd at 1200°C, relative stabilities of tbe separate su~~~u~s and ZOkb, from glasses. The high temperature plagioclase 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

Palmeda Hawker Pmmda Alp, hr8.3 Cl,+$tah Pl (#harp b,c, No b domains, c Gay (1953, 1951) mineral Pama VLlley, in vu** of d rcflcctionr dauina rcvcral UQinwright 6 StQrkcy (1971) coIl9ction Auntrio tharmlly urn in diumrtcr ?mllQr 6 w*nk (1973) no. 3776 m?tQmorphoQed (MUlIar h Uenk. MUllc1 et Ql. (1973) linrntone: Di- 1973) Frey Qt al. (1977) Ct-An-Phf-SP- Adlhart Qt al. (19SOa.b) (Prf- eklt nDnta liarker VrQuviur, Volcanic ejecta, Py (rbarp b.c, Nc b dmina. c Gay (1953) sarv tiin4?r2ll ItQly rberlM11y d rcflQctionQ) dooll‘nr SGD- Elmw ct al. (1962) collection meteaorphoscd 3a3$ in K&r.r et .i. (lb621 limeorone: Pa:- diwr.Qr (Czmk Cuak et al. (1973) An-Phl-Ct-Sp at al. 1973) GruadF & Brown 0974) Bruno Qt al. (1976) 115G82a lkrker s. of Elultyra Cranulitt (cc- Pl (&a+~ b-c. coll*ctim NymQlmd kn intergrOWth) d raf leerions) no. 115082

217Ma Harka Vlakfmtixin, AoorthoQitt xi (harp b 1QolQt.d b This iQ Q-As tlo. 902 of collection Bwhtnld rQflactionQ, antiptuQQ ua8lxQr 1192r>. no. 21704 Cn*rlQX, diffum aad bound*risr TrmrvaQl Qtrakad c r*fl*ctionr)

101377r Harkar Silver Bay, Anorchorite Ii (*hrp b Iwilated b Similar **la fra St. collectioa L&r frm large rrf lw!.tionQ, QntipiwQ LouiDco., lnri.*att, ilo. 1013T7 Superior, inclu*ion in diff+mQ ad beusdQriQ8 deQeribQd by cly (19§3) XilLmrotr Brwer River 3trmkQ.d c Ilest at Ql. (1966) &abbro rQflictfonr) cryrta1 P. Gay Aaarthoaite IT (rlurp b Yracek 6 NQuvar,en (1952) B*Y rQfl*ctionQf 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)

42771b Narkmr Hoslyk, Norite “a” (aharp Q, SqlQ with aimilat CollQctim BushvQld f r.flQCtiOM) cosositioa from BuQhwld no. 42771 CaplQX, darcribed by WeLarca (1970) E. TrmrvQQl

i&e co P. Gay, LILsga Ii (WY StewQrt Qt 41. (1966) U.S.N.M. phQl%oGrysts rliCtly ncL.arm k nQZQbQl1 a9741 collcctioa in basalt QlmQtQ b It&my I UQ& (1978) no. 115900 rQflQCtiOW) WQnk lt r1. (19BG) WeaL 8 Naluji6~ (19%) TQgQi 6 lhrckrvr 0981)

Earkar SluarSutd Gabbro “Q” faarp L, NW 11976) cotlQction intrusion, E. f raflQCtiOW. Siril&r QQI@Q &tc+ikd no. 118724 GrcNnland, *oa VQry W&II* L Nakajiss (19SG) drill COrQ ‘I, diffuQQ f 968’ (Bidden raflrctionr) ZOflQ)

67799 Harker Anartboeitic “C” (Dh.Kp Qr lbmptmaur collection grmulitc f r.flQCtiOnQ) piaD, , mei- no. 67796 phwa dariss

1104401* Ilaker Ouluth, Anorthoritc “C” (*bmp . . w rmeiphQaQ ncconu*lf (197&l CollQtticd ximlQ*ctr f rQf1QCtiWbQ) daninr Capa~tar 6 IMon~Qlf (1984) no. 11046

91413b Barker StirlinS Pill, Aaphibolitc “C” fDtwp l t x&By gmin* Bioac fX96b. 1965) coll*otion SraLm Bill. f rQflQCtiWQ> coI)‘cIiIplow no. 91413 Ha South r*1ituds UQlQ!.

T-12-22* lbrkrr BelleAW Rscrystalli8ed Philpotts (1966) collection DQ*WlUi8rl norite no. 9bb60 .t.‘. Qmba! (sync*ctomic t*cry*calli*rtioa at hi&h prQQ.ure)

91315c RQrkor Worth Nine, "Qw (wQk and Biarl (1964. 1965) coll~etioa Broken Hill. diffum Q SlidaB (1976) (.rplQ P2) no. 91315 New South raf 1QctioaQ) li*1**

9749G p. WY, NQad of LivLlQ clear cry*tlls “a*‘ (wry Krwak 4 Nsuvonen (1952) U.S.N.X. Rock Creak, fro5 pQ_titc diffueQ . Gsy $1956) no. 97090 Mtcb*ll Co. * reflection*) Phillipa Qt al. (1971) N. Carolina Ordering enthalpy in plagioclase 949

SUplC Source Locality structure niero#tructure Kef*rmce*

Uaokb Earvard buk Nine. Clear crystal8 Extramlg veti No obviow Kracek b Ncuvvmn (1952) University B&*rsville. fro0 pegmntite md diffrue c sisns of CW (1956) dIl*rnl l4itcbe11 co., reflections, luolution c&l& 1 Llm (1974) collactiori 1. Urolitu not alii~p~ McLarm (1976) (described no. 97606 detectable faint adulation) Aulia Barker hlia Clear crymtals Ci low albite Wny ref*renc*a, c.*. see: Ab miraral Courthoure, from pegvtite strWzture PWguMD at al. (1958) collection Arlia Co., Ribbe at al. (1%9) Virginia Smith (1974) liarlow 6 nrmm (1960) Kibbc (1983) Kroll 6 Kibbc (1963) 8797% Urkcr Sittcmpundi, !3ytmmitc- Ii (sharp b Subr-iu (1956) collection Plldras edcnitc gnciss reflections. no. 07975 diffuse and- Itretied c rcfl*ctions) Nikaj iu Prof. R. c. abbroic 11 (ghrrp b char1u et al. (1978) NeVt0n nodule*, reflections. Newton et al. (1980) ejected from streaked c VOlCUlO reflections)

ited objective in mind we have undertaken a prelim- was used only for cell parameter determination (87975a) inary study of the order/disorder properties. In this and a final sample, anorthite from Mikajima volcano, Japan, which was generously provided by Professor R. C. Newton paper we present the results of heat of solution to allow some interlaboratory comparison of calorimetric measurements made at -970K in Pb&O5 on natural data. Table 1 gives the provenance and structural states of “low” and heat treated “high” plagioclase samples. the natural samples. Many have been described in the It is not possible to reproduce the lowest structural literature and, as far as possible, references are given to the states seen in natural plagioclases on a laboratory principal papers in which the same or very similar material has been used for structural, TEM or experimental studies. time scale, and a study of ordering properties must depend on ordered crystals separated from rocks. For Separation and purification solution calorimetry, particularly pure and homogeneous material is required so that considerable em- We have attempted tc remove all impurities from the phasis has had to be placed on selection and purifi- plagioclase powders. First, the rocks or pegmatite crystals cation; the final suite of plagioclases used for the were crushed and passed through a 76 rm sieve and the solution measurements came from a very wide range dust fractions separated by flotation in water. A feldspar concentrate was then obtained using standard heavy liquid of geological environments. In addition, conventional methods and this was further split into nanow density powder X-ray diffraction methods of characterisation fractions with tetmbromoethane (TBE) diluted as necemary would not, alone, have been adequate either to with acetone. At this stage the most promising density ascertain the homogeneity of the crystals or to divide fraction, i.e. the fraction with the cleanest looking grains and the narrowest composition range (as determined by them into more specific categories than simply “high” electron microprobe analysis of gram mounts), was selected or “low” structural states. Every sample has therefore for the final purification. In each case about I g of powder also been examined by transmission electron micros- was wrapped in a small envelope of thin Pt foil and heated copy (TEM). for up to 12 hours in air at -800°C. The purpose of this Enthalpy differences associated with changes in treatment was to decrepitate any fluid inclusions, dehydrate any aheration products in the feldspar grams and oxidise cation order between the natural and heat treated any Fe-bearing impurities. Rather small heavy and light samples have turned out to be significant at every fractions were then removed in diluted TBE, generally composition examined. The implications and rami- leaving -0.8 g of plagioclase grains spanning a very narrow fications are numerous but we have restricted our range of density. As the last step, any remaining grams analysis only to the most immediate issues. Readers which were obviously not plagioclase were removed by handpicking under a high powemd, incident-light micromope. are referred to Reviews in Mineralogy, Volume 2, By the end of this procedure the powders contained much second edition (1983, RIBBE, ed.) and to SMITH less than 1% of impurity grains and consisted of equidimen- (1974) for detailed descriptions of the plagioclase sional or slightly acicuhtr plagioclase crystals in the size feldspars, their structures, superstructures and micro- range -SO-X@ pm (none, of course, being wider than 76 pm, the sieve size). structures. The density of quartz is very close to that of plagioclase in the composition range AnM-Ah, making it virtually SAMPLE DESCRIPTION AND METHODS impossible to obtain a complete separation of the two. This restricts the range of sodic-plagioclase samples available for Natural plagioclase feldspan were selected for this dori- calorimetry to those taken from quartz free rocks, unless metric study on the basis of their composition, structund the grain size is large enough to allow an initial separation state, homogeneity and lack of alteration or inclusions. A by hand picking. 8imiiatiy, the density of natural calcites very large number of ~ampks from a wide range of geological spans that of anorthites and the Pasmeda anorthite could environments was examined. From these, a final set of only be obtained as a pure powder because the 8OO’C heat nineteen was sekted, consisting of seventeen prepared treatment caused decarbonation of the calcite and, therefore, specifically for heat of solution measurements, one which 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 Some of the powders also became slightly sintered dunng separated even from pegmatite or phenocryst crystals which the high temperature annealing, but no glass was observed appeared to be translucent and clean (e.g. Amelia albite. in them by transmission electron microscopy and they were oligoclase from North Carolina, Lake County labradorite easily disaggregated by very light pressure in an agate mortar and Mikajima anorthite). A final difficulty was that in a few and pestle. In view of the extremely slow rates of .41/Si cases (91315c, 91413b, llO44pl*) the 800°C anneal caused order/disorder at relatively low temperatures and under dry the grains to develop a pale brownish tinge. On inspection conditions it is unlikely that any of the 800°C heat treatments under an optical microscope this colouration appeared to caused a modification of the structural states of the crystals. be due to a red surface coating, presumably of iron oxide, on many of the crystals. The worst grains could be removed Characterisation (!I structural .\raw by hand picking but the 9 I3 15~ powder still looked brownish. The I 1044pl* and 91413b powders used for calorimetry Grain mounts of the powders used for solution calonmetry had only a very pale brown colour while the other powders were prepared for electron microprobe analysis. Approxi- were not visibly affected and remained white. mately 10 grains per sample were analysed for Na. Mg. Al. Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, using an energy dispersive Heat treatments system following the procedures described by SWEATMAN and LONG (1969) and STATHAM(1976). Only Na, Al. Si. Each of the purified plagioclase powders was split into K, Ca and Fe were detected. Proportions of Ab, Or and An two batches, one for calorimetry without any further treat- molecules were calculated from the Na. Ca and K contents ment, and the second for disordering at high temperatures for most of the plagioclase range. The Amelia albite crystals. prior to a second set of calorimetric measurements. The however, appeared to suffer Na loss in the electron beam disordering was accomplished by wrapping the powders in and it was necessary to assume that Na = I.0 - (Ca + K) Pt foil envelopes and suspending them in vertical Pt. MO or atoms per formula unit. In addition, compositions more kanthal wound furnaces, at temperatures of between 107O’C An-rich than --AQ,~ consistently had Na + K + Ca < I.0 and 1350°C and for times of up to 48 days (Fig. 1). per eight oxygen& a feature which has been ascribed to Annealing times were selected on the basis of previously incomplete stripping of the Na peak in the energy spectrum published experimental data. For example, Amelia albite is at low Na contents (CARPENTERand MCCONNELL,1984). generally consiieted to reach an equilibrium state of disorder For these samples, An = Ca. Or = K and Ab = I.0 - (Ca after about one month at 1050°C (MCKIE and MCCONNELL, + K) have been assumed, giving satisfactory agreement with 1963; HOLM and KLEPPA, 1968) and labradorite of com- the measured Al and Si contents, and analyses obtained by position -Anlo after about l-2 weeks at 1300°C (CARPEN- other means (CARPENTERand MCCONNELL, 1984). Mean TERand MCCONNELL,1984). Kinetic experiments to ensure molecular proportions of Ab, Or. An and the range of that an equilibrium state of disorder was achieved at each compositions in each sample are given in Table 2. Iron was composition, however, were not undertaken. only detected in the more calcic plagioclases but was present After the high temperature heat treatments. each sample at rather low levels (given as Fe0 in Table 2). Two samples was held in air at 800°C for a few hours in order to try to were reanalysed after the high temperature heat treatments return any Fe-bearing impurities to the same oxidation state (T-l2-22a/l, 1190°C, 110377a/l. 1370°C and 1300°C): no as in the starting material. All the samples emerged looking changes in composition due, for example. to evaporation pure white in colour, including those which had started with could be detected. a brownish tinge. This suggests that some irreversible change Crushed grains from all the calorimetric samples were in oxidation state of the small amount of iron in these examined in an AEI EM6G transmission electron microscope samples (91315~ 91413h 11044pl*) may have occurred. 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 CARPENTERand MCCONNELL,1984). Horizontal spread of each point represents range of composition. vertical spread represents range of annealing temperature. Ordering enthalpy in piagioclase 951

field conditions in the electron microscope. The only piagioclases with obvious signs of exsolution were 4277 1b, 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 PMU& n.d. the TEM observations on the natural and heat treated lbmt. sou 0.14 samples are included in Tables 1 and 4. nikajir cioj 0.42 115092* (10) 0.22 Unit cell parameter data were also collected for the S7975a (10) s.d. calorimetric samples, and are given in Table 3. Up to fifty 21704a (10) 0.29 101377s (10) 0.3k re5ections in X-my powder ditfmction traces were indexed c4ste1 Ilay 0.28 with the assistance of the calculated powder patterns given b277tb I:; 0.20 Laka co. (10) 0.36 by BORG and SMITH(1969) and the tables of BAMBAUER et SKSHW (20) 0.31 al. (1967a). The X-ray traces were collected at a scan rate 677%b (10) 0.d. 11044e1* iioi 0.34 of 118’ 28 per minute over the range 20-60’ 28 (CuK, 91413b (9) xl.*. radiation), and pure silicon was used as an internal standard. I-12-22a (8) n.d. 91315c (1.0) l&d. The least squares computer programme used for the ceil 97490 110) Ld. parameter refinements was written by Profffsor C. T. Pmwitt Il.*. B.wkb (8) and is based on the method of BURNHAM (1962). Conven- Amelia Ab (11) “.d. 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 reflections at h _+ k = odd, I = odd Positions which were heat treated samples have “high” structural states (Fig. 2). either absent (Cl structure), diffuse (Cl structure with short There was insufficient material to measure the heat of range cation ordering), single (type b reflections of the Ii solution of anneald Pasmeda anorthite (Pasmeda/l) but struchm)or paired (type e iefkctions of the incommensurate, cell parameters of this and of two extra samples (87975a. intermediate plagioclese ~tmCtaue). In addition, the presence 87975a/l) are included in Table 3. 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 The calvet type twin calorimeter and the techniques used and examined in more detail under bright field and dark 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 kbnteSomall (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

8.176(l) 12.676(l) 14.%75(2) 93*11(l) 115.69(l) 91*28(l) 1336.7(4) 0 8*100(l) 12.8730) 14.176f2) 93.16(l) 115.65(l) 1339.50) 0 B.183M 12.674(2) 14.176(2f 93.19(l) 115.650) x:*:;::; 1340*3(S) 0 mIlta sou/2 6.imMilj 12.678(t) 14.18Ot2) 93.15(l) 115.69(l) 91:23(l) 1340.2(4) 0 115oB2~ 8.179(l) 12.876(l) 14.1&I(2) 93.16(l) 115.YZW 91,17(l) 1339.5(4) 0 115062a/l 8.185(l) 12.878(2) 14*lBO(3) 93.23(l) 115.64w 91*16(l) 1341.4(5) 0 67975a S.lSlilj 12.876(l) 14.166(2) 93.32(l) 116,01(l) 90.94(l) 1339.5(4) 0 fJ7975a 8.181(l) 12.673(l) 14.167(2) 93*35(l) 115.6S(l) 90*99(l) 1340.60) 0 21704a 8.184(l) 12.678(2) 14.197(2) 93.37(l) 116.050) 90.63(l) 1340.5(5) 0 21704all 6.181(l) 12.87siij 14.193(2) 93.38(l) 115.92(l) 90*65(l) 1341.2(4) 0 101377a 8.179(l) 12.679(2) 14.201(2) 93.45(l) 116.05(l) 90.67(l) 1340.3(S) 0 101377*/l 8.1BO(l) 12.676(l) 14.201(2) 93.46(l) 115.95(l) 90.67(l) 1341.2(S) 0 Cryeta nay 8.175(l) 12.673(Z) 14.205(2) 93.45(l) 116.09(l) 90.56(l) 1339.0(5) 0 Cnatal Bay/l 8.1790) 12.874(2) 7.100(l) 93.45(2) iis.oiiij 90.55(2) 670.2(2) 0 42771b 8.181(l) 12.871(l) 7.104(2) 93.43(l) 116.150) 90.43(l) 669.6(2) 1 -0.5 669.3(2) 4277lb12 &180(l) 12.880(2) 7.102(l) 93.45(l) 116.01(l) 90.560) 670.7(3) 1 -0.5 670.2(3) L&t Couaty 8.173(l) 12.874(l) 14.205(2) 93.460) 116.06ilj 9oo.sl(l) 1339.2(4) 0.5 -0.5t 1336.7(4) Lake alnt~/l 8.176(l) 12.876(l) 7*102(l) 93.460) 116.03(l) 90.51fl) 670.1(2) 0.5 -0.3 669.8(2) BKllElP B. 180(l) 12.873[2) 7.104w 93.460) 116.13(l) 90.40(l) 669.9(2) 1 -0.6 669.3(2) Sxaiwll 8.177(l) 12.677(l) 7.101(l) 93*45(l) 116.01(1) 90*55(i) ~~~~~~ 1 -0.6 669.7 (2) b2779b 8.176(2) 12.656(2) 7.105(I) 93*50(l) 116.20(l) 90.07(l) 1 -0.6 667.863) 62779btl &172(l) 12.875(2) 7.102(l) 93.46(l) 126.06(l) 90.47(l) 669:5(2) 1 -0.6 66S.9(2) 11044@1* 6.162(l) 12.870(l) 7.106(l) 93*47(l) 116.16(11 W.lOW 670.3(2) 2.5 -1.4 668.9(2) 11044g1*/1 6.176(l) 12.876(l) 7.103(l) 93.44(l) llS.O6(ll 90.45(l) 670.2(2) 2.5 -1.4 666.6(2) 91413b 6.167(l) 12.854(2) 7.111(l) 93.61(l) 116.2411).~. 89.83(l) 666.0(2) 0 _ 91413b/l 6.169(l) 12.675(2) 7*106(l) 93.50(l) 116.15(l) 90.37(l) 669.1(2) 0 T-12-22a 6.163(2) 12.857(2) 7.116(l) 93.57(l) 116.26(l) 89.790) 66&O(3) 1 -0.7 667.3(3) T-12-22a/l 6.167(l) 12.877(2) 7.106(l) 93.42(l) 116.23(l) 90.31(l) 669.0(2) 1 -0.7 666.3(2) 9131% 6.162(l) 12.853(2) 7.116(l) 93.65(2) 116.36(l) 69.67(l) 667,5(2) 1 -0.7 666.8(Z) 91315c/l 8.164(l) 12.875(2) 7.109(l) 93.47(l) 116.24(l) 90.27(l) 666.6(2) 1 -0.7 667.9(2) 9749Q 6.165(l) 12.849(l) 7.127(l) 93.71(l) 116.36(l) 69.25(l) 666.4(2) 2.5 -1.8 666.6(2) 9749011 6.169(2) 12.880(2) 7.111(l) 93.37(l) 116.26ilj 90.26(l) 669.4(3) 2.5 -1.6 667.6(3) 6.168(l) 12.8390) 7.136(l) 93.63(l) 116.45(l) 66.91(l) 668.6(2) 3 -2.2 666.4(2) E $1 6.166(2) 12.681(l) 7.113(l) 93.37(l) 116.33(l) 90.24(l) 669.2(2) 3 -2.2 667.0(2) Amlia Ab 6.141(l) 12.791(Z) 7.160(l) 94.25(l) 116.56(l) 67.710) 664.9(2) 1 -0.6 664.1(2) _lia Ab/2 6.162(2) 12.876(l) 7*111(l) 93.46(l) 116.42(l) 90.26(l) 667.7(2) 1 -0.6 666.9(Z) 952 M. A. Carpenter, J. D. C. McConnell and A. Navrotsk!

X” 90

88 a IO 20 30 40 50 60 m so 90 Ab mol % An An b I L 10 20 30 LO 50 60 70 60 90 A HO motYe 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 to very different P-T histories this is hardly surprising. the p&&&se powders were dissolved in R&OS at Much of the variation must be due to differences in -7tiC. T&y. 3-5 batches (i.e. 90-150 w in all) structural state and cumposition though it is necessary wm dissolved in each 30 a batch of flux. The plagiociase powdm dissolved rapidly, giving reaction times df l&s than to consider the effects of the minor components 30 minutes and actual heats of solution of - 1.5-2.0 calories. (principally potassium and iron) which also varied A correction lo allow for the effect of stirring was applied between samples. to each measurement This was determined in dummy runs with no sample and was frequently rechecked; it amounted The mean Or content of each sample is between 0 and 3 to a few % of the total heat of solution signal. Runs which mok%. Unfortunately a heat of solution value for K feldspar did not return to a steady background were t+%ed, leaving in lead borate at 700-C does not appear lo be avaibbk, but 4-6 rcceptlbk heat of soh&on values per sample. Calibfation there m data for albite and K-fel&ar glasses. which show was by the Pt drop method, allowing for 1% heap pick up a di5erence of -6 k&/mole between the two (HERVIGand by the Pt nugget during its drop. No special prrautions NAVROTSKY, 1984). This would imply a 60 &mote increase were taken to dry the powders before loading them into the in AIf, for each mde % Or addal and thcrrfore. a total calorimeter. They each spent between 5 and I5 hours at effect Of O-180 Cal/mole variation between sampks. Even -700’C in the calorimeter while it equilibrated, however, though the estimation is very approximate and is based on which should have been sufficient 10 drive off any adsorbed glasses rather than crystalline felbpan it serves to show the water. It is unlikely that any change in StNaural state order ofmagnitude of the likely contribution. In the present occurred during this equilibration period. context such a small tire can be ignored. Although the heat of solution measurements were spread Potenlially more serious is the e%cl of changing the over three main sessions spanning several months. a sin& ox&ion s!ate of imn. SMmr (1983a). in nGewi% pub&ed master stock of i%&O, was used thro@out. There was data for iron in pla@oclam!s. has concluded that in terrestrial some variation in the calorimeter temperature between samples it is pnsmt as a mixture of Fti+ and Fe’+. Half the sessions, however, from 708’C in the first, 704°C in the calorimetric smpks had no &ectabk iron and only 5 had second, to 694°C in the third. This variation of up to 14’C more than 0.30 wts (given as FeO). The amount of iron probably did not contribule significant drift to the Ai&+ available to be reduad or oxidised during diodution runs values but the calorimeter temperature is specifrd for each was therefore sma& and it rtms likeiy that the principal sample in Table 4. The complete calorimetric dala are given elTect of variations in Fe content would only have ban to in Table 4 and Fig. 3. introduce a degree of scatter. One exception may be 9 I3 15c which lies off the main “high” aru~rural state trend and DlSCU!SSlON OF THE DATA which was distinct from the other samples by virtue of the brownish colour of its grains. Enthalpies of solution Although the u and Arizona State high temperature sdution calorimdar are very similar in design and 0puistion There is clearly a fair degree of scatter in the AH&. there mry be variations between W&s of flux A?icicnt data plotted in Fig. 3, but in view of the fact that the lo give slightly different AH* results. Nevertheless, some samples are not cogenetic and have been subjected useful comparison may be made between the data for Ordering enthalpy in plagioclase 953

synthetic “high” piagiociases given by NEWIQN ef al. ( 1980) short range ordering in the Cf stability field, just outside and the heat treated natural samples presented here. The the li field, and the Anrr-Anroo samples have sharp b data of Newton PI al. are plotted in P&t. 4 along with the relkrctions, indicative of long range ordering. CARWml$ lines drawn from the data in Fig. 3. For the range from and MCCONNELL( 1984) also suggested that the “high” C I nure albite to -Ana them is a remarkable degree of overlap solid solution can be described as having zero heat of but for Aqe-An,ea&!re is a distinct divergence. Newton et mixing, i.e. that the AH,, values lie on a straight line. If al. svnthesised their olaaioclases from glass at 1200°C. 20 this assumption is accepted, a simple, linear e$rapolation kb for three hours. ‘T6e difference in heats of solution of the Ci solid solution to a fictive disordered (Cl) anorthite between the natural anorthites (this study) and the synthetic end-member can be made. Deviations from the straight line anorthite (NEWTONet al., 1980) could be explained either are then due to the enthalpy change on ordering from Cl by some effect of calorimeter conditions or by a degree of to the Ii structure. In Fig. 3 the Cl solid solution line is a metastabie Al/Si disorder in the synthetic samples. The heat Ieast squares fit for the A&, values of heat treated samples of solution values for two natural anorthites reported by with compositions between An, and Anrr. Sample no. &HARLUet al. (1978) (a volcanic sample horn Mikajima 9 131SC (An& was excluded from the fit because of possible volcano, Japan, and a metamorphic sample from the Sittam- problems due to an iron oxide coating on the grains (see pundi complex, Madms) agree well with Newton et al.% above), and only one of the values for annealed An,, synthetic anorthite. We have found a distinct difference (4277 1b/2, 1300°C) was used. 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 Enthalpies of ordering irrespective of differences in the calorimeter conditions. Our value of LV&, for Mikajima anorthite, prepared in the With the exception of Monte Somma anorthite, a same way as all our other samples, plots exactly on our trend for Ii structures annealed at high temperatures. We change in Ai&, was detected between every natural believe that the anorthite data of CHARLUef al. (1978) and sample and its heat treated equivalent. This difference NEWTONet a!. (1980) am slightly low due to metastable Al/ will be referred to as the enthaipy of ordering (A.&& Si disorder in their synthetic sample, and perhaps to impu- and is simply the enthalpy change at 7OPC on rities in their natural samples (the purification procedures were not described). Metastable Al/Si disorder has also been transforming from a relatively ordered state to a found in cot&rite cqstalhsed from glass (CANWTEI~ef relatively disordered state (see Table 5 and Fig. 5). al., 1983). Our results for the natural samples are very While there may be some scatter in the absolute different from those of KRACEK and NEUVONENf 1952). A&+, values introduced by variations in Or and iron They used acid calorimetry, however, and reported the presence of precipitates in their acid alter solution runs on contents, AH& should be much less dependent on anotthite-rich compositions. impurity contents because it is given by the difference Systematic trends shown by the A&,, data in Fig. 3 must between two samples with identical compositions. be explicable in terms of the mixing and order/disorder For albite this change in order corresponds to the behaviour of the system. In the most general terms, the high difference between low and high albite, both of which structural state (CT) samples plot along a fairly well defined line. The Ii structums annealed at - 13OO’Cdefine a second have Ci symmetry. Between Anrs and Ant,, with the trend and the natural Ii structures a third. The natural e exception of Lake County labradorite, AE& is for structures are the most scattered and the straight line drawn the e piagioclase structure transforming to the Ci, in for them is rather arbitrary. An important difference high albite structure, For Lake County (An& and between the “high” and “low” series is that the former represents samples which have been subjected to known Crystal Bay (An& samples the symmetry change is heat treatments and tberefore forms a more internally from Ii to Ci. The anorthite-rich samples (An,,- eons&tent set. The natural “low” samples come from meta- An& were annealed below the Ci * Ii transfor- morphic, vokanic and plutonic rocks, and pegmatites. They mation Iine (Fig. I), however, and AE& therefore have very variable states of order and this may be reflected does not include the effects of an actual change in in some of the scatterof the results for e plagieclases. In the “low” Ii series, the Anloo, An% samples are metamorphic, symmetry; in these cases it describes only a change the An%, Ann and Anw samples an: from slowly cooled of structural state within the stability field of Ii igneous rocks and the A&,*sample (Lake County) is volcanic ordering. in origin. The metamorphic and plutonic igneous samples should perhaps lie on the same trend for highly ordered Ii The present value of AEiH,, for low * high albite (3.08 structuresbut the Lake County sample need not, because it + 0.30 kc&/mole) is in good agreement with previous is representative of a relatively high ~u~ibmtion tempemtum. dete~inatjons at 700°C (3.4 * 0.25 kc&/mole, HOLM and A straight tine is therefore an alternative to the curved trend KLEPPA,1968; 2.86 & 0.23 k&/mole, NEWON et al., 1980; shown in Pig. 3. It should also be noted that the calorimeter 2.80 2 0.29 kcal/mole, BL~NOVAand KJSELEVA,I982) and temperature of -7OO_“C is well above the temperature of at 5VC (2.60 C 0.30 k&/mole, WALDBAU~~~and ROBE, the displacive Ii S PI transformation in anorthite (see, for 1971; 2.63 f 0.40 kc&/mole, THOMPSONet al.. 1974). The exampk, FREY er al., 1977; ADLHARTet a/., 198Oa,b). ~11 only published value known to the present authors for the heats-of solution of ano~ite-~ch ctystals are therefore di~rde~~ a plagioeiase of int~~iate ambition is for the I1 structure; low temperature atomic displacement given by BLINOVAand KEELEVA (1982). They obtained effectsassociated with the Pi structure can be ignored. 2.66 + 0.59 k&/mole as the enthalpy difference at -700°C A marked change in the trend of A&,,. values for “high” between andesine of composition An&b,&& and the same Ph@CfascS occurs between -A&, and -AnM, (Fig. 3). sample annealed at 1250°C for 22 hours. Their sample was CARPENTERand MCCONNELL( 1984) have argued that this reported to show a blue iridescence indicating the presence feature, which also appears at about the same composition of exsolution lamellae within the crystals. This compares in the data of NEWTONet al. (1980), is due to a discrete with 2.14 f 0.2 I kcalfmole for the An,,* specimen described order/disorder fci * ii) t~nsfo~ation. The new data are hen and a complete range of PJi, values for “e‘ plagioclams consistent with such an in~~~~tion in that the anneal& of l-3 kcal/mole (Table 5, Pii. 5) for the intermediate At+A% samples have diffuse b reflections, indicative of 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 Origin and structural state Physical Mean An Molecular Mean AH (solution at appe.¶riXlce content veight (kcal/d?tP T, ‘Cl 700°ct (9) (No. of lXX?asUre!ZlelltS)

Pesmeda Metamrphic, Ii White loo 278.211 17.90 * 0.24 (704) powder (5) Paameda/l 20 days 1302 * 4'C. Ii (sharp White ii b reflections, elongate c powder reflections) Monte Volcanic, Ii White 90 277.891 17.28 t 0.17 SOnma powder (6) (704) nonte 21 daya 1360 t 4%. Ii (sharp White 98 277.891 17.05 * 0.23 S-/l b reflections, streaked c powder (5) (704) reflections) Hikaj ima Volcanic. Ii White 97 277.731 17.03 t 0.33 (694) powder (4) 115082a Metamorphic, Ii White 96 277.572 17.81 t 0.14 (704) powder (6) 115082a/l 21 days 1306 + 4’C, Ii (sharp White 96 277.572 16.98 ? 0.17 (704) b reflections, streaked c powder (6) reflections) 87975a Metamorphic. Ii White 89 tt powder 87975all 21 days 1300 f 10°C, Ii White 89 -t 1 (sharp b reflection, very povder diffuse and streaked c reflections) 21704a Igneous, plutonic, Ii White 86 275.973 17.53 * 0.14 (704) powder (5) 21704aJl 23 days A366 s 8’C + 21 days White 86 275.973 16.13 f 0.19 (704) 1306 i 4 C, 11 (*harp weak b powder (6) reflections, very diffuse and streaked c reflections) 101377a Igneous, plutonic, Ii White 78 274.964 17.21 + 0.14 (704) powder (5) 1013771/l 23 days A366 ; B’C + 21 days Slightly 78 274.964 15.46 + 0.23 (704) 1306 t 4 C, I1 (*harp weak b riatered. (5) reflections, extremly diffuse white c reflections)

Crystal Igneous, plutonic. Ii White 72 273.735 16.66 ? 0.24 Bay pOWd*t (6) (704) Crystal 23 days 1346 f 7’C, Ci (SOW Slightly 72 273.735 14.77 + 0.10 Bay/l faint diffuse intensity at wintered, (6) (704) positions of b reflections) vhite 42771b Igneous, plutonic. “e” (amoe White 71 273.736 17.12 t 0.15 (704) exsclved grains) powder (6) 42771bll 14 days &373 $ 3’C + 23 days Slightly 71 273.736 14.54 * 0.24 (704) 1346 ? 7 C, Cl (possibly ointered, (6) some very diffuse intensity white at positions of b reflections) 4277Lbl2 14 days 1273 *_3’C + 21 days Slightly 71 273.736 14.70 f 0.11 (704) 1300 2 10 C, Cl (some faint sincered, (5) diffuse intensity at positions white of b reflections) 4277lbl6 21 days 1312 f 3’C. Ci Uhite 71 273.736 14.73 ?:0.31 (694) (diffuse intensity at positions pOVder (4) of b reflections) Lake co. Volcanic, Ii Uhite 68 273.257 16.01 f 0.17 (704) pOWder (5) Lake Co.11 21 days 1300 f 1oOc. ci (ame Slightly 68 273.257 15.33 * 0.18 (704) diffuse intmmity at positions riatered, (5) of b reflections) white SKHlW Igneous, plutonic, “e” Off-white 67 273.096 17.36 ? 0.13 (708) powder (6) SKHHw/l 10 days 1334 * 5%. Ci (SO= Slightly 67 273.096 15.57 ? 0.23 (708) diffuse intensity at poaitiOIm sistered. (6) of b reflections) white 67796b Hetemorphic, “e” White 60 271.978 17.77 i 0.16 (704) powder (6) 67796bf 1 21 days 1300 i 1O’C. Cl Slightly 60 271.970 14.97 t 0.23 (694) sincered. (6) white Ordering enthalpy in plagioclase 955

Mean An no1ecu1llr Mean AH1 1 Sample Origin and structural state Physical (kcal/mo!eP (solution at appearance content weight (No. of T. ‘0 700°ct (g) messurem?nts)

110440 1* Igneous, plutonic, “e” Pale brown 60 272.140 17.15 f 0.27 (708) powder (5) 11044P1*11 lo-14 days 1278 f 6’C. Ci (some Slightly 60 272.140 15.42 f 0.18 (708) diffuse intcnsitv .% msitions aintered, (6) of b reflectionsj white 91413b Metamorphic, “e” (with some very pale 49 270.058 17.76 f. 0.16 (704) B&gild exsolution) brown (5) powder 91413b/l 25 days 1250 * 10°C, Ci Slightly 49 270.058 15.62 f 0.14 (704) sintered, (5) white T-12-22a Metamorphic, “e” White 40 268.780 17.25 f 0.19 (694) p0Wder (6) T-12-228/1 35 days 1192 * 2Oc, cl White 40 268.700 15.85 f 0.19 (694) powder (5) 91315c Metamorphic. “e” Brownish 35 267.981 18.09 + 0.23 (704) powder (5) 91315c/l 37 days 1172 f l°C, Ci White 35 267.981 16.53 f 0.10 (704) povder (6) 97490 Pegmatite, “e” White 27 266. a63 17.59 t 0.20 (694) povder (6) 97490/l 38 days 1148 f 2Oc; ci Uhite 266. a63 15.95 t 0.09 (694) powder (5) Hawk b Pegmatite, “e” White 265.905 18.61 f 0.17 (694) powder (6) Hawk b/l 48 days 1110 * 20°C. ci White 265.905 16.31 f 0.23 (694) powder (5) Amelia Ab Pegmtite, ci (low albite) White 262.546 20.26 f 0.22 (694) p0Wder (5) Amelia Ab/2 42 days 1067 + 4’C, Ci (high White 262.546 17.18 f 0.20 (694) albite) powder (4)

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 the albite end of the solid solution, AHd increases from stabilities of the different-ordered plagioclase structures to - I.5 kcal/mole at An2, to -3 k&/mole at A%. 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 Volumes of mixing and so, at these compositions, it is not possible to produce Small but distinct volume differences were detected crystals with equilibrium_ Cl states. Values for the enthalpy change on going from I I to C I can. however, be extracted between the natural ordered samples and the same using the extrapolated Ci line shown in Fig. 3. This line samples after they had been heat treated. These gives an estimate of what the heats of solution of C? volume changes (AL’,) are given in Fig. 6a and structures between An,, and Anloo would be if they could Table 5 and, for comparative purposes, all refer to a be prepared. The difference between this extrapolated value and the value measured for the natuml, ordered sample is unit cell with c = 7A. An important difference be- then Hord, the enthalpy change for II G=Ci, as would be tween the enthalpy and volume measurements is that measured at 700°C (see Table 5 and Fig. 5). For pure the former were obtained at 700°C and the latter at anorthite AH& = 3.7 & 0.6 kca I/ moe,I where the error in- room temperature, -25°C. While it is possible to cludes an estimate of the uncertainty in the extrapolation. ignore the Pi G Ii dispiacive transformation in the The ovemll trends shown by the data should be more reliable than any conclusions based on a single composition interpretation of the enthalpy data, the volume data and in this respect a number of distinct features are evident may include effects of the lower symmetry (Pi) of from Fig. 5. Firstly, A&, and AH&, for the Ii - Ci the most calcic samples below -250°C. transformation show a rapid decrease in magnitude from Anloo to AQ~. (This trend is apparent even if the volcanic The largest volume change associated with a change in sample, Lake County, Ass, for which AH& = 0.7 kcal/ order was found for albite: 2.8 + 0.4A” (-0.45%). This mole, is excluded). Secondly, in_ the composition range compares with the value of 2.63A’ given by KROLL and Anb,-An,2 AH, values for e - Cl are signilicantly greater RtBllE (1983). For e plagioehrses the volume change, with than for Ii - Ci. There is also some variation in AIf_, for the exception of I 1044~1. and SKHHM* samples, is rela- e - CT at these compositions (A-An&, where more tively eonstant at -I + OSA’ (-0.15%). 11044pl* and than one sample has been measured. Thirdly, the most SKHHM* are both igneous in origin so their smaller volume ordered e plagioclases show a difference in LM&, according change on disordering may indicate either that they did not to whether they have An > -50 mole% (-2.8 kcal/mole) achieve as high a degree of order as the metamorphic or An < 50 mole% (- I .5 kcal/mole). The sample at An,9 samples or that their experimental heat treatments were gave an intermediate value of -2.1 kcal/mole. Finally, at 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 NEWTONet al. (1980) for synthetic hi structural state pla&clases in relation to trends from Fig. 3. Trian@es = Mikajima volcanic anorthite, open triangle = from CHARLUer al. (1978), filled triange = from this study. lnwzted triangk = rne~~c anorthite, from CHARLUn al. (1978). Ordering enthalpy in plagioclase 957

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

A&d Structural AV (at 25' ) (at 700%) 2580% Saqle change (93, c - 7)li (kcalfmlc) (kcallmole)

Pasocda Ii + Ii w4dc~ 100 0.6 f 0.6 3.7 * 0.6 Monte sm 11 * Ij mOO%) 98 0.05 + 0.65 0.23 + 0.29 3.1 * 0.5 115082a 11 * 11 wOo"c) 96 0.95 + 0.65 0.83 + 0.22 3.5 t 0.6 S7975a Ii + 11 wJO~c) 0.55 t 0.65 21706a Ii * 1-1(13000C) :z 0.35 f 0.65 1.60 + 0.26 3.0 t 0.6 101377a 11 * 11 mOOoc) 78 0.65 * 0.5 1.75 + 0.27 2.6 f 0.6 Crystal Say 11 l cl (13500C) 72 0.65 f 0.5 1.89 f 0.26 62771bll e *cl (13500C) 71 2.58 f 0.28 62771bl2 p aci (13000C) 71 0.9 f 0.5 2.62 t 0.19 LakC Camty 11 l Cl (13000C) 60 0.65 2 0.6 0.68 ? 0.25 SKHRW c *CL (1335,C) 67 0.6 f 0.6 1.79 f 0.26 67796b I?+ Cl (13000C) A0 1.1 f 0.5 2.80 * 0.28 11066Pl* c *cl (12SOoC) 60 -0.1 f 0.6 1.73 f 0.32 91413b e “Cl (12500C) 69 1.1 f 0.6 2.16 ? 0.21 T-12-22a e *cl U1900C) 60 1.0 * 0.5 1.60 t 0.27 91315c e z Cl (11700C) 35 1.1 f 0.6 1.56 i 0.25 97690 e + Cl (11500C) 27 1.0 + 0.5 1.66 * 0.22 Hawk b S *cl (1110 C) 20 0.6 + 0.6 2.30 * 0.30 Anlia Ab Cl *cl (1070°C) 1 2.8 f 0.6 3.06 r 0.30

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 correction factors were obtained by taking the slope of the found for some of the calcic samples annealed within the Ii volume versus composition curves at small Or contents field. 115082a, a metamorphic sample, gave the largest from the At&r data given by KROLL and RIBBE( 1983) and volume change of 1.O + OSA” and Monte Somma. a vokxutic 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

b 3.0 AHO, (gAH&) 2.5 kcol lmde 2.0

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) - Ci (high albite), solid error bars = e - Ci, dashed error bars = Ii - 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 M. A. C‘arpenter. J. D. C McConnell and A. Navrotskb

20 v A&xi

cR1 t5

10 20 30 10 50 60 70 60 90 b Ab mol % An An 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). the common sigmoidaI vdume of mixing behaviour observed To get the volume change at intermediate compositions, a for many solid solutions in terms of the size effects of the linear ~mbination of the two end member values was cations substituting for each other in each crystal structure. adopted, giving corrections to the measured cell volumes of In the case of the ptaleioclasesthe &ect of tetmhedrrd cation up to 2.2A3. The corrections are clearly arbitrary but give order may be more important, because changes in slope of volume-composition curves which are considerably more the volume-composition curves coincide approximately with systematic than the raw data alone (Fig. 6b). changes in ordering scheme. The actual volume changes In Fii. 6b two lines are drawn in, representing, approxi- tiated with disordering are small in comparison with mately, the trends for the low temperature, ordered samples, the total effect of #motion across the soIid solution, and the high temperature, annealed samples. The unit cell however. 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 IMPLICATIONS lie on two parallel straight lines. Low albite fatls well below the trend for ordered structures, probably because of its It might seem at first sight that the new calorimetric different (Ci) ordering scheme, and there is also a distinct data should be sufficient to establish quantitative change in slope at calcic compositions. The reduction in volume from a maximum at -An@ towards pure anonhite enthalpy models for both ordering and mixing in the coincides with the increasing intensity and sharpness of e plagioclase solid solution at a range of temperatures. reelections in electron diffraction patterns from the crystals. However, because of the highly variable or~r/~~~r and could. therefore, be due to structural changes in the If behaviour the most straight-forward mixing models, s Pi transformation. It also corresponds with an increasing i.e. regular sotutions, sub-regutar solutions etc., are degree of Al/S order. however. and this is a more likely cause of the volume fusion. There is no evidence for a not strictly appropriate. Moreover, estimating config- marked break in the volume~om~itioR relations for the urational entropies in a system which has three, “hi so&l solution so the Ci/Ii tmnsfo~ation could be possibly contin~us, order/di~rder tmRsfo~ations continuous in V. would be a major task in itself even if the usual Volume data for natural plagioc~ given by BAMBAUER et al. (1967b) have a greater scatter than shown in Fig. 6b problems of determining Al/Si distributions in feld- but show essentially the same trend. The data for high spars did not exist, The implications of this study structural state (synthetic) plagiocIasesof KROLL and MOLLER relate, therefore, more to identifying the stability (1980) and NEWTON et al. ( 1980) give comparabie variations relations of the different structures, with refenmce to with composition. The synthetic sampIeapnpared at 12OO”C, the overall solid state behaviour, than to producing 20 kb by NEWTON tl al. ( 1980) plot along the “low” trend of Fig. 6b for An,-Anta, and atong the “‘high” trend for free energy models of immediate petro~ogicai use. Ah--An%. NEWTON and WOOD (1980) have accounted for Our calorimetric measurements provide the first sys- Ordering enthalpy in plagioclase 959

tematic enthalpy dataset for “high” = “HOW”tmnS- AIJSi order for the temperature at which it was formations in plagioclases and are used here to annealed ( 12OO’C) and our different LW~, values identify some of the compositional and temperature are. caused by the difference between their calorimeter constraints on the different ordering processes. 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 AI/Si disorder in anorthite effect of ordering. It is interesting to note that the We have found that volcanic anorthites give differ- value for AH: derived by HELGESON et al. (1978) ent heats of solution from metamorphic anorthites from phase equilibrium studies is more negative than and that the values for volcanic samples correspond the calorimetric values of CHARLU et al. (1978, closely to the values obtained for a metamorphic natural samples), ROBIE et al. (1978) and NEWTON sample annealed at 1300°C for three weeks (115082a/ et af. (1980) by -900 Cal/mole. 1). Both the high temperature and low temperature These small adjustments to the enthalpy of anor- crystals have sharp b reflections and should have Ii thite can be associated very approximately with an symmetry at the calorimeter temperature. We suggest entropy change if the order/disorder transformation that the change in AHml, is due to a continuous (ci = Ii) in anorthite is treated as being first order. variation in the degree of Al/Si order with temperature An estimate of the equilibrium order/disorder (Ci within the stability field of long range Ii ordering. = Ii) transformation temperature for pure anorthite There is indirect evidence of some Al/B disorder (T0,.J is made by extrapolating the experimental in anorthite quenched from high temperatures. This results of CARPENTER and MCCONNELL ( 1984) for takes the form of small variations in unit cell param- the transformation at intermediate compositions to eters and spectroscopic properties (SMITH, 1972, 1974; Anlo,. Using this approach gives TOd = 2000-2250 BRUNO and FACCHINELLI,1974) and of variations in K. An 800 calorie correction due to ordering would the character of c reflections. The type c reflections then correspond approximately with an entropy cor- can be diffuse or absent, depending on the temperature rection of -800/2000 = 0.4 cal/mole* K while a from which anorthite crystals are quenched, and it 1700 calorie adjustment would give -1700/ has been proposed that the low temperature displacive 2000 = 0.8 cd/mole - K for the related entropy change. transformation, which gives rise to them, is sensitive In this context it is interesting to note that the third to the degree of Al/Si order (LAVESand GOLDSMITH, law entropy of anorthite given by ROBIE et al. (1979) 1954, 1955; GAY, 1954; GOLDSMITH and LAVES, is in good agreement with phase equilibrium data if 1956; MEGAW, 1962; SMITH, 1974; BRUNO and a configurational contribution of - 1 &/mole - K is FACCHINELLI,1974; BRUNO et al., 1976). Ii symmetry added to it (HELGESON et al., 1978; GOLDSMITH, is maintained up to the melting point, however, 1980, 1981; PERKINS et al., 1980; WOOD and HOL- (LAVES and GOLDSMITH, 1955; LAVES et al., 1970; LOWAY, 1982, 1984). BRUNO et al., 1976) and the degree of disorder even The value of AHbti = 3.7 + 0.6 kcal/mole, extracted in samples quenched from 1530°C is probably small for the enthalpy of disordering of pure anorthite from (BRUNO et al., 1976). Thus the enthalpy difference an Ii (ordered) state to an extrapolated Ci (disordered) of -800 Cal/mole observed between high and low state, as measured at 700°C seems “reasonable” temperature anorthites can arise only from a limited when compared with a total of -3 kcal/mole for number of Al/Si exchanges, disordering in albite (HOLM and KLEPPA, 1968; NEW- Published values for the enthalpy of formation of TON et al., 1980; BLINOVA and KEELEVA, 1982; anorthite from oxides (AH&,), as determined by THOMPSONet al., 1974). If again, as a first approxi- solution calorimetry, are not all in agreement. Some mation, the transformation is treated as being first of the scatter could be due to differences in Al/Si order, the total entropy of disordering is given by order between different synthetic and natural samples, AHaT,, = 1.4-2.2 cal/mole - K. This is rather less though the most recent value of NEWTON et al. than the maximum possible value (for complete (1980) on a synthetic sample (24.06 + 0.31 kcal/ order to complete disorder) of 5.5 &/mole - K, prob- mole) is consistent with the value obtained by CHARLU ably because the Ci solid solution at high temperatures et al. (1978) for two natural samples (23.93 f 0.48 has significant short range ordering of Al and Si kcal/mole) and also with a value based on acid (KROLL, 1978; KROLL and RIBBE, 1980). calorimetry of a synthetic sample by ROBIE et al. (1978) (23.89 + 0.82 kcal/mole, interpolated to 970 Solid solution at high temperatures K by NEWTON et al., 1980). We have not measured heats of solution for the oxides but if our anorthite The suite of synthetic high structural state plagio- AH,, values can be compared directly with those of clams preparedby NEWTON efal. ( 1980) was subjected Newton et al. our data suggest that AH&,0 for high to a uniform heat treatment. In the present study the temperature anorthites should be more negative than ‘*high” samples were not all annealed at the same their value by 820 + 330 Cal/mole and for low tem- temperature and the natural crystals also had different perature anorthites by 1670 + 330 Cal/mole. If the origins and impurity contents. AH&, values for the Newton et al. synthetic anorthite had equilibrium 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- anorthite”. MCCONNELL ( 1974) suggested that the ing at high temperatures are consistent with both. incommensurate superlattice develops metastably The data are somewhat scattered, so that small under conditions of substantial undercooling when. deviations from linear composition-U,,, relations for kinetic reasons. the stable commensurate super- cannot be ruled out, but they otherwise conform to lattice fails to develop. and this argument has been the interpretation of two ideal (zero excess enthalpy supported, on the basis of microstructural evidence. of nixing) segments related by a non-first order by WENK and hbiK.AJIhlA (1980) and WFN~ cv ui transformation (CARPENTER and MCCONNELL. 1984). ( 1980). GROVE et N/. ( 1983) chose to treat the e Mmeral solid solutions commonly show positive de- structure “as a thermodynamically stable phase which viations from ideality (NEWTON TV al.. 198 1) and has ordered with the IT structure” and suggested that these can be understood in terms of strain as a it “may be produced by at least two mechanisms’*. structure adapts to the substitution of cations with The present calorimetric data are the first directly different sizes (NAVROTSKY, 1971: NEWTON et al.. measured thermodynamic properties available for c 1980). Where cation ordering occurs at intermediate plagioclases and they appear to point to a rather compositions the net enthalpy of mixing at low different set of conclusions from these views. Without temperature tends to zero. as in the jadeite-diopside the appropriate entropy and heat capacity (Cp) data solid solution (WOOD et al.. 1980). or might even absolute statements on the relative stabilities are not become negative, as in enstatite-ferrosilite (CHATIL- possible. but the trends are clear. LON-COLINET et al.. 1983). Both the Ci and Ii The enthalpy stabilisation due to ordering on the segments of the plagioclase solid solution at high basis of the e structure is significant for all intermediate temperatures have a degree of Al/Si order which compositions and the maximum observed value (-2.8 varies with composition. Thus the apparently zero kcal/mole at An,& is comparable to the cnthalpy excess enthalpies of mixing for the CT and Ii segments change associated with commensurate ordering in may be the net result of both mixing and ordering albite and anorthite. Thus. whatever the precise cation contributions. configurations of e plagioclases might actually be, A change from short range ordering to long range they are energetically favoured (in terms of enthalpy) ordering at -Am4 (where the break in slope has to a remarkable degree. It is also evident. from Fig. been placed in Fig. 3) is exactly as would be predicted 5, that Ii ordering is viable only for a limited for a non-first order transformation, and analogous composition range from An,,x, towards albite. LHoti diffraction effects have been observed in natural and AH&,, for Ci = 11 ordering decrease rapidly sodium-rich pyroxenes (CARPENTER and SMITH, with increasing albite content and would extrapolate 198 1). AH,,, values in the composition range of the to zero at -An6” (Fig. 5). For the composition range crossover between Ci and 17 structures are not -An,,-An,o both e and 11 structures arc possible sufficiently precise to pin down the mixing curve but the enthalpy of e ordering is significantly greater unambiguously. For a “mixed or X” transformation, than for 11 ordering. For compositions more albite as defined by THOMPSON and PERKINS ( 198 I ) a step rich than -An,. the 11 scheme is simply not available would occur in the enthalpy of mixing curve at the as an alternative to the e structure. crossover point (Fig. 3) but for a classical second What are then the sequences of ordering at each order transformation the curve would simply have a composition within the plagioclase solid solution? A cusp. If the enthalpy of mixing is continuous then range of possibilities is shown in Fig. 7. In pure the entropy of mixing probably will be also. albite, of course. the sequence with falling temperature NEWTON et al. (1980) used their enthalpy data to is monalbite (C2/m) - high albite (CT) - low albite set up a convenient mixing model for petrological (CT) with some kinds of transition between them applications. The new data suggest that a slightly (Fig. 7a). At -Anur the only ordering scheme possible larger excess heat of mixing should be used, because at low temperatures appears to be of type e. so the of the different results for anorthite. Some of the sequence must be Cl - e (Fig. 7b). At Anh. ii and overall constraints on the mixing behaviour. to take e ordering are alternative possibilities and three dif- account of the Ci/li transformation. have been ferent sequences (Fig. 7c-e) must be considered. It is outlined by CARPENTER and FERRY ( 1984). known from experiments that -Anhs crystals with the e structure will transform to the Ii structure at - 1000°C (MCCONNELL, 1974: CARPENTER and Stability qf‘the “e” plagioclase structwc MCCONNELL, 1984). The relatively high temperature In treatments of the phase transformations and part of the sequence must therefore be CT -+ 11. The stability relations of plagioclase feldspars it is generally low temperature part of the sequence could have e assumed that the e structure represents a metastable as a metastable alternative to 11 (Fig. 7c) or as a state. For example, SMITH ( 1983b) states that “only stable ordered state replucing 17 (Fig. 7d). The cnthalpy low albite and P-anorthite are stable at low temper- advantage for e ordering is greater than for Ii ordering atures”, and that “the ‘e’ structure type is merely a so it seems inevitable that at some low temperature coherent small-scale intergrowth of domains which the former must become the stable state. The sequence locally have structures like those of low albite and must therefore be CT ---- 1: -- e (Fig. 7dL though Ordering enthalpy in plagioclase 961

there may be a small temperature range over which impossible for pure anorthite but most viable in the e is just metastable relative to Ii (Fig. 7e). At Anloo intermediate composition range. The effectiveness of the sequence would be Ci - Ii - Pi if the melting type e ordering as a means of lowering the free energy relations were ignored (Fig. 7f). increases as the albite component is added to anor- A very approximate estimate of the entropies in- thite. while the effectiveness of the Ii structure is volved may be made by treating the ordering reactions greatly reduced. As borne out by our enthalpies of as being first order in character and ignoring AC,, ordering, one structure simply becomes more stable effects. The enthalpy of ordering (A&,) for each than the other. with a range of intermediate compo- transformation can be taken from Fig. 5 and values sitions (-An~-An,5) at which both are possible. At for the transformation temperatures (Toti) from the the pure albite end, of course, low albite becomes the experimental results of CARPENTERand MCCONNELL stable ordered state. The stability of the e structure (1984) though it is necessary to guess a value _for with respect to two phase mixtures is considered in T$$f’. Taking Ar+,r as an example, AHz&$=” a later section. z - 1500 &/mole and 7$,$=li = 12OO’C. This gives An important consequence of these arguments is AS~~F=li ~ -1500/1473 = - 1 Cal/mole - K. A max- that natural samples with compositions of --Atim- imum value for AH:;=’ is - -2600 Cal/mole, so Anr0 would, on cooling from solidus temperatures, that AH?:’ is -(-2600 + 1500) = - 1100 Cal/mole. pass through the stability fields first of the Ci structure, If T$zc is assumed to be -800°C (and the precise then of the Ii structure and, finally, of the e structure. temperature selected makes little difference to the Serious kinetic problems with regard to ordering at general argument), A$,$’ = - 1 lOO/ 1073 = - 1 Cal/ temperatures above - 1000°C do not appear to exist mole-K. Thus e ordering might result in a signifi- (CARPENTER and MCCONNELL, 1984) and natural cantly lower configurational entropy than Ii ordering samples quenched from high temperatures can, in- at this composition. We may use the total entropy deed, have Ii order (STEWARTet al., 1966; MCLAREN change for e ordering from a Ci structure, AS$,Ge and MARSHALL, 1974; WENK et al. 1980). There is, ET 1 + 1 = 2 Cal/mole * K, and the total enthalpy therefore, little reason to doubt that slowly cooled change, AH:&=’ = -2600 Cal/mole, to estimate a igneous plagioclase crystals in this composition range value for Z$Aze (see Fig. 7e) of w-2600/2 = 1300 K; order first on the basis of the Ii structure before i.e., the Ci structure would transform to the “e” developing the e structure at lower temperatures. The structure at - 1000°C if Ii ordering failed to occur. evidence of microstructures in plagioclases is not The entropy values in these rough approximations necessarily inconsistent with this. should not be taken too seriously except insofar as In the context of the structural states of slowly they point to the reason why the “e” structure takes cooled plagioclases with compositions of -AnTo, the over from Ii at low temperatures when the compo- Crystal Bay sample is highly unusual. On the basis sition deviates from anorthite towards albite. Ii or- of other TEM studies (HEUER et al., 1972: NISSEN, dering is most appropriate for an Al:5 ratio of 1: 1. 1974; NORD et al., 1974; MCLAREN, 1974; MCCON- Solid solution involves a change in this ratio and the NELL,1974; MCLARENand MARSHALL,1974: CLIFF Ii scheme becomes increasingly ineffective as a means el al., 1976; GROVE, 1976, 1977a; WENK and NA- of distributing Al and Si in an ordered manner. The KAJIMA, 1980) exsolution lamellae would have been free energy. enthalpy and entropy changes associated expected but, instead, it is homogeneous with the Ii with the ordering progressively decrease, as does the structure. The thermal histories of the anorthosites equilibrium order/disorder temperature. On the other from which the sample came, however, are highly hand, the incommensurate structure involves an in- unusual. They had a complex crystallisation and teraction between AI/Si ordering and Na/Ca ordering, metamorphic history before being incorporated as according to MCCONNELL (1978), and is therefore rafts in a gabbroic intrusion (MORRISONef al., 1983).

C21m CT ci CT CT - T Cl - Ii IT Ii T --___ ci00~) e IS (e) e PI

*nll - An,, -A% - *“ss ..*“I3 (a1 (b) (C1 (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 1I. (d) -An65. e structure becomes_stablerelative 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 enthalpies must reflect some difference between c plagioclase reflects the unusual heat treatment to structures which form within the Ii field and those which it was subjected. which form in the Cl field. perhaps due to the different driving forces for AI/Si order. Temperature dependence q/‘ ordering MCCONNELL (1974) has argued that a structural break at - AnSo relating to the “e” ordering behaviour Values for the enthalpy of ordering (AHO, and could be responsible for the Beggild miscibility gap. AH&) described in this paper refer only to the We can rule out exsolution driven simply by non- difference in enthalpy between two rather arbitrary ideal mixing in this range. because there is no evtdence states, one substantially ordered and the other equil- for a large positive excess heat of mixing for the e ibrated at some high temperature. The order/disorder solid solution. The model proposed by MCCONNELL behaviour of each structure appears to involve at (1974) is therefore not inconsistent with our results least some component of a continuous variation with and the correct answer to the problem must surely temperature. The Ci Z Ii transformation seems to involve differences in ordering behaviour with com- be continuous with respect to composition and, position. therefore, also with respect to temperature (see CAR- If, as has been commonly proposed (see review by PENTER and MCCONNELL, 1984). We have argued SMITH, 1983b). the e plagioclase structure is simply that the AH=,. values obtained with different anor- a fine scale intergrowth of albite-like and anorthite- thites are consistent with a continuous change in Al/ like slabs, the latter having an antiphase relationship Si order. For e plagioclases we might expect the same from one slab to the next. a simple linear variation kind of result. Igneous and metamorphic e samples in AH,, with composition might be expected, reflect- in the composition range - Anm-AnTO gave different ing merely the changing proportions of the Ab and heats of solution which could be explained in this An units. Extrapolation to the pure end members way but, because they were not all subjected to should then give AHwd for pure albite and pure identical heat treatments in the laboratory, the con- anorthite, less a small contribution for interfacial clusion is not as certain. End member albite also effects. Clearly the calorimetric data suggest a more shows signs of having continuous order/disorder complicated picture than this and. for thermodynamic properties (summarised by SMITH, 1983b). purposes, the e structure is rather more than a simple While the transformations between the different intergrowth of two phases. structures may or may not involve a first order break Electron diffraction patterns from the ordered sam- actually at their equilibrium transformation temper- ple at -AnM (Hawk Mine, Bakersville, N. Carolina) atures, it is clear that physically realistic models of had e reflections which were extremely weak and the ordering will need to account for a temperature diffuse, indicating only limited short range ordering dependent ordering contribution at every composition on the basis of the e structure. Its AH,,, value, in the solid solution. however, shows an increase relative to the value at Ant, (Fig. 4), as if tending towards the value for e and Ii solid solutions at low temperatures low - high albite. It is possible that the ordering too AH=,” values for the “low” Ii series show a fairly is tending towards that of low albite. Curves linking well defined trend in Fig. 3. If the curvature of the the e and low albite data have not been drawn in on Fig. 3 and Fig. 5 because the thermodynamic relations trend is real it probably reflects a reduction in the degree of order at compositions away from pure between the Ci (low Ab) and e structures are not anorthite. As with the “high” series, separating out well understood. For the same reason no attempt has been made to show a definite relationship between the contributions of straight mixing from those of the e and low Ii curves. ordering will not be a simple matter. Similarly, compositional changes in the low temperature intermediate solid solution are accompanied Subsolidus phase relations by changes in order, as evidenced by variations in the orientation and spacing of the type e antiphase Having decided that the e structure might be stable domains (GAY, 1956; SMITH, 1974; GROVE, 1977b; relative to an Ii structure at the same temperature WENK, 1979a; RIBBE, 1983). It is evident from Fig. and composition, the next question to ask is whether 5 that the enthalpy associated with ordering of the it can also be stable relative to two phase mixtures most ordered (metamorphic) An-rich e-plagioclases and therefore have a true equilibrium field of stability. is greater than for ordering of Ab-rich samples. The In the past the general view has been that at inter- apparent change through - Anw coincides with breaks mediate compositions and low temperatures the equi- in other structural parameters (SMITH and GAY, librium assemblage is low albite plus anorthite (SMITH, 1958; DOMAN et al., 1965; BAMBAUERet al., 1967a,,b; 1974, 1983b; WENK, 1979& GROVE et al., 1983). SLIMMING, 1976) and also with the approximate Again, the calorimetric data are not quite so unequiv- position of the experimental Cl/Ii transformation ocal. line when extrapolated to low temperatures (CARPEN- Treating the low temperature end-members, low TER and MCCONNELL, 1984). This difference in the albite and anorthite, as having complete Al/Si order Ordering enthalpy in plagioclase 963

(i.e. zero configurational entropy) and excluding pos- rium and stability. We have deliberately tried to sible AC’, contributions provide a very approximate assess the implications of the enthalpy and experi- means of assessing stable equilibrium assemblages at mental results independently of previous accounts of intermediate compositions. If a line is drawn between the plagioclase subsolidus phase relations. Interpre- the A&l, values of Amelia albite and Pasmeda tation of the many and complex microstructures anorthite in Fig. 3, most of the e samples plot within observed by transmission electron microscopy, for - 1500 Cal/mole below it. For the e structures to be example, involves a different set of subjective judge- stable as homogeneous phases in place of Ab + An ments. Perhaps the questions raised here, particularly this apparent positive excess enthalpy of mixing must relating to the sequences of ordering, will lead to be balanced by a configurational (and/or vibrational) some reconsideration of these observations, but in entropy contribution to the free energy. Thus at 25’C the future it will obviously be necessary to reconcile the excess entropy of the e structures relative to the all strands of the microstructural. petrographic, ex- pure ordered end members must be at least 1500/ perimental and calorimetric evidence. 298 = 5 Cal/mole - K. Corresponding values at 300 Our principal conclusions are as follows: and 500°C are -2.6 cal/mole- K and -1.9 cal/ 1. The enthalpy differences between ordered and mole - K respectively. The configurational entropy of disordered plagioclase feldspars vary in the range l- e plagioclase is unknown, but if it is zero, i.e. if the 4 kcal/mole. structure has complete cation order, albite plus an- 2. Enthalpies of mixing for the “high” structural orthite will almost certainly be the stable assemblage stale series are consistent with the interpretation of at all three temperatures. A configurational entropy two ideal segments, Ci and Ii, related by a non-first of 5 caj/mole . K is unreasonably large given that the order transformation. maximum possible configurational entropy at Anso 3. Pure anorthite has a temperature dependent is 6.7 cal/mole - K; Ab + An is probably the stable variation in its degree of Al/Si order which contributes assemblage at 298 K. Values of 2.6 and 1.9 Cal/ to an enthalpy difference (as measured at 7OO’C) of mole - I( (1.7 and 1.3 Cal/mole - K for 67796b, Anm, -800 Cal/mole between metamorphic anorthite and which gave a larger AH,,,,,, value), however, are not anorthites equilibrated at - 1300°C. impossible and, therefore, e plagioclases might be 4. The enthalpy of ordering for a symmetry change truly stable at these temperatures. ri s Ci decreases in magnitude from a value esti- Estimates arrived at in this way are sufficient only mated at -3.7 T 0.6 kcal/mole at Anloo and extrap to demonstrate that, because of the large enthalpies elates to zero at -Anm. of ordering, true equilibrium stability for the e struc- 5. In the composition range AGS-Anlo the enthalpy tures is a real possibility. Some means of assessing change associated with ordering on the basis of the e the entropies properly is needed to resolve the issue structure is greater than for Ii ordering. It has been but, because of the general problems of distinguishing argued that this implies a field of true stability for Al and Si atoms with X-rays and of obtaining only the e structure relative to the Ii structure at low average structures (see WENK et al., 1980, and review temperatures. by RIBBE, 1983). conventional structure refinements 6. Type e ordering at anorthite-rich compositions may not be very helpful in this respect. Clearly, gives a larger enthalpy effect than e ordering in more however, e ordering is likely to be highly influential albite rich compositions. This change in the enthalpy in the BBggild and Huttenlocher exsolution reactions of ordering, at -AnSO, may be important for the and, if the e structures do have a significant stability origin of the B0ggild miscibility gap. range in temperature and composition space, then every equilibrium phase diagram which does not 7. The assemblage Ab + An is probably stable in specifically include them is wrong. The volume effects, place of an intermediate structure at room tempera- both of ordering and mixing, are too small for ture, but it is not inconceivable that e plagioclase pressure to be an important factor. could become the stable state at higher temperatures. Only moderate ( - l-2 Cal/mole - K) configurational entropies for the e structures may be required. CONCLUSIONS

Although a great deal of data has been generated Acknowledgements-We are most grateful to P. Gay, C. Francis and R. C. Newton for generously providing us with in this study, the ability to calculate exact thermo- important samples for this study, and to J. V. Smith and dynamic properties is, as yet, rather limited. The G. L. Hovis for their critical comments on the manuscript. problems are firstly to separate out mixing and or- We also thank A. R. Abraham for assistance with collecting dering contributions, secondly to distinguish between the X-ray data. Financial support from the Natural Envi- ronment Research Council of Great Britain (grant no. GR3/ the thermodynamic behaviour of the different possible 4404 to JDCMcC) and from the National Science Foundation ordered structures and, finally, to produce physically (NSF grant no. DMR 8 106027 to AN) is gratefully acknowl- realistic, continuous ordering models for every com- edged. This is Cambridge Earth Sciences Contribution no. position in the solid solution. The data do, however, Es 517. provide the first systematic set relating to the ordering and provide a basis for testing hypotheses of equilib- Editorial handling: P. C. Hess 964 M. 4. Carpenter, J. D. c‘. McConnell and :\. \javrotsh\

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