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Entbalpies of Ordering in the Plagioclase Feldspar Solid Solution

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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.
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