Sapphirine-bearing assemblages in the system...

Sapphirine-bearing assemblages in the system MgOndashAl2O3ndashSiO2a continuing ambiguity

Konstantin K PODLESSKII1 Leonid Y ARANOVICH1 2 Taras V GERYA3 and Natalia A KOSYAKOVA1

1Institute of Experimental Mineralogy Russian Academy of Sciences 142432 Chernogolovka Moscow RussiaCorresponding author e-mail kkpiemacru

2Institute of Geology of Ore Deposits Petrography Mineralogy and Geochemistry (IGEM)Russian Academy of Sciences Staromonetny per 35 119017 Moscow Russia

3Institute of Geophysics Department of Geosciences Swiss Federal Institute of Technology (ETH ndash Zurich)ETH ndash Honggerberg HPP 8093 Zurich Switzerland

Abstract Based on experimental data for reactions involving sapphirine in the system MgOndashAl2O3ndashSiO2 (MAS) thermodynamicproperties of the sapphirine solid solution end-members have been optimized for both ideal and non-ideal models and internallyconsistent PndashT grids are proposed According to the calculated PndashT relationships in MAS the sapphirine + quartz assemblage whichis widely recognized as indicative of ultrahigh-temperature metamorphism can be stable down to 840 C and 067 GPa ie attemperatures up to 150 C lower than estimated by Kelsey et al (2004) The sapphirine + kyanite assemblage has been found stableat temperatures below 860 C and 113 GPa whereas the sapphirine + forsterite assemblage may be stable below 800 C only underspecific conditions of a very low activity of water Calculations of PndashT relationships involving sapphirine using the Thermocalc

dataset revealed its inconsistency with both experimental and natural assemblages

Key-words sapphirine MgOndashAl2O3ndashSiO2 thermodynamic data ultrahigh-temperature metamorphism

Introduction

This work is dedicated to the memory of Werner Schreyerwho came to an understanding about the importance of sap-phirine for estimating metamorphic conditions long agowhile working on the phase relationships of cordierite inthe beginning of 1960s (Schreyer amp Yoder 1960 Schreyeramp Schairer 1961a and b) Schreyer later made significantcontributions that illuminated different aspects of sapphirinebehavior both in experimental systems and in natural rocks(Schreyer 1967 1968 Schreyer amp Seifert 1969a and b1970 Schreyer amp Abraham 1975 Schreyer et al 19751984 as well as other publications with co-workers citedbelow)Assemblages of sapphirine once considered to be a rare

mineral have in recent years been recognized as importantindicators of relatively high-temperature metamorphismSapphirine occurs in rocks that have undergone different tec-tono-metamorphic histories with the PndashT range of forma-tion being estimated from below 700 C and 05 GPa toabove 1100 C and 15 GPa Sapphirine associated withquartz is attributed exclusively to the highest temperatureconditions of crustal metamorphism referred to as lsquolsquoultra-high-temperature metamorphismrsquorsquo (Harley 1998)Although experimental data involving sapphirine extend

over an even wider PndashT range the quantitative interpretationof sapphirine-bearing assemblages remains ambiguous and

thermodynamic properties of sapphirine are still poorlyunderstood In part this is because experimental reversalsof equilibria involving sapphirine have been obtained onlyin the model system MgOndashAl2O3ndashSiO2 (MAS) and onlyHollis amp Harley (2003) have attempted to measure the com-positions of sapphirine solid solution in such experimentsBecause the experimental data were insufficient to substan-tiate an unambiguous thermodynamic description of theMAS system Gasparik (1994) and Podlesskii (1995) inde-pendently deduced petrogenetic grids differing from oneanother both quantitatively and qualitatively Internally con-sistent thermodynamic datasets (Gerya et al 1996 Hollandamp Powell 1998) with their later modifications (Ouzeganeet al 2003 Gerya et al 2004a Kelsey et al 2004) alsoimply significantly different phase relationships for keysapphirine-bearing assemblages even in this simple systemIn this contribution we consider only MAS assemblages

because experimental studies of more complex systemsbeginning with the pioneering work by Hensen amp Green(1971) are restricted to synthesis or crystallization experi-ments with no bracketing by reversal which do not providea firm foundation for thermodynamic modeling (Berman ampAranovich 1996) It is noteworthy that those who carriedout experiments involving sapphirine in more complexsystems such as FeOndashMgOndashAl2O3ndashSiO2 (FMAS) K2OndashFeOndashMgOndashAl2O3ndashSiO2 and K2OndashNa2OndashFeOndashMgOndashAl2O3ndashSiO2 did not attempt to use their own data to extract

Eur J Mineral2008 20 721ndash734Published online October 2008

This paper is dedicated tothe memory of Werner Schreyer

DOI 1011270935-122120080020-18700935-1221080020-1870 $ 630

2008 E Schweizerbartrsquosche Verlagsbuchhandlung D-70176 Stuttgart

thermodynamic characteristics of sapphirine end-membersor mixing properties but restricted application to qualitativegraphical analysis of phase relations (Hensen 1986 Hensenamp Harley 1990) although these were plotted on quantitativePndashT diagrams (Harley 1998 2008 Das et al 2001 2003)Calibrations of FendashMg exchange reactions between sapphi-rine and other minerals based on experimental crystallizationof complex mixtures did not provide evidence for achievingequilibrium relationships and conflicting results wereobtained in experiments using the same approach (Daset al 2006 Sato et al 2006)Other components that enter sapphirine ie Fe3+ Be

(Christy et al 2002 Christy amp Grew 2004 Grew et al2006 and the references therein) B (Grew et al 19901992) Ca (Grew et al 1992) Cr (Friend 1982 Brigidaet al 2007) and Ti (Harley amp Christy 1995 Sheng et al1991) undoubtedly play important roles in expanding the sta-bility of sapphirine relative to the model system MAS butthese components are also beyond consideration in this paperHere we present new models of the thermodynamic prop-

erties of magnesian sapphirine Phase relations involvingsapphirine in MAS derived with these data are tested withexisting experimental constraints Our parameters differfrom the data used in Thermocalc (Kelsey et al 2004)and seem to describe the experimentally studied reac-tions better For the estimated stability of assemblages ofsapphirine with quartz kyanite and forsterite to which wehave paid special attention due to their petrologic impor-tance these differences are dramatic and may change theinterpretations of petrogenetic processes

Sapphirine solid solution and itsthermodynamic model in MAS

After the structure determination by Moore (1969) sapphi-rine has been recognized as a chain silicate with the simpli-fied crystallographic formula M8T6O20 where M and Trepresent octahedral and tetrahedral sites respectively InMAS octahedral positions in sapphirine are occupied byMg and Al while tetrahedral positions belong to Al andSi and substitutions occur along the join Mg + Si =Al + Al as implied by Higgins et al (1979) Followingour previous publications (Podlesskii 1995 Gerya et al1996 2004a) we accepted end-members Mg4Al8Si2O20(SprS) and Mg3Al10SiO20 (Spr

A) to represent the sapphirinesolid solution for thermodynamic modeling in MAS Theseend-members have also been chosen for the latest version ofthe Thermocalc database (Kelsey et al 2004) with whichwe will compare our results (earlier Thermocalc usedend-members Mg4Al8Si2O20 and Mg35Al9Si15O20) Interms of the Tschermak type substitution Mg + Si = Al +Al a wider range of the sapphirine solid solution will prob-ably be considered in future studies since sapphirines withmore than 2 Si per formula unit have been observed both inexperiments (Milholland amp Presnall 1998 Liu amp Presnall2000 Das et al 2001 2003 Liu amp OrsquoNeill 2004) andin nature namely cracks in pyrope megablasts from Dora-Maira Italian Alps (Simon amp Chopin 2001) whereas

sapphirine with less than 1 Si per formula unit has beenencountered in eclogites from the South Carpathians Roma-nia (Sabau et al 2002)In our approach sapphirine solid solution (Spr) was ini-

tially considered as an ideal mixture of end-members SprS

and SprA (often referred to as lsquolsquo221rsquorsquo and lsquolsquo351rsquorsquo respec-tively in the literature) Unlike Kelsey et al (2004) wefound that describing the existing experimental data didnot require non-ideality However non-ideality can be intro-duced by analogy with other minerals that demonstrate theTschermak substitution and we have also tried the non-idealmodel although this approach has added parameters notconstrained by measurements A composition parameterXSS that denotes the mole fraction of SprS ranging from 1to 0 has been used to represent the activity relationshipsA regular solution parameter WSpr has been used for thenon-ideal model to express the excess energy dependenceon composition as WSpr AElig XSS (1 XSS)Our experience (Podlesskii 1995) shows that variation in

sapphirine composition plays a decisive role for thermody-namic modeling of sapphirine-bearing equilibria and it isimpossible to describe the whole spectrum of existingexperimental data with a constant composition as was triedby Gottschalk (1997) with sapphirine Mg9Al18Si3O40(lsquolsquo793rsquorsquo) or Jung et al (2004) with an out-of-date formulaMg4Al10Si2O23 (lsquolsquo452rsquorsquo) of Foster (1950) that is in conflictwith the structure determinations (Moore 1969 Higgins ampRibbe 1979 Merlino 1980 Barbier amp Hyde 1988)At present effects of polytypism and stacking disorder on

stability of sapphirine (Christy 1989a and b) cannot beconsidered in terms of its thermodynamic properties dueto the lack of data for the experiments under considerationand this simplification undoubtedly introduces additionaluncertainties

Experimental data and their thermodynamicdescription

Experimental investigations related to MAS have a long his-tory since this system has become important for metallurgyand the ceramic industry (Jung et al 2004 and the refer-ences therein) and petrology (Schreyer amp Seifert 1969aGasparik 1994 and the references therein) However exper-iments on melting and crystallization of various mixtureswidely used in ceramic studies often did not provide evi-dence for achieving equilibrium relationships They alwaysinvolved phases of undefined composition like melt withthe sapphirine composition having never been determinedalthough sometimes claimed to be close to 221 or 793In the absence of fluid reactions involving MAS phasesare notoriously sluggish because the phases are so refractoryand thus often proceed via metastable intermediates so withinsufficient run durations incompatible phase assemblagesmay be observed even when the temperatures are relativelyhigh For example Shu et al (2002) and Menchi amp Scian(2005) reported the joint appearance of sapphirine and cris-tobalite at temperatures between 1200 C and 1280 C incrystallizing cordierite ceramics Conversely crystallizing a

722 KK Podlesskii LY Aranovich TV Gerya NA Kosyakova

mixture of cordierite + spinel with stoichiometry of sapphi-rine 221 at 900 C and 15 GPa Schreyer (1967) initiallyobtained orthopyroxene + corundum + spinel in a 22 h longrun and only in longer runs did the mixture yield 40 ofsapphirine Forsterite was found in metastable coexistencewith quartz in experiments at temperatures up to 1350 Cand pressures up to 2 GPa (Arima amp Onuma 1977)To facilitate reactions in MAS and to get the run product

grains large enough for electron microprobe analysisPerkins et al (1981) and Gasparik amp Newton (1984) usedfluxes which yielded grains large enough to characterizesolubility of alumina in orthopyroxene coexisting withpyrope or forsterite + spinel in detail and with confidenceThis in due course resulted in a more reliable descriptionof thermodynamic properties of orthopyroxene solid solu-tion in MASThe only experimental determination of equilibrium sapphi-

rine compositions is reported by Hollis amp Harley (2003)which was not sufficient to get a confident description ofthe compositional variation with P and T (this issue will beaddressed later in this paper) Most other authors either didnot analyze product sapphirines or reported that the sapphirinegrains were too small to be analyzed with the electron micro-probe (eg Chatterjee amp Schreyer 1972 Ackermand et al1975) Consequently we used XSS as one of the parametersthat describes sapphirine-bearing reactions while fitting theexperimental brackets This was only possible for the reac-tions with minerals whose thermodynamic properties hadalready been well constrained by measurements and determi-nation of sapphirine-free reactions Equilibria with phasessuch as melt and mullite which have variable and undeter-mined compositions had to be excluded from the treatmentFor the same reason the reversals of sapphirine breakdownreactions involving chlorite in the system MgOndashAl2O3ndashSiO2ndashH2O (MASH) obtained by Seifert (1974) were alsonot treated since equilibrium compositions of chlorite hadnot been determined in experiments and could not be con-

strained from thermodynamic data The onlyMASHreversalstreated were those of reaction orthopyroxene + sapphirine =cordierite + spinel It is noteworthy that with the addition ofwater the stability fields of MAS sapphirine-bearing assem-blages either shrink dramatically towards higher temperaturesor disappear because H2O-containing phases become stableThis hampers the experimental investigations under moderatetemperatures and elevated pressures since fluid-absent crys-talline mixtures either do not react or react unacceptablyslowly under these conditions Podlesskii (1996) tried tocircumvent this problem by adding anhydrous MgCl2 tocrystalline starting materials while investigating reactionorthopyroxene + spinel + corundum = sapphirine at 700ndash750 C and 15ndash16 kbar However reaction was still slowand sapphirine grains were still very small so it was only pos-sible to obtain preliminary constraintsThermodynamic properties of sapphirine end-members and

other minerals have been estimated in terms of semi-empiri-cal Equation (1) introduced by Gerya et al (1996 19982004a and b) for expressing the Gibbs free energy (Jmol)

Gs frac14 H 2981 T S2981 thorn V os Wthorn c1 frac12RT lneth1 e1THORN

H os1 eth1 T=T 0THORN e01=eth1 e01THORN

RT lneth1 e01THORN thorn c2 frac12RT lneth1 e2THORNH o

s2 eth1 T=T 0THORN e02=eth1 e02THORN RT lneth1 e02THORN eth1THORN

where T is temperature (K) standard temperature T0 =29815 K P is pressure (bar) universal gas con-stant R = 8313 (Jmol K) ei frac14 exp frac12ethHo

si thornV os WTHORN=

RT e0i frac14 exp frac12Hosi=ethR T 0THORN W = 5 AElig (1 + )15 AElig

[(P + )45 (1 + )45]4 (bar) other parameters areshown in Table 1 In the case of non-ideal sapphirine itsend-member properties were obtained consistent with theintegral and partial mixing energies

Table 1 Parameters of end-members for calculating the Gibbs free energy with Equation (1)

End-member Formula DH2981Jmol

S2981JK mol

c1 DH os1 J V o

sJbar

bar DV os AElig 103Jbar

c2 DHos2 J

Qtz SiO2 910303 4159 47513 3074 2305 71406 40049 41759 10523En MgSiO3 1545471 6420 143025 5362 3124 250489 46603 28493 24657OK Al2O3 1672830 5086 148854 5896 2872 456483 30291 19663 27556Crn Al2O3 1673827 4979 150305 5989 2553 522202 35226 18833 30645Fo Mg2SiO4 2169792 9514 200065 5144 4350 250668 53655 42036 28433Spl MgAl2O4 2297275 8491 208017 5575 3967 404263 37376 36415 29111Sil Al2SiO5 2583627 9596 223771 5641 4980 231391 21234 40337 26131Ky Al2SiO5 2590904 8354 233306 5992 4406 241723 44135 36993 29794And Al2SiO5 2587717 9134 229572 5838 5139 258753 40850 29233 25055Prp Mg3Al2Si3O12 6281474 26260 569093 5384 11295 323482 35274 84103 24574Crd Mg2Al4Si5O18 9161186 40314 804590 5496 23302 160366 09514 162280 28331SprS Mg4Al8Si2O20 11018469 41375 988224 5405 19861 387809 33426 139675 22093SprA Mg3Al10SiO20 11168243 37967 980793 5383 19667 460442 33064 166157 20434

SprS Mg4Al8Si2O20 11003035 42532 980066 5405 19861 387809 41798 169013 22093SprA Mg3Al10SiO20 11181724 36794 962787 5146 19651 460442 21747 140486 20434

Fictive orthopyroxene end-member end-member properties for the non-ideal solid solution

Sapphirine assemblages in MAS 723

Gm frac14 RT frac12eth1 X SSTHORN ln eth1 X SSTHORN thorn X SS ln ethX SSTHORNthorn eth1 X SSTHORN X SS W Spr eth2THORN

GmSprS frac14 RT ln X SSeth THORN thorn eth1 X SSTHORN2 W Spr eth2aTHORN

GmSprA frac14 RT ln 1 X SSeth THORN thorn X 2

SS W Spr eth2bTHORN

where WSpr = 4343 9832 AElig T 006355 (P 1)We have extracted the volume characteristics of sapphirine

from measurements of unit cell parameters of synthetic sap-phirines by Schreyer amp Seifert (1969a) Seifert (1974)Newton et al (1974) Bishop amp Newton (1975) Charluet al (1975) Steffen et al (1984) and Christy et al(2002) As no analytical data have been provided by theauthors for sapphirines 221 and 793 we believe thatthe crystallographic parameters correspond to the nominalcompositions of starting mixtures within the uncertainty ofplusmn005 Si per 20 oxygens In case of the most aluminous sap-phirine synthesized and analyzed by Bishop amp Newton(1975) the composition was calculated to correspond to1367 Si per 20 oxygens The unit cell volumes were calcu-lated for the space group P21a which in some cases dif-fered from those calculated for the space group P21n byauthors of the measurements (eg Schreyer amp Seifert1969a) The derived VndashXss dependence is shown in Fig 1In the case of non-ideal sapphirine this dependence wasdescribed using a small negative volume of mixing param-eter (see WSpr above) consistent with the other thermody-namic properties which were obtained by simultaneoustreatment of all experimental data Obviously existing con-straints on the VndashXss dependence are relatively loose whichadds ambiguity to thermodynamic modelingTable 1 also presents an internally consistent set of param-

eters for other minerals relevant to MAS They have beenobtained by processing experimental data for the systemCaOndashFeOndashMgOndashAl2O3ndashSiO2ndashK2OndashNa2O by Gerya et al(2004a) In the treatment of experimental data and calcula-tions with this dataset mixing properties of En and OK

respectively the MgSiO3 and Al2O3 end-members of ortho-pyroxene solid solution (Opx) were obtained as correspond-ing derivatives of the integral mixing energy

Gm frac14 RT 1 2 XOKeth THORN ln 1 2 XOKeth THORNfrac12thorn2 XOK ln 2 XOKeth THORN=2thorn 1 2 XOKeth THORN XOK W MA thornW AM 1 4 XOKeth THORNfrac12

eth3THORN

ie

GmOK frac14 Gm thorn eth1 XOKTHORN oGm=oXOK eth3aTHORN

GmEn frac14 Gm XOK oGm=oXOK eth3bTHORN

where XOK is the mole fraction of OK ie the number of Alatoms per 3 oxygens WMA = 536165 226953 AElig T 008454 AElig (P 1) WAM = 161705 + 36511 AElig T 003899 AElig (P 1) To account for the activity change ofcordierite under water-saturated conditions the model pro-posed by Aranovich amp Podlesskii (1989) was used ieGw = 100994 + T AElig 81833 155751 AElig (P 1) wasadded toGs values obtained for Crd with Eq (1) The Gibbsfree energy for quartz was calculated using Eq (1) withaddition of energy relative to the ordered state (Geryaet al 1998 2004b) defined as

Ga frac14 c1 ethRT lnffrac121 expethH os1=R=T THORN

=ethfrac121 expethH os1=R=T THORN

thorn expethfrac12H sk X 2a V sk W=R=T THORN

frac12expethH os1=R=T THORN

expethfrac12H os1 V o

s W=R=T THORNTHORNgTHORNthorn c2 ethRT lnffrac121 expethH o

s2=R=T THORN=ethfrac121 expethH o

s2=R=T THORNthorn expethfrac12H sk X 2

a V sk W=R=T THORN frac12expethH o

s2=R=T THORN expethfrac12H o

s2 V os W=R=T THORNTHORNgTHORN

thorn fRT frac12X a lnX a thorn eth1 X aTHORN lneth1 X aTHORNthorn ethH ord thorn V ord WTHORN X a

thorn ethWH thornWv WTHORN X a eth1 X aTHORNg=eth1thorn 2 X aTHORNeth4THORN

where Xa denotes the mole fraction of ordered clustersin quartz at Ga = min DHsk = 125946 Jmol DVsk =021922 Jbarmol Hord = 26228 Jmol Vord =012889 Jbarmol WH = 142362 Jmol WV = 0637Jbarmol Other minerals were treated as end-memberphasesIn comparison to modeling with a sufficient set of indepen-

dent reactions a lsquolsquofull-scalersquorsquo equation of state allows more

Fig 1 The compositional dependence of sapphirine molar volumein MAS Rectangles represent the measurements with uncertaintiesplusmn02 for molar volume and plusmn005 for Si per 20 oxygens Heavyline ndash our model for the ideal solution dashed line ndash our model forthe non-ideal solution thin line ndash Kelsey et al (2004)


freedom for extrapolations However it also adds uncertain-ties related to additional parameters that are not constrainedby direct determinations of individual thermodynamic prop-erties and composition changes Unfortunately data onsapphirine itself are confined mostly to characterization ofordering phenomena optical properties or spectroscopy(Burzo 2004 and the references therein) which at themoment are of little help for describing the phase equilibriaLow- and high-temperature heat capacities were measuredonly for a natural sapphirine from Fiskenaeligsset thatcontained significant quantity of Fe2+ and Fe3+ and minoradmixtures of Cr Mn Na and K (Nogteva et al 1974Kiseleva amp Topor 1975) Such an impure sapphirine isnot suitable for calculating thermodynamic parametersMeasurements of low- and high-temperature heat capacitiesof synthetic sapphirines with well-determined compositionsand determinations of their PndashVndashT behavior could make theestimates of EOS parameters less uncertain and hopefullywill be available in the futurePetrogenetic grids calculated with thermodynamic data of

Table 1 for both ideal and non-ideal sapphirine are shown inFig 2 Based on our internally consistent dataset of mineralproperties they have a topology similar in many importantdetails to that calculated by Podlesskii (1995) with the setof internally consistent reaction properties The grids involvetwo phases of variable composition Opx and Spr and sevenphases of constant composition Crd Crn Fo Prp Qtz Sil(Ky And) and Spl Modeling with the ideal sapphirine pro-duces six invariant points (named using absent-phase nota-tion [ ]) I1 ndash [Crd Spr Qtz Sil (Ky And)] I2 ndash [Crd Fo

Qtz Sil (Ky And)] I3 ndash [Crd Fo Qtz Spl] I4 ndash [FoPrp Crn Spl] I5 ndash [Crn Prp Qtz Sil (Ky And)] andI6 ndash [Qtz Fo Prp Spl] within a PndashT range of 00001ndash25 GPa and 500ndash1400 C The non-ideal sapphirine modelexcludes I6 because reactions Opx + Crn = Spr + Ky(Sil)(r31) and Spr + Sil = Crd + Sil (And) (r61) do not intersectsince the reaction lines curve away from one another(Fig 2) with sapphirine becoming more aluminous at lowertemperatures Reactions (r51) and (r21) involving non-idealsapphirine must intersect at an invariant point [Crd PrpQtz Sil (Ky And)] not far below 500 C (actually estimatedat 451 C and 1446 GPa) whereas with the ideal sapphi-rine these two reactions probably intersect at temperaturestoo low to be accessible in Earthrsquos crust

Comparison of experimental data andcalculated phase relations

Table 2 shows how our thermodynamic parametersdescribe experimental data on univariant reactions in MASTable 2 has been deposited and is freely accessible as supple-mentary material on the GSW website of the journal (httpeurjmingeoscienceworldorg) This description is also com-pared with calculations based on the data from Kelsey et al(2004) Where high-pressure cells other than NaCl cells hadbeen used in runs with piston-cylinder equipments weapplied friction corrections to the nominal pressures reportedby the original authors The corrections were made accordingto recommendations of Perkins et al (1981) and Gasparik amp

Fig 2 PndashT diagrams for the system MgOndashAl2O3ndashSiO2 calculated with data of Table 1 involving (a) ideal and (b) non-ideal solid solutionmodels of sapphirine Invariant points are marked as in the text Reaction curves (high-pressure assemblage on the left-hand side)KS ndash Ky = Sil SA ndash Sil = And r11 ndash Fo + Crn = Opx + Spl r12 ndash Prp = Opx + Crn r13 ndash Prp + Spl = Fo + Crn r14 ndash Prp +Fo = Opx + Spl r21 ndash Opx + Spl + Crn = Spr r22 ndash Prp + Spl + Crn = Spr r23 ndash Prp + Spl = Opx + Spr r31 ndash Opx + Crn =Spr +Sil(Ky) r32 ndash Prp + Crn = Opx + Sil r33 ndash Prp = Opx + Spr + Sil r41 ndash Opx + Sil(Ky) + Qtz = Crd r42 ndash Opx + Sil = Spr + Qtz r43ndash Spr + Opx + Qtz = Crd r44 ndash Spr + Qtz = Crd + Sil r45 ndash Opx + Sil = Crd + Spr r51 ndash Opx + Spl = Spr + Fo r52 ndash Opx + Spr =Crd + Spl r53 ndash Opx + Spl = Crd + Fo r54 ndash Spr + Fo = Crd + Spl r55 ndash Opx + Spr = Crd + Fo r61 ndash Spr + Sil(Ky And) = Crd +Crn r62 ndash Opx + Crn = Crd + Spr r63 ndash Opx + Ky = Crd + Crn r71 ndash Prp + Qtz = Opx + Ky(Sil) Qtz ndash andashb Qtz transition Thisnumbering system is used throughout the text tables and figures


Newton (1984) for different types of cells All pressurevalues were further adjusted by decreasing low-pressurelimits and increasing high-pressure limits by 5 thus takinginto account the uncertainties of the pressure measurementsSuch an adjustment is reasonable (Gerya et al 1998)bearing in mind that we did not correct the temperature val-ues for uncertainties The nominal corrected and acceptedvalues are shown in Table 2 Runs for which the originalauthors had reported lsquolsquono reactionrsquorsquo were not consideredbecause there could be many reasons for the reactions notto proceed and thus the corresponding values may not referto equilibrium conditions Reaction brackets based on runswhere directions of reactions had been observed proved tobe sufficient for the analysis of phase relationshipsThe two thermodynamic datasets demonstrate good agree-

ment with sapphirine-free experiments except that thedescription of reaction Opx + Spl = Crd + Fo with theThermocalc dataset is not quite satisfactory Significant

differences show up when sapphirine-bearing reactions areconsidered As can be seen from Table 2 and Fig 3 posi-tions of the reactions calculated with our thermodynamicparameters agree with all the experimental brackets whereasthose calculated by Kelsey et al (2004) are inconsistent withthe experimental data of Seifert (1974) for the reactionOpx + Spr = Crd + Spl The reactions calculated by Kelseyet al (2004) also miss almost all brackets reported byNewton (1972) and Perkins et al (1981) for Spr + Qtz(+Opx) = Crd including that obtained at 1250ndash1280 Cand 72 plusmn 02 kbar in a gas apparatus as well as bracketsfor Opx + Sil = Spr + Qtz at 1080 1150 and 1200 C byChatterjee amp Schreyer (1972) at 1100ndash1150 C by Arimaamp Onuma (1977) at 1200 and 1400 C by Newton(1972) and at 1400 C by Hensen (1972) As a result theSpr + Qtz stability field calculated with the Thermocalc

dataset is smaller than the field calculated from our datasetand I4 is shifted towards higher temperatures (986 C vs our839 C for ideal sapphirine and 835 C for non-idealsapphirine Fig 4)Although differing in calculated equilibrium compositions

of sapphirine our datasets and the dataset of Kelsey et al(2004) produced coincident orthopyroxene compositionswhich implies that the thermodynamic parameters of ortho-pyroxene must have been well constrained by sapphirine-free experiments ie the Al solubility in Opx associatedwith Prp Spl + Fo or Crd + Qtz (Danckwerth amp Newton1978 Perkins et al 1981 Gasparik amp Newton 1984Aranovich amp Kosyakova 1987) based on compositionalbrackets and electron microprobe analyses of Opx In con-trast compositions calculated for the Opx + Spr + Qtzassemblage (Table 3) do not agree with the experimentaldata reported by Hollis amp Harley (2003) The calculatedAl contents of Opx differ from those experimentally deter-mined both in values and in trends of their changes withT and P Compared to the experimental determinations thecalculations give NOK (a) a more gentle increase withincreasing temperature at 12 GPa and (b) an increase withincreasing pressure at 1325 C (where NOK = 100XOK)This result implies negative slopes of the NOK isoplethsie slopes opposite to those reported by Hollis amp Harley(2003) Figure 4 illustrates the discrepancy between the cal-culated and the experimental compositions The discrepancyis not related to the sapphirine solution model and originatesfrom an irresolvable contradiction between the data byHollis amp Harley (2003) and calculated isopleths for Opx +Sil + Qtz and Opx + Crd + Qtz which are tightlyconstrained by the above-mentioned data for Opx + PrpOpx + Fo + Spl and Opx + Crd + Qtz Having locatedreaction curves (r42) and (r43) directly from experimentaldata shown in Fig 3 and using the data in Berman amp Arano-vich (1996) we have estimated the isopleths for Opx +Spr + Qtz (Fig 4c) which also are contradictory to Hollisamp Harley (2003) experiments Unfortunately there is noother source of measured compositions for Opx coexistingwith Spr + Qtz to serve as a basis for comparisonWe can only assume that experiments by Hollis amp Harley

(2003) did not attain equilibrium with regard to composi-tions of orthopyroxene and sapphirine These authors

Fig 3 Comparison of experimental data and calculations forreactions r21 r22 r42 r43 r44 and r52rsquo (reaction r52 involvingwater-saturated cordierite) with (a) ideal and (b) non-ideal solidsolution models of sapphirine Experimental points representbrackets with the pressure values corrected for friction with 5 error bars (see accepted values in text and in Table 2) Dotted area ndashpreliminary experimental data for reaction r21 (Podlesskii 1996)Heavy lines ndash our model thin lines ndash Kelsey et al (2004) Numbersshow the calculated sapphirine composition (Nss = 100Xss) at endsof corresponding lines


reported that no significant compositional trend in sapphi-rine could be resolved from the experimental data due tosluggish kinetics of this mineral This may give us a possibleexplanation for the discrepancy between the calculated andthe experimental data If sapphirine reacted incompletelythen full equilibration of Opx is also in doubt It shouldbe noted that heterogeneity of the run product compositionsslightly increased with increasing temperature which mayindicate that sapphirine started to react more readily onlyat higher T It is noteworthy that the highest temperature runsat 12 GPa by Hollis amp Harley (2003) did produce the Opxcompositions consistent with the calculations (see Table 3)To interpret experiments involving more than one phase ofvariable composition with confidence it is desirable thatall solid solutions give a consistent sense of reaction direc-tion (Berman amp Aranovich 1996) which seems not to bethe case here Experimental reversals of alumina contentof orthopyroxene in assemblage with sillimanite and quartzwhich is potentially a good geothermometer (Aranovich ampPodlesskii 1989 Podlesskii 2003) could probably illumi-nate this problem but they are not available at present

Using criteria based on the appearance of different phasesArima amp Onuma (1977) estimated experimental Al contentsof Opx associated with Spr + Qtz Their lower temperaturedata indicate significantly lower Al contents compared tothose derived with our models However it is unlikely theirlower temperature Al contents are equilibrium values sincerun products below 1350 C contained Fo + Qtz The high-temperature datawith no Fo + Qtz present in the run productsagree with our calculations experimentally estimated con-tents of 15ndash16 wt Al2O3 at 1400 C and 16ndash17 wtAl2O3 at 1450 C correspond to 154 and 160 wt Al2O3respectively calculated with ideal Spr and to 154 and 161wt Al2O3 respectively with non-ideal SprOur calculations are in reasonable agreement with experi-

ments by Anastasiou amp Seifert (1972) who estimated3ndash5 wt Al2O3 at 900 C and 5ndash7 wt at 1000 C forOpx associated with Spr + Crd from the angular positionof two X-ray reflections in the powder diffraction patternsof run products We obtained with both ideal and non-idealSpr 54 plusmn 04 (uncertainty is related to PndashT measurementerrors) and 68 plusmn 04 respectively

Fig 4 Isopleths of the alumina content in Opx for assemblages Opx + Sil + Qtz Opx + Crd + Qtz and Opx + Spr + Qtz in the systemMgOndashAl2O3ndashSiO2 (a) Calculated with data of Table 1 (non-ideal solid solution of sapphirine) (b) calculated with the data from Kelseyet al (2004) (c) calculated with data from Berman amp Aranovich (1996) for assemblages Opx + Sil + Qtz and Opx + Crd + Qtz andestimated for Opx + Spr + Qtz Heavy lines ndash reaction curves thin lines ndash isopleths (numbers denote mol Al2O3) solid lines ndash calculateddashed lines ndash estimated H amp H ndash isopleths for assemblage Opx + Spr + Qtz (dashed lines with encircled numbers) and experimental points(filled squares with numbers showing the ranges of determined alumina contents) from Hollis amp Harley (2003)


Petrogenetic grids and implicationsfor metamorphic rocks

Possible application of derived grids to petrologic interpreta-tion may be related to sapphirine assemblages with quartzkyanite and forsteriteAs shown in Fig 2 and 5 the Spr + Qtz field extends to

temperatures below 850 C which is closer to the initialestimate by Newton (1972) who supposed the invariantpoint to be located below 800 C under anhydrous condi-tions and whose experimental data together with that ofChatterjee amp Schreyer (1972) form a basis of our knowledgeon stability limits of this assemblage in MAS (see Table 2)The sequence of reactions that belongs to this invariant pointpredicted by Chatterjee amp Schreyer (1972) from graphicalanalysis proves to be qualitatively valid but the calculatedrelations differ from the prediction in important quantitativedetails It can be seen in Fig 5 that reactions Opx + Sil +Qtz = Crd (r41) and Opx + Sil = Spr + Crd (r45) lie veryclose to each other in pressure as do reactions Spr + Opx +Qtz = Crd (r43) and Spr + Qtz = Crd + Sil (r44) Thisimplies that the presence of quartz must have little influenceon the stability of Opx + Sil and an addition of orthopyrox-ene to Spr + Qtz changes the stability of this assemblageonly slightly Under water-saturated conditions the Spr +Qtz stability field shrinks due to the enhanced stability ofhydrous cordierite with the calculated invariant point I4being shifted along the Opx + Sil = Spr + Qtz reactioncurve from 835 C 0667 GPa to 1066 C 117 GPa (withideal sapphirine from 839 C 0669 GPa to 1046 C116 GPa) This agrees with experiments and further

considerations of Newton (1972 1995) concerning the mag-nesian cordierite stability and H2O activity in high-grademetamorphism Figure 5 also shows the effect of stabiliza-tion of cordierite at P frac14 PCO2estimated with the model ofAranovich amp Podlesskii (1989) However if we considerexperimental data on reaction (r42) which are available onlyfor temperatures above 1050 C (Table 2 Fig 3) and nottheir present interpretation we would have to entertain thepossibility that reaction curves (r41) and (r42) do not inter-sect to form I4 This situation could result from the unpre-dictable non-ideal behavior of sapphirine wherebySpr + Qtz is stabilized relative to Opx + Sil and reactioncurve (r42) acquires a concavity analogous to curve (r22)in Fig 3b With other experimental data unavailablefor MAS at lower temperatures the preliminary data ofPodlesskii (1996) for reaction Spr = Opx + Spl + Crn(r21) provide the only obstacle for such modeling Onecould easily dismiss these data by assuming that equilibra-tion must have not been achieved in experiments at 700ndash750 C If such a model were proposed it could have dra-matic consequences related to elimination of I4 which isthe starting point of FMAS reaction Spr + Qtz = Opx +Sil + Crd (Podlesskii 1997 Kelsey et al 2004)Another issue which may be important for interpretation of

petrogenetic conditions is the stability field of Spr + KyThis has been calculated with our data to exist below816 C and 1015 GPa with ideal sapphirine and below862 C and 1128 GPa with non-ideal sapphirine wherereaction curve (r31) intersects the sillimanitendashkyanite transi-tion curve (Fig 6) According to our calculations ideal sap-phirine must become significantly more aluminous along

Table 3 Experimental determinations of mineral compositions in assemblage Opx + Spr + Qtz by Hollis amp Harley (2003) compared tocalculations with internally consistent datasets

T C Orthopyroxene NOK Sapphirine NSS

Experimental Calculated Experimental Calculated

Low High Median 1 2 Low High Median 1 2

12 GPa1250 81ndash98 104ndash113 91ndash105 126 646ndash764 656ndash758 705 613

127 128 723 7551275 93ndash10 114ndash118 103ndash11 130 593ndash663 612ndash738 612ndash636 615


134 135 745 7551325 113ndash114 131ndash14 125 137 577ndash803 665ndash789 646ndash695 618


141 143 764 754

14 GPa1325 10ndash122 105ndash13 11ndash115 140 63ndash724 638ndash756 675ndash713 617

140 143 747 753


143 147 739 752

Low starting lower-alumina orthopyroxene high ndash starting higher-alumina orthopyroxene the composition data were taken from Fig 2 and3 of Hollis amp Harley (2003) 1 calculated with data of Table 1 (upper value ndash ideal sapphirine lower value ndash non-ideal sapphirine) 2 ndashcalculated with data from Kelsey et al (2004) Calculated values for pressures with 10 correction for friction Calculated valuesshown bold fall outside of the experimentally determined ranges


(r31) starting with NSS = 60 (NSS = 100XSS) at invariantpoint I3 and ending with NSS = 33 at I6 Our estimates showthat with the non-ideal sapphirine solid solution the Spr +Ky stability field is wider at lower temperatures with NSS

changing from 49 at I3 to 20 at 500 C Reactions involvingthe non-ideal sapphirine (r31) and (r61) do not intersecteither under fluid-absent condition when reaction (r61)enters the And stability field at low temperatures nor atP frac14 PH2O and P frac14 PCO2in the Ky stability field becausesapphirine becomes very alumina-rich and the reaction linescurve away from one another thus excluding the existenceof invariant point I6 While investigating the stability field

of gedrite in MASH Fischer et al (1999) observed theappearance of this phase together with Spr and Ky at850 C and 11 GPa which can be considered as evidencesupporting our non-ideal sapphirine model In MASHSpr + Ky must have a very limited stability field due toappearance of competing assemblages involving otherhydrous phases such as chlorite (Seifert 1974) or boron-free kornerupine (Wegge amp Schreyer 1994)The third remarkable assemblage (Spr + Fo) has been cal-

culated to be stable at temperatures below invariant point I5(769 C 0209 GPa with the ideal sapphirine model and801 C 0215 GPa ndash non-ideal compare to 735 C

Fig 5 Reactions belonging to invariant point I4 calculated with the data of Table 1 (non-ideal sapphirine) for fluid-absent conditions (solidlines) P frac14 PH2O(dashed lines) and P frac14 PCO2 (dotted lines) using the cordierite model proposed by Aranovich amp Podlesskii (1989)Predictions from graphical analysis by Chatterjee amp Schreyer (1972) are shown for comparison in the upper left corner

Fig 6 The Spr + Ky stability field calculated with the data of Table 1 (non-ideal sapphirine) for fluid-absent conditions (solid lines)P frac14 PH2O(dashed lines) and P frac14 PCO2 (dotted lines) using the cordierite model proposed by Aranovich amp Podlesskii (1989)


0209 GPa estimated by Podlesskii 1995) Under water-saturated conditions this assemblage does not appear inour calculations with the ideal sapphirine model at petrolog-ically significant temperatures due to stabilization of cordie-rite and the decrease of its stability calculated with thenon-ideal model is shown in Fig 7 In addition hydrousphases like chlorite may also preclude the coexistence of sap-phirine and forsterite in the wet system Gasparik (1994) alsoobtained a narrow field of Spr + Fo at low temperatures butwe could not check its compatibility with experimental dataon Opx + Spr = Crd + Spl and Opx + Spl = Crd + Fobecause the dataset presented by this author did not allowcalculations with water-bearing cordieriteFigure 8 presents our calculation of phase relations in

MAS with the thermodynamic datasets of Holland amp Powell(1998) and Kelsey et al (2004) The former dataset being aprecursor of the latter would not have been considered if ithad not been used by Ouzegane et al (2003) as a basis fordeveloping their own grid As can be seen in Fig 8 the dataof Holland amp Powell (1998) imply that Spr + Fo must pre-clude stability of competing assemblage Opx + Spl almostover the whole PndashT space considered which is in obviousconflict with much experimental and natural evidence ofthe wide PndashT stability range of Opx + Spl Grew et al(1994) who predicted existence of invariant point I5 atlow temperatures under decreased water activity empha-sized a rarity of natural occurrences of Spr + Fo Theexaggerated stability of Spr + Fo implied by the Thermo-

calc dataset might have been overlooked by Holland ampPowell (1998) when they incorporated the sapphirineproperties The successor version presented by Kelseyet al (2004) also implies an unrealistically wide stability fieldfor Spr + Fo although smaller than that of Holland ampPowell (1998) Ouzegane et al (2003) attemptinglsquolsquoto derive pseudosections suitable for the studied rocks and

the estimated PndashT conditionsrsquorsquo with the Thermocalc

data from Holland amp Powell (1998) found that this couldbe achieved by adding 5 kJmol to the enthalpy ofMg2Al8Si2O20 and 10 kJmol to that of Mg35Al9Si15O20thus increasing the stability of sapphirine Obviously thisenergy adjustment must cause even greater stability of Spr +Fo in relation to Opx + Spl which has not been consideredby the authors who restricted themselves to consideration ofcertain granulites Although their petrogenetic grid is under-mined by the arbitrary adjustment comparable to univariantreaction energies it should be noted that Ouzegane et al(2003) must have considered the extended stability of Spr +Qtz in MAS (~ 850 C 07 GPa in their Fig 12a) accept-able In their interpretation of MAS Spr + Ky must be sta-ble at ~ 750ndash1100 C and 08ndash17 GPaIn other aspects phase relations implied by successive

versions of the Thermocalc dataset look very similar toeach other As outlined above in relation to Spr + Qtz thedata presented by Kelsey et al (2004) shift the stability fieldof this assemblage towards high temperature This shift islikely due to inadequate fitting of thermodynamic propertiesto experimental data At least in MAS sapphirine can coex-ist with quartz at temperatures significantly below 900 CCalculations with the Thermocalc dataset as can be

seen in Fig 8 do not allow sapphirine to coexist withkyanite in MAS Evidently this implies that stability ofSpr + Ky must be even less possible in FMAS becauseFe preferably enters orthopyroxene which must inevitablystabilize the competing Opx + Crn assemblage Baldwinet al (2007) tried to overcome the discrepancy betweenthe observed presence of sapphirine + kyanite in natureand predictions by Thermocalc just by excluding silli-manite from the list of phases involved in calculation of PndashTrelations and interpreted their textures based on the CandashFeOndashMgOndashAl2O3ndashSiO2 grid where lsquolsquoreactions dominantly

Fig 7 The Spr + Fo stability field calculated with the data of Table 1 (non-ideal sapphirine) for fluid-absent conditions (solid lines) andP frac14 PH2O(dashed lines) using the cordierite model proposed by Aranovich amp Podlesskii (1989) The low-temperature stability limit ofsapphirine at P frac14 PH2O(dotted line) is shown according to experimental data of Seifert (1974)


lie within corundum + quartz stabilityrsquorsquo (their Fig 10)Asami et al (2007) described an FMAS sapphirine +spinel + kyanite + garnet (XMg 055) assemblagemeetingtextural and chemical criteria of equilibrium in a granulitefrom the Soslashr Rondane Mountains East Antarctica but theywere not successful in finding a stability field for this assem-blage using ThermocalcIn our opinion the outlined features of the Thermocalc

dataset simply reflect still insufficient knowledge ofsome major phase energetics and in this regard the state-ments of Powell amp Holland (2008) that lsquolsquopseudosectionsare likely to be the most effective form of thermobarometryfor granulite facies and UHT rocksrsquorsquo and of Kelsey (2008)that lsquolsquocalculated phase diagrams allow the true complexity ofmineral reaction topologies to be seenrsquorsquo appear to be some-what premature The example of sapphirine demonstratesthat calculations of phase relations in complex chemical sys-tems involving poorly constrained thermodynamic proper-ties of minerals do not yet hold the advantage overlsquolsquoconventionalrsquorsquo (the term introduced by Powell amp Holland2008) thermobarometry based on either direct calibrations orthermodynamic datasets for experimentally well-constrainedphases

Concluding remarks

Critical assessment of phase equilibria involving sapphirinein MAS as calculated using thermodynamic datasets showsthat there remain major inconsistencies between calculatedand observed stability relationships Better constraint is stillneeded for equilibria and thermodynamic properties usingreversal equilibration experiments and determination of

equilibrium compositions of minerals Optimized thermody-namic properties of the sapphirine solid solution imply thatSpr + Qtz could be stable below 900 C Narrow stabilityfields of Spr + Ky are predicted at lower temperaturesand for Spr + Fo at very low water activity in accord withobserved natural occurrencesAlthough our description seems to fit most relevant exper-

imental data it should be noted that it is based on modeledcompositions of sapphirine and thus much uncertaintyremains One possible improvement to the sapphirine modelwould be an extension of the solid solution range beyondour limits of 2 gt Si gt 1 per 20 oxygens consistent withsome natural sapphirines and Ge contents in GandashGe analogsreported by Barbier (1998)Recently Kelsey (2008) recognized that experimental data

for systems other than MAS are not applicable to modelingof thermodynamic behavior of iron-bearing sapphirine andswitched to natural FendashMg partitioning data between sapphi-rine and coexisting orthopyroxene and garnet to get a bridgeto FMAS and beyond In this respect he followed anapproach earlier applied for the same purpose and reasonsby Podlesskii (1995 1997) However the latter publicationshave demonstrated that even for FMAS this method canproduce alternative PndashT diagrams with much lower temper-atures of Spr + Qtz assemblages They also had commonfeatures with the grid of Kelsey et al (2004) such as thevery close spacing of invariant points which are to a greatextent inherited from phase relationships defined by reac-tions belonging to the MAS invariant point I4For modeling reactions involving sapphirine outside of

MAS it would be most desirable to obtain data that coulddefine how other major constituents of this mineralFe2+ and Fe3+ affect its properties The thermodynamic

Fig 8 PndashT diagrams for the system MgOndashAl2O3ndashSiO2 calculated (a) with data from Kelsey et al (2004) and (b) Holland amp Powell(1998) Additional invariant points I8 ndash [CrdOpxQtzSil(KyAnd)] and I9 ndash [CrdCrnQtzSil(KyAnd)] Additional reaction curvesr81 ndash Prp + Spl = Spr + Fo r82 ndash Fo + Crn = Prp + Spr r83 ndash Fo + Spl + Crn = Spr r91 ndash Prp + Fo = Spr + Opx r92 ndashPrp + Spl = Spr + Opx


description of the effect of composition and PndashT conditionson the stability of different polytypes of sapphirine could beanother improvement Since sapphirine in most cases has thehighest Fe3+Fetotal ratio among coexisting FendashMg mineralsincluding aluminous spinel these data could also shed lighton redox conditions of metamorphism

Acknowledgments This work was inspired by WernerSchreyer at initial stages when KKP was an Alexandervon Humboldt research fellow in Bochum The authorsare grateful to Simon Harley and Andrew Christy for theirthorough and constructive reviews that stimulated us to im-prove the clarity of this paper Our thanks are due also toEdward Grew for his extremely helpful comments and edi-torial assistance The study was funded by RFBR grants06-05-65069 06-05-64976 and 06-05-64098-a

References

Ackermand D Seifert F Schreyer W (1975) Instability ofsapphirine at high pressures Contrib Mineral Petrol 5079-92

Anastasiou P amp Seifert F (1972) Solid solubility of Al2O3 inenstatite at high temperatures and 1ndash5 kb water pressureContrib Mineral Petrol 34 272-287

Aranovich LY amp Kosyakova NA (1987) The cordierite = orth-opyroxene + quartz equilibrium laboratory data on and ther-modynamics of ternary FendashMgndashAl orthopyroxene solidsolutions Geochemistry Int 24 111-131

Aranovich LY amp Podlesskii KK (1989) Geothermobarometry ofhigh-grade metapelites simultaneously operating reactions inlsquolsquoEvolution of Metamorphic Beltsrsquorsquo RA Cliff BWD YardleyJS Daly eds Geol Soc London Spec Publ 43 45-62

Arima M amp Onuma K (1977) The solubility of alumina inenstatite and the phase equilibria in the join MgSiO3ndashMgAl2SiO6 at 10ndash25 kbar Contrib Mineral Petrol 61 251-265

Asami M Grew ES Makimoto H (2007) Relict sapphi-rine + kyanite and spinel + kyanite associations in pyropicgarnet from the eastern Soslashr Rondane Mountains East Antarc-tica Lithos 93 107-125

Baldwin JA Powell R Williams ML Goncalves P (2007)Formation of eclogite and reaction during exhumation to mid-crustal levels Snowbird tectonic zone western Canadian ShieldJ metamorphic Geol 25 953-974

Barbier J (1998) Crystal structures of sapphirine and surinamiteanalogues in the MgOndashGa2O3ndashGeO2 system Eur J Mineral10 1283-1293

Barbier J amp Hyde BG (1988) Structure of sapphirine its relationto the spinel clinopyroxene and b-gallia structures ActaCrystallogr B44 373-377

Berman RG amp Aranovich LY (1996) Optimized standard stateand solution properties of minerals I Model calibration forolivine orthopyroxene cordierite garnet and ilmenite in thesystem FeOndashMgOndashCaOndashAl2O3ndashTiO2ndashSiO2 Contrib MineralPetrol 126 1-24

Bishop FC amp Newton RC (1975) The composition of lowpressure synthetic sapphirine J Geol 83 511-517

Boyd FR amp England JL (1959) Pyrope Carnegie Inst WashYear Book 58 83-87

Brigida C Poli S Valle M (2007) High-temperature phaserelations and topological constraints in the quaternary system

MgOndashAl2O3ndashSiO2ndashCr2O3 an experimental study Am Min-eral 92 735-747

Burzo E (2004) Sapphirine and related silicates in lsquolsquoMagneticproperties of non-metallic inorganic compounds based ontransition elements orthosilicatesrsquorsquo HPJ Wijn ed Landolt-Bornstein - Group III Condensed Matter vol 27 subvol II(Orthosilicates) Springer-Verlag 339-354

Charlu TV Newton RC Kleppa OJ (1975) Enthalpies offormation at 970 K of compounds in the system MgOndashAl2O3ndashSiO2 from high temperature solution calorimetry GeochimCosmochim Acta 39 1487-1497

Chatterjee ND amp Schreyer W (1972) The reaction ensta-titess + sillimanite = sapphiriness + quartz in the system MgOndashAl2O3ndashSiO2 Contrib Mineral Petrol 36 49-62

Christy AG (1989a) The effect of composition temperature andpressure on the stability of the 1Tc and 2M polytypes ofsapphirine Contrib Mineral Petrol 103 203-215

mdash (1989b) A short-range interaction model for polytypism andplanar defect placement in sapphirine Phys Chem Minerals16 343-351

Christy AG amp Grew ES (2004) Synthesis of berylliansapphirine in the system MgOndashBeOndashAl2O3ndashSiO2ndashH2O andcomparison with naturally occurring beryllian sapphirine andkhmaralite Part 2 a chemographic study of Be content as afunction of P T assemblage and FeMg1 exchange AmMineral 89 327-338

Christy AG Tabira Y Holscher A Grew ES Schreyer W(2002) Synthesis of beryllian sapphirine in the system MgOndashBeOndashAl2O3ndashSiO2ndashH2O and comparison with naturally occur-ring beryllian sapphirine and khmaralite Part 1 experimentsTEM and XRD Am Mineral 87 1104-1112

Danckwerth A amp Newton RC (1978) Experimental determina-tion of the spinel peridotite to garnet peridotite reaction in thesystem MgOndashAl2O3ndashSiO2 in the range 900ndash1100 C and Al2O3

isopleths of enstatite in the spinel field Contrib MineralPetrol 66 189-201

Das K Dasgupta S Miura H (2001) Stability of osumilitecoexisting with spinel solid solution in metapelitic granulites athigh oxygen fugacity Am Mineral 86 1423-1434

mdash mdash mdash (2003) An experimentally constrained petrogenetic gridin the silica-saturated portion of the system KFMASH at hightemperatures and pressures J Petrol 44 1055-1075

Das K Fujino K Tomioka N Miura H (2006) Experimentaldata on Fe and Mg partitioning between coexisting sapphirineand spinel an empirical geothermometer and its applicationEur J Mineral 18 49-58

Doroshev AM amp Malinovsky IYu (1974) Upper pressurelimit of stability of sapphirine Dokl Akad Nauk SSSR 219136-138

Fawcett JJ amp Yoder Jr HS (1966) Phase relations of thechlorites in the system MgOndashAl2O3ndashSiO2ndashH2O Am Mineral51 353-380

Fischer H Schreyer W Maresch WV (1999) Synthetic gedritea stable phase in the system MgOndashAl2O3ndashSiO2ndashH2O (MASH)at 800 C and 10 kbar water pressure and the influence ofFeNaCa impurities Contrib Mineral Petrol 136 184-191

Foster WR (1950) Synthetic sapphirine and its stability relationsin the system MgOndashAl2O3ndashSiO2 J Am Ceram Soc 33 73-84

Friend CRL (1982) AlndashCr substitution in peraluminous sapphi-rines from Bjornesund area Fiskenaeligsset region southern WestGreenland Mineral Mag 46 323-328

Gasparik T (1994) A petrogenetic grid for the system MgOndashAl2O3ndashSiO2 J Geol 102 97-109

Gasparik T amp Newton RC (1984) The reversed alumina contentsof orthopyroxene in equilibrium with spinel and forsterite in thesystem MgOndashAl2O3ndashSiO2 Contrib Mineral Petrol 85186-196


Gerya TV Perchuk LL Podlesskii KK Kosyakova NA(1996) Thermodynamic database for the system MgOndashFeOndashCaOndashAl2O3ndashSiO2 Exp Geosci 5 N2 24-26

Gerya TV Podlesskii KK Perchuk LL Swamy V Kosyak-ova NA (1998) Equation of state of minerals for thermody-namic databases used in petrology Petrology 6 511-526

Gerya TV Perchuk LL Podlesskii KK (2004a) Database andmineral thermobarometry program lsquolsquoGEOPATHrsquorsquo in lsquolsquoExperi-mental mineralogy some results on the Centuryrsquos FrontierrsquorsquoVA Zharikov amp VV Fedrsquokin eds Nauka Moscow 2 188-206(in Russian)

Gerya TV Podlesskii KK Perchuk LL Maresch WV(2004b) Semi-empirical Gibbs free energy formulations forminerals and fluids for use in thermodynamic databases ofpetrological interest Phys Chem Minerals 31 429-455

Gottschalk M (1997) Internally consistent thermodynamic data forrock forming minerals in the system SiO2ndashTiO2ndashAl2O3ndashFe2O3ndashCaOndashMgOndashFeOndashK2OndashNa2OndashH2OndashCO2 Eur J Mineral 9175-223

Grew ES Chernosky JV Werding G Abraham K MarquezN Hinthorne JR (1990) Chemistry of kornerupine andassociated minerals a wet chemical ion microprobe and X-raystudy emphasizing Li Be B and F contents J Petrol 311025-1070

Grew ES Yates MG Romanenko IM Christy AG SwihartGH (1992) Calcian borian sapphirine from the serendibitedeposit at Johnsburg NY USA Eur J Mineral 4 475-485

Grew ES Pertsev NN Yates MG Christy AG Marquez NChernosky JV (1994) Sapphirine + forsterite and sapphi-rine + humite group minerals in an ultra-magnesian lens fromKuhi-Lal SW Pamirs Tajikistan are these assemblagesforbidden J Petrol 35 1275-1293

Grew ES Yates MG Shearer CK Haggerty JJ SheratonJW Sandiford M (2006) Beryllium and other trace elementsin paragneisses and anatectic veins of the ultrahigh-temperatureNapier Complex Enderby Land East Antarctica the role ofsapphirine J Petrol 47 859-882

Harley SL (1998) On the occurence and characterization ofultrahigh-temperature crustal metamorphism in lsquolsquoWhat DrivesMetamorphism and Metamorphic Reactionsrsquorsquo PJ Treloar ampPJ OrsquoBrien eds Geol Soc London Spec Publ 138 81-107

mdash (2008) Refining the PndashT records of UHT crustal metamorphismJ metamorphic Geol 26 125-154

Harley SL amp Christy AG (1995) Titanium-bearing sapphirine ina partially melted aluminous granulite xenolith Vestfold HillsAntarctica geological and mineralogical implications Eur JMineral 7 637-653

Hensen BJ (1972) Phase relations involving pyropess enstatitessand sapphiriness in the system MgOndashAl2O3ndashSiO2 CarnegieInst Wash Year Book 71 421-427

mdash (1986) Theoretical phase relations involving cordierite andgarnet revisited the influence of oxygen fugacity on the stabilityof sapphirine and spinel in the system MgndashFendashAlndashSindashOContrib Mineral Petrol 92 362-367

Hensen BJ amp Essene EJ (1971) Stability of pyropendashquartz in thesystem MgOndashAl2O3ndashSiO2 Contrib Mineral Petrol 30 72-83

Hensen BJ amp Green DH (1971) Experimental study of thestability of cordierite and garnet in pelitic compositions at highpressures and temperatures I Compositions with excessalumino-silicate Contrib Mineral Petrol 33 309-330

Hensen BJ amp Harley SL (1990) Graphical analysis of P-T-Xrelations in granulite facies metapelites in lsquolsquoHigh-temperaturemetamorphism and crustal anatexisrsquorsquo JR Ashworth amp MBrown eds Unwin Hyman United Kingdom 19ndash56

Herzberg CT (1983) The reaction forsterite + cordierite = alumi-nous orthopyroxene + spinel in the system MgOndashAl2O3ndashSiO2Contrib Mineral Petrol 84 84-90

Higgins JB amp Ribbe PH (1979) Sapphirine II A neutron andX-ray diffraction study of (MgndashAl)VI and (SindashAl)IV ordering inmonoclinic sapphirine Contrib Mineral Petrol 68 357-368

Higgins JB Ribbe PH Herd RK (1979) Sapphirine I Crystalchemical contributions Contrib Mineral Petrol 68 349-356

Holland TJB amp Powell R (1998) An internally-consistentthermodynamic dataset for phases of petrological interestJ metamorphic Geol 16 309-343

Hollis JA amp Harley SL (2003) Alumina solubility in orthopy-roxene coexisting with sapphirine and quartz Contrib MineralPetrol 144 473-483

Jung I-H Decterov S A Pelton A D (2004) Critical thermo-dynamic evaluation and optimization of the MgOndashAl2O3 CaOndashMgOndashAl2O3 and MgOndashAl2O3ndashSiO2 systems J Phase Equilib25 329-345

Kelsey DE (2008) On ultrahigh-temperature crustal metamor-phism Gondwana Res 13 1-29

Kelsey DE White RW Holland TJB Powell R (2004)Calculated phase equilibria in K2OndashFeOndashMgOndashAl2O3ndashSiO2ndashH2O for sapphirine-quartz-bearing mineral assemblages Jmetamorphic Geol 22 559-578

Kiseleva IA amp Topor ND (1975) High-temperature heat-capacity of sapphirine (in Russian) Geokhimiya 2 312-315

Liu T-C amp Presnall DC (2000) Liquidus phase relations in thesystem CaOndashMgOndashAl2O3ndashSiO2 at 20 GPa applications tobasalt fractionation eclogites and igneous sapphirine J Petrol41 3-20

Liu X amp OrsquoNeill HStC (2004) Partial melting of spinellherzolite in the system CaOndashMgOndashAl2O3ndashSiO2 plusmn K2O at11 GPa J Petrol 45 1339-1368

Malinovsky IYu ampDoroshev AM (1975) Low-pressure boundaryof the stability field of pyrope Experimental studies inmineralogy(in Russian) Akad Nauk SSSR Novosibirsk 95-100

Merlino S (1980) Crystal structure of sapphirine 1Tc Z Kristal-logr 151 91-100

Menchi AM amp Scian AN (2005) Mechanism of cordieriteformation obtained by the solndashgel techniqueMaterials Lett 592664-2667

Milholland CS amp Presnall DC (1998) Liquidus phase relationsin the CaOndashMgOndashAl2O3ndashSiO2 system at 30 GPa the alumi-nous pyroxene thermal divide and high-pressure fractionation ofpicritic and komatiitic magmas J Petrol 39 3-27

Moore PB (1969) The crystal structure of sapphirine AmMineral 54 31-49

Newton RC (1972) An experimental determination of the high-pressure stability limits of magnesian cordierite under wet anddry conditions J Geol 80 398-420

mdash (1995) Simple-system mineral reactions and high-grade meta-morphic fluids Eur J Mineral 7 861-881

Newton RC Charlu TV Kleppa OJ (1974) A calorimetricinvestigation of the stability of anhydrous magnesian cordieritewith application to granulite facies metamorphism ContribMineral Petrol 44 295-311

Nogteva VV Kolesnik YN Paukov IE (1974) Low-temper-ature heat-capacity and thermodynamic properties of sapphirine(in Russian) Geokhimiya 6 820-829

Ouzegane K Guiraud M Kienast JR (2003) Prograde andretrograde evolution in high-temperature corundum granulites(FMAS and KFMASH systems) from the In Ouzzal terrane(NW Hoggar Algeria) J Petrol 44 517-545

Perkins III D (1983) The stability of Mg-rich garnet in the systemCaOndashMgOndashAl2O3ndashSiO2 at 1000ndash1300 C and high pressureAm Mineral 68 355-364

Perkins III D Holland TJB Newton RC (1981) The Al2O3

contents of enstatite in equilibrium with garnet in the systemMgOndashAl2O3ndashSiO2 at 15ndash40 kbar and 900ndash1600 C ContribMineral Petrol 78 99-109


Podlesskii KK (1995) Stability relations involving sapphirine inthe system FeOndashMgOndashAl2O3ndashSiO2 Bochumer Geol GeotechArb 44 146-151

mdash (1996) Sapphirine-bearing equilibria in the system MgOndashAl2O3ndashSiO2 Terra Abstracts 8 Suppl 1 51

mdash (1997) PndashTndashXMg relations in aluminous granulites Exp Geosci6 1 22-23

mdash (2003) Hypersthene in assemblage with sillimanite and quartzas an indicator of metamorphic conditions Trans (Doklady)Rus Acad SciEarth Sci Sec 389 248-251

Powell R amp Holland TJB (2008) On thermobarometry Jmetamorphic Geol 26 155-179

Sabau G Alberico A Negulescu E (2002) Peraluminoussapphirine in retrogressed kyanite-bearing eclogites from theSouth Carpathians status and implications Int Geol Rev 44859-876

Sato K Miyamoto T Kawasaki T (2006) Experimental calibra-tion of sapphirinendashspinel Fe2+ndashMg exchange thermometerimplication for constraints on PndashT condition of Howard HillsNapier Complex East Antarctica Gondwana Res 9 398-408

Schreyer W (1967) A reconnaissance study of the system MgOndashAl2O3ndashSiO2ndashH2O at pressures between 10 and 25 kbar Carne-gie Inst Wash Year Book 66 380-392

mdash (1968) Stability of sapphirine Carnegie Inst Wash Year Book67 389-392

Schreyer W amp Abraham K (1975) Peraluminous sapphirine as ametastable reaction product in kyanite-gedrite talc shist fromSar e Sang Afghanistan Mineral Mag 40 171-180

Schreyer W amp Schairer JF (1961a) Stable and metastable phaserelations in the system MgOndashAl2O3ndashSiO2 Carnegie Inst WashYear Book 60 144-147

mdash mdash (1961b) Compositions and structural states of anhydrousMg-cordierites a re-investigation of the central part of thesystem MgOndashAl2O3ndashSiO2 J Petrol 2 324-406

Schreyer W amp Seifert F (1969a) High-pressure phases in thesystem MgOndashAl2O3ndashSiO2ndashH2O Am J Sci 267A Schairervolume 407ndash443

mdash mdash (1969b) Compatibility relations of the aluminium silicates inthe system MgOndashAl2O3ndashSiO2ndashH2O and K2OndashMgOndashAl2O3ndashSiO2ndashH2O at high pressures Am J Sci 267 371-388

mdash mdash (1970) Pressure dependence of crystal structures in thesystem MgOndashAl2O3ndashSiO2ndashH2O at pressures up to 30 kb PhysEarth Planet Interiors 3 422-430

Schreyer W amp Yoder HS (1960) Instability of anhydrous Mg-cordierite at high pressure Carnegie Inst Wash Year Book 5990-91

Schreyer W Abraham K Behr HJ (1975) Sapphirine andassociated minerals from the kornerupine rock of WaldheimSaxony N Jb Mineral Abh 126 1-27

Schreyer W Horrocks PC Abraham K (1984) High-magne-sium staurolite in a sapphirine-garnet rock from the Limpopobelt South Africa Contrib Mineral Petrol 86 200-207

Seifert F (1974) Stability of sapphirine a study of aluminous partof the system MgOndashAl2O3ndashSiO2ndashH2O J Geol 82 173-204

Sheng YJ Hutcheon ID Wasserburg GJ (1991) Origin ofplagioclase-olivine inclusions in carbonaceous chondrites Geo-chim Cosmochim Acta 55 581-599

Shu C Mingxia X Cailou Z Jiaqi T (2002) Fabrication ofcordierite powder from magnesium-aluminum hydroxide andsodium silicate its characteristics and sintering Materials ResBull 37 1333-1340

Simon G amp Chopin C (2001) Enstatite-sapphirine crack-relatedassemblages in ultrahigh-pressure pyrope megablasts Dora-Maira massif western Alps Contrib Mineral Petrol 140 422-440

Steffen G Seifert F Amthauer G (1984) Ferric iron insapphirine a Mossbauer spectroscopic study Am Mineral69 339-348

Wegge S amp Schreyer W (1994) Boron-free kornerupine its upperpressure stability limit in the system MgOndashAl2O3ndashSiO2ndashH2O(MASH) Eur J Mineral 6 67-75

Received 19 March 2008Modified version received 17 June 2008Accepted 7 July 2008


thermodynamic characteristics of sapphirine end-membersor mixing properties but restricted application to qualitativegraphical analysis of phase relations (Hensen 1986 Hensenamp Harley 1990) although these were plotted on quantitativePndashT diagrams (Harley 1998 2008 Das et al 2001 2003)Calibrations of FendashMg exchange reactions between sapphi-rine and other minerals based on experimental crystallizationof complex mixtures did not provide evidence for achievingequilibrium relationships and conflicting results wereobtained in experiments using the same approach (Daset al 2006 Sato et al 2006)Other components that enter sapphirine ie Fe3+ Be

(Christy et al 2002 Christy amp Grew 2004 Grew et al2006 and the references therein) B (Grew et al 19901992) Ca (Grew et al 1992) Cr (Friend 1982 Brigidaet al 2007) and Ti (Harley amp Christy 1995 Sheng et al1991) undoubtedly play important roles in expanding the sta-bility of sapphirine relative to the model system MAS butthese components are also beyond consideration in this paperHere we present new models of the thermodynamic prop-

erties of magnesian sapphirine Phase relations involvingsapphirine in MAS derived with these data are tested withexisting experimental constraints Our parameters differfrom the data used in Thermocalc (Kelsey et al 2004)and seem to describe the experimentally studied reac-tions better For the estimated stability of assemblages ofsapphirine with quartz kyanite and forsterite to which wehave paid special attention due to their petrologic impor-tance these differences are dramatic and may change theinterpretations of petrogenetic processes

Sapphirine solid solution and itsthermodynamic model in MAS

After the structure determination by Moore (1969) sapphi-rine has been recognized as a chain silicate with the simpli-fied crystallographic formula M8T6O20 where M and Trepresent octahedral and tetrahedral sites respectively InMAS octahedral positions in sapphirine are occupied byMg and Al while tetrahedral positions belong to Al andSi and substitutions occur along the join Mg + Si =Al + Al as implied by Higgins et al (1979) Followingour previous publications (Podlesskii 1995 Gerya et al1996 2004a) we accepted end-members Mg4Al8Si2O20(SprS) and Mg3Al10SiO20 (Spr

A) to represent the sapphirinesolid solution for thermodynamic modeling in MAS Theseend-members have also been chosen for the latest version ofthe Thermocalc database (Kelsey et al 2004) with whichwe will compare our results (earlier Thermocalc usedend-members Mg4Al8Si2O20 and Mg35Al9Si15O20) Interms of the Tschermak type substitution Mg + Si = Al +Al a wider range of the sapphirine solid solution will prob-ably be considered in future studies since sapphirines withmore than 2 Si per formula unit have been observed both inexperiments (Milholland amp Presnall 1998 Liu amp Presnall2000 Das et al 2001 2003 Liu amp OrsquoNeill 2004) andin nature namely cracks in pyrope megablasts from Dora-Maira Italian Alps (Simon amp Chopin 2001) whereas

sapphirine with less than 1 Si per formula unit has beenencountered in eclogites from the South Carpathians Roma-nia (Sabau et al 2002)In our approach sapphirine solid solution (Spr) was ini-

tially considered as an ideal mixture of end-members SprS

and SprA (often referred to as lsquolsquo221rsquorsquo and lsquolsquo351rsquorsquo respec-tively in the literature) Unlike Kelsey et al (2004) wefound that describing the existing experimental data didnot require non-ideality However non-ideality can be intro-duced by analogy with other minerals that demonstrate theTschermak substitution and we have also tried the non-idealmodel although this approach has added parameters notconstrained by measurements A composition parameterXSS that denotes the mole fraction of SprS ranging from 1to 0 has been used to represent the activity relationshipsA regular solution parameter WSpr has been used for thenon-ideal model to express the excess energy dependenceon composition as WSpr AElig XSS (1 XSS)Our experience (Podlesskii 1995) shows that variation in

sapphirine composition plays a decisive role for thermody-namic modeling of sapphirine-bearing equilibria and it isimpossible to describe the whole spectrum of existingexperimental data with a constant composition as was triedby Gottschalk (1997) with sapphirine Mg9Al18Si3O40(lsquolsquo793rsquorsquo) or Jung et al (2004) with an out-of-date formulaMg4Al10Si2O23 (lsquolsquo452rsquorsquo) of Foster (1950) that is in conflictwith the structure determinations (Moore 1969 Higgins ampRibbe 1979 Merlino 1980 Barbier amp Hyde 1988)At present effects of polytypism and stacking disorder on

stability of sapphirine (Christy 1989a and b) cannot beconsidered in terms of its thermodynamic properties dueto the lack of data for the experiments under considerationand this simplification undoubtedly introduces additionaluncertainties

Experimental data and their thermodynamicdescription

Experimental investigations related to MAS have a long his-tory since this system has become important for metallurgyand the ceramic industry (Jung et al 2004 and the refer-ences therein) and petrology (Schreyer amp Seifert 1969aGasparik 1994 and the references therein) However exper-iments on melting and crystallization of various mixtureswidely used in ceramic studies often did not provide evi-dence for achieving equilibrium relationships They alwaysinvolved phases of undefined composition like melt withthe sapphirine composition having never been determinedalthough sometimes claimed to be close to 221 or 793In the absence of fluid reactions involving MAS phasesare notoriously sluggish because the phases are so refractoryand thus often proceed via metastable intermediates so withinsufficient run durations incompatible phase assemblagesmay be observed even when the temperatures are relativelyhigh For example Shu et al (2002) and Menchi amp Scian(2005) reported the joint appearance of sapphirine and cris-tobalite at temperatures between 1200 C and 1280 C incrystallizing cordierite ceramics Conversely crystallizing a

















S2981JK mol

c1 DH os1 J V o

sJbar


c2 DHos2 J








SS W Spr eth2bTHORN






eth3THORN

ie























































141 143 764 754


140 143 747 753


143 147 739 752





















Concluding remarks










References






















































































































S2981JK mol

c1 DH os1 J V o

sJbar


c2 DHos2 J








SS W Spr eth2bTHORN






eth3THORN

ie























































141 143 764 754


140 143 747 753


143 147 739 752





















Concluding remarks










References










































































































SS W Spr eth2bTHORN






eth3THORN

ie























































141 143 764 754


140 143 747 753


143 147 739 752





















Concluding remarks










References









































































































































141 143 764 754


140 143 747 753


143 147 739 752





















Concluding remarks










References

























































































































Concluding remarks










References









































































































References







































































































Sapphirine-bearing assemblages in the system...

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