American Mineralogist, Volwne 71, pages 277-300, 1986

Phaseequilibria and thermodynamic properties of mineralsin the BeO-AlrO3-SiO2-H2O(BASH) system,with petrologicapplications

Mlnx D. B.qnroN Department of Earth and SpaceSciences, University of California, Los Angeles,Los Angeles,California 90024

Ansrru,cr The phase relations and thermodynamic properties of behoite (Be(OH)r), bertrandite (BeoSirOr(OH)J, beryl (BerAlrSiuO,r),bromellite (BeO), chrysoberyl (BeAl,Oo), euclase (BeAlSiOo(OH)),and phenakite (BerSiOo)have been quantitatively evaluatedfrom a com- bination of new phase-equilibrium, solubility, calorimetric, and volumetric measurements and with data from the literature. The resulting thermodynamic model is consistentwith natural low-variance assemblagesand can be used to interpret many beryllium-mineral occurTences. Reversedhigh-pressure solid-media experimentslocated the positions of four reactions: BerAlrSiuO,,: BeAlrOo * BerSiOo+ 5SiO, (dry) 20BeAlSiOo(OH): 3BerAlrsi6or8+ TBeAlrOo+ 2BerSiOn+ l0HrO 4BeAlSiOo(OH)+ 2SiOr: BerAlrSiuO,,+ BeAlrOo+ 2H2O BerAlrSiuO,,+ 2AlrSiOs : 3BeAlrOa + 8SiO, (water saturated). Aqueous silica concentrationswere determined by reversedexperiments at I kbar for the following sevenreactions: 2BeO + H4SiO4: BerSiOo+ 2H2O 4BeO + 2HoSiOo: BeoSirO'(OH),+ 3HrO BeAlrOo* BerSiOo+ 5H4Sio4: Be3AlrSiuOr8+ loHro 3BeAlrOo+ 8H4SiO4: BerAlrSiuOrs+ 2AlrSiO5+ l6HrO 3BerSiOo+ 2AlrSiO5+ 7H4SiO4: 2BerAlrSiuOr8+ l4H2o aBeAlsioloH) + Bersio4 + 7H4sio4:2BerAlrsiuors + 14Hro 2BeAlrOo+ BerSiOo+ 3H4SiOo: 4BeAlSiOr(OH)+ 4HrO. The results of these experiments were combined with the heat capacitiesand entropies determined in a companion study and with data from the literature to estimate the ther- modynamic and thermophysical properties of the beryllium minerals. Eighty-six data sets containing over 1000independent observations were evaluatedusing the program eHASE2o. The values of G!rr,, (kJ/mol) and S!rr., (J/(K.mol)) with their uncertainties(2o) arebehoite -827.3(6.7),45.57(0.18);bertrandite -4300.6(1.6), I72.ll(0.77); anhydrousberyl -8500.4(3.8),346.7(4.7);chrysoberyl -2176.2(1.5), 66.25(0.30); euclase -2370.2(r.l), 89.1 l (0.a0);and phenakite- 2028.4(0.9), 63.43(0.27). The effect of water on the stability of beryl is treated as a purely volumetric interaction

: - Gr, r ty*ous ueryt Gp, Ii anlyarcu. t" ryl P noiaZu, where Z*" is an effective volume for the fluid occluded by the beryl structure (14.1 cm3/ mol). Not only doesthis simple "bottle model" work as well for beryl as alternative, more sophisticatedmodels, in part owing to the lack of hydration data, it also appearsto work reasonablywell for cordierite. Beryl has a wide stability range consistent with its relative natural abundance. The stability field of beryl is greatly increasedby high fluid pressuresand by alkali-containing solid solutions. The assemblagechrysoberyl + quartz is stableonly at relatively high tem- peratures(>600"C) or with Poro ( P,.o,. At moderate pressureswith decreasingtemper- ature, euclasebecomes stable around 400'C. Euclasereplaces beryl in aluminum silicate- bearing assemblagesin the 300'C range. In the 200"C range, phenakite and bromellite hydrateto bertrandite and behoite,respectively, and pure beryl reactsto euclase+ quartz + bertrandite or phenakite. The predicted phaserelations are in good qualitative agreement

0003-004x/86/0304427 7 $02.00 277 278 BARTON:PHASE EQUILIBRIA IN THE BeO-A1,O,-SiO,-H,OSYSTEM

with natural associations.Consideration of the effect of solid solution in beryl improves the agreement. In most geologicenvironments, externally imposed chemical potentials govern the sta- bility of beryllium minerals. As a result, minerals in this system are perhaps most useful as indicators of metasomatic variables. Euclase,for example, has a broad P-f shbility range, but is stable only in environments with unusually high alumina activities, thus accounting for its relative rarity.

INrnoouc:ttoN kinds ofsubstitutions: speciesoccupying channel sites and Beryllium, one of the geochemicallyless abundant ele- speciessubstituting for the tetrahedral or octahedral cat- ments, occursas an essentialcomponent in more than 50 ions. The alkaliesoccur in beryl in amounts up to several minerals (Fleischer, 1980). Although rarely composing weight percent, occupying channel sites with chargebal- more than a small portion of any rock, beryllium minerals ancebeing accomplishedby substitution of other cations are fairly common, especiallyin differentiated felsic ig- in the framework sites and accompanied by adjoining neousand related metasomatic rocks. The variety of be- water moleculesin the channels(Hawthorne and eerni, ryllium minerals and their widespread occurrencemean 1977).Lithium, in coupled substitution with the channel that they can be useful petrologic indicators. alkalies,replaced beryllium (Hawthorne and eern!, 1977). Ofthe beryllium minerals,the most common and hence Water, in addition to accompanyingthe alkalies, also oc- the most petrologically and economically important be- cupiesthe channel cavities in a manner similar to that in long to the system BeO-AlrOr-SiOr-HrO (BASH). Over cordierite (Schreyerand Yoder, 1964; Aines and Ross- twenty phasesoccur in this system,and those of geologic man, 1984).Both of these substitutions have a profound interest are listed in Table l. The AlrOr-SiOr-HrO system effect on the stability of beryl. In addition to the alkalies has been intensively studied, and its phase relations are and water, up to several weight percent of magnesium, well understood (seerecent data and reviews by Hemley manganese,iron, and chromium are found in someberyls. et al., 1980; Perkinset al., 19791,thermodynamic com- The iron occupiesboth channel positions and substitutes pilation by Robinson etal.,1982). Thesestudies propose for aluminum in the octahedralsites, whereas chromium, similar phasediagrams for the ASH systemand thus pro- magnesium,and manganesesubstitute for the aluminum vide a reasonablebasis for addition of the component (Goldman et al., 1978;Nassau and Wood, 1968; Wood BeO. and Nassau,1968; Gibbs et al., 1968).Helium, carbon Relatively few thermochemical data are available for dioxide, and argon have also been found in beryl (Beus, beryllium-bearingphases. In this study, phaseequilibrium l 966). and solubility experimentswere performed to provide the Spectroscopicdata on beryl and cordierite suggestthat information neededalong with data from the literature to water occupiestwo distinct positions at low temperatures, derive an internally consistentthermodynamic model for one with the H-H vector normal to c and "sitting" in the phasesin the system.This model, in turn, is used to cal- rings betweenthe large cavities (Type II), the other with culate a petrogeneticgrid for the BASH system and re- the H-H vector parallel to c and located in the large cav- actions in more complex systems. From these calcula- ities (Type I, Wood and Nassau,1968). The abundance tions, deductions can be made about the petrogenesisof of Type II water correlateswell with the alkali content; rocks containing minerals in this system. this correlation leadsto the suggestionthat there is bond- Figure I showsthe phasesconsideredin this study pro- ing betweenthem (Goldman et al., 1978; Hawthorne and jected from water. Of thesephases, only beryl hasa widely Cern!, 1977;Wood and Nassau,1968). Aines and Ross- variable composition. The aluminum silicatescan accept man (1984) have shown that with increasingtemperature small amounts of transition metals, and kaolinite, pyro- thesetwo types of water decreasein quantity with equi- phyllite, and diasporemay contain small amounts ofhalo- librium partitioning favoring a third state that has the gens substituting for hydroxyl, but their compositional spectroscopiccharacteristics ofa gas (not bonded or ori- variations are small. Chrysoberyl can contain substantial ented on an IR time scale). iron, chromium, and manganesesubstituting for alumi- Many crystallographicstructure refinementshave been num, but in most casesthis substitution is small (Vlasov, done on phasesin this system(Ross, 1964; R. M. Hazen 1967,Vol. III). Euclaseand phenakiteare uniformly close et al., unpub.). None suggeststhat there is any disorder to their ideal compositions.Ganguli and Saha(1967) re- among the tetrahedralspecies. The mineralogic properties ported that synthetic bertrandite can contain Be(OH) of the beryllium minerals were recently reviewed by Burt considerablyin excessof the ideal formula, but analyses (1982). of natural bertrandites show no evidence of nonstoi- Thermodynamic data have been measuredfor some of chiometry. the beryllium phasesin this system.Low-temperature heat- Beryl, in contrast to the other phases,exhibits a wide capacity measurementshave been performed on phen- rangeof compositional variation, as a consequenceoftwo akite by Kelley (1939) and on chrysoberyl by Furukawa BARTON: PHASE EQUILIBRIA IN THE BeO-AIO,-SiO,-H,O SYSTEM 279

Table 1. Names, abbreviations,and formulas of some phases of geologicinterest in the BeO-AlrOr-SiOr-HrO system

Abbr. Fomula Abbr. Fomul a

Diaspore Di Aro(olr) Aluminumsilicate Als At2sio5 Andalusite And Al^si0. Euclase Eu BeAlSi 04(0H)

Gibbsite Gb Ar(0H) Behoite Bh Be(0H)z 3 And,Kya,Sil Bertrandite Bt Be4siz07(0H)2 Kaolinite Ka Ar2siZ0s(oH)4 Kyanite Kya AI Beryl Be Be3Al25i 601 8.nH20 25i05 Beryl l'i te Bl Be3si04(0H)2.H20Phenakite Ph 8e,,5i 0,

BromelI ite Br Be0 Pyrophyllite Py Ar2si4oto(oH)2 chrysoberyl ch BeAl Quartz Qz si02 "0, Alro, Corundum co At^0. Si I I imanite Si I At2si05 Proiected {rom water Fig. l. Minerals in the BeO-AlrO3-SiOr-H2Osystem pro- jected Abbreviations from Table l. and Saba( I 966).Hemingway et al. ( I 9 86),in a companion from HrO. study, measuredthe low- and high-temperatureheat ca- pacities of bertrandite, beryl, chrysoberyl, euclase,and other investigators, although Miller and Mercer (1965) phenakite.Ditmars and Douglas(1967) measured the heat did synthesize what they believed to be a beryllian mullite content of chrysoberylfrom 298 to I173 K. Enthalpiesof in their experiments at higher temperatures. formation of chrysoberyl are available from Robie et al. In addition to the experimental data, a number of com- (1978) and from Holm and Kleppa (1966). Holm and pilations of the observed parageneses of beryllium min- Kleppa (1966),Bamberger and Baes(1972), and Schuiling erals exist (Beus, 1966; Vlasov, 1967, Vols. II and III; etal. (1976)gave values for the reaction2BeO + SiOr: Burt, 1978). Burt (1978) derived a detailed topological BerSiOoobtained from oxide-melt calorimetry and phase model for phase equilibria in the system based on natural equilibria. Thermodynamic data for BeO are tabulated in associations.Burt (1 975a, 1975b, I 976, I 980), Franz and Robie et al. (1978).Kiseleva et al. (1984)reported heats Morteani (1984), and Kupriyanova (1982) discussed the of formation of bertrandite, beryl, chrysoberyl, euclase, stability ofthese minerals in more complex systems. Franz and phenakite basedon heat-of-solution experiments. and Morteani (1981) also discussed the topology of the Although there are many studies concernedwith syn- reactions in the system in light of their experimental re- thesis of phasesin this system,especially beryl, few have sults. dealt with elucidating the phase relations. In the system ExpnnrvrnNTAl- METHoDS AND RESULTS BeO-SiOr-HrO, Bukin (1967) synthesizedbertrandite up to temperaturesof 500'C and phenakite at similar and High-pressure,solid-media phase-equilibriumand silica-buff- higher temperatures.Ganguli and Saha(1967) performed ering solubility experiments were done to provide the basis for system. a number of synthesisexperiments in the quaternary sys- derivation of a thermodynamic model for the BASH Dikov (1967) and Syromatnikov et al. tem. Beus and Starting materials (1971) studied the stability of beryl and other beryllium with the phasesin the presenceof alkalies and fluorine-bearing Natural materials were used in all the experiments bromellite (l- to 3-mm crystalsgrown in species,but none of their experimentswere reversed. exception of synthetic lithium molybdate flux, NMNH 115234). Cell parametersfor The high-temperaturestability of beryl and its melting the starting materials are listed in Table 2. A cell refinement was investigatedby Franz and Morteani relations have been not done for BeO becauseof its toxicity. Crystallographicdata (1984),Ganguli (1972),Ganguli and Saha(1967), Miller were collected by powder-ditrraction methods using Ni-filtered and Mercer(1965), Van Valkenbergand Weir (1957),and Cu Ka radiation and were reduced with the program ofBurnham Munson (1967). Unfortunately, these and other studies (1962). Qrafiz and corundum servedas internal standards. on beryl and related phaseshave been synthesisstudies Partial chemical analyseswere performed on an automated and are therefore of limited value in the derivation of ARL-EMX microprobe at the University of Chicago.Spectrom- thermodynamic properties. eter analyseswere done under operatingconditions of I 5 kV and cor- Franz and Morteani (1981),Hsu (1983),and Seckand l5 pA. Data reduction was accomplishedusing a ZAF-type program by I. M. Steele of the University of Okrusch (1972) studied severalreactions by reversal ex- rection written Chicago. Standardsused were fluor-topaz (for A1, Si, and F in periments. Their data, although limited to only a few bertrandite and euclase),hematite (for Fe in all but beryl), albite, place limits on the upper thermal stabilities of reactions, microcline, pollucite, and synthetic Rb-feldspar (for Na' K' Cs' ( beryl, euclase,and bertrandite. Franz and Morteani 198l) and Rb, respectively,in the Minas Geraisberyl). Only the chryso- reported a new phase in the quaternary system that is beryl and beryl contain detectable impurities' The chrysoberyl more aluminous than chrysoberyl plus quartz and more contains 3.1 wto/oFerO, correspondingto about 2.5 mo10/oBe- silicious than beryl plus andalusite. This phase was not FerOo.Analyses of the beryl used are given in Table 3. Water observedin this study and has not been reported by any contents of the Brazilian beryl and the bertrandite were deter- 280 BARTON: PHASE EQUILIBRIA IN THE BeO-AIO,-SiO,-H,O SYSTEM

Table 2. Crystallographicdata for the BeO-AlOr-SiOr-H,O starting materials

Cel I Parametersl source2

l'4ateniaI a (A) b (A) c (A) e(il v (43)

Andalusite 7.7e28114) 7.8980(te) 5.5539(l9) - 341.83(12) Black Hills, South 0akota

Bertrandi te B.7192( l6) t5,2722(23) 4.s624(17) - 607,9r{16) Albany, t4aine ( F.14.N.H. #6969) Beryl 9.2104(t4) 9.r903(r3) - 675.18(22\ Robertson ToMship, Quebec (N,trl.N.H.#123207) Beryl 9.2I 84{09 ) 9.r 678(s8) - 674.70(37) Minas Gerais, Brazi I (provided by R. Gaines) Chrysobe ryl 5.480r(03) 9.4lle(07) 4.4288(03) - 228.43(02) Colatina, Espirito Santo, Brazil (N.14.N.H.#R15231) Euclase 4.7703l.29) r4.3235(s4) 4.6317(39) r00.416(62) 3il.26(39) !1inas Gerais, Brazil (N.14.N.H.#l2l 350) Phenakite 12.4722(10\ 8.2532(19) |il.82(2r) San ltliguel di Pi raci caba, Brazil (N.l'l.N.H.#B2ll52) Quartz unknown(University of Chicago #2099)

I Errors given in parentheses (lo) I tl.ll.ru.l. = National Museumof Natural History, F.M.N.H. = Field Museumof Natural History

mined by loss on ignition (averageof 5 sampleseach). The ber- atilization ofthe mix could not be done at 1400t becausethe trandite results are consistent with normal bertrandite stoichi- starting mix reacted rapidly to give beryl (which should be meta- ometry and limit the substitution of Be(OH), for SiO, (proposed stable relative to chrysoberyl + cristobalite + phenakite). by Ganguli and Saha,1967) to <30/0.The Quebecberyl was used Two pressureassemblies were used. An assemblyconsisting in the silica-buffering experiments and the Brazilian beryl was almost entirely of NaCl was used for the water-bearingexperi- used in the high-pressureexperiments. mentswhich were all conductedbelow 800qC.This salt assembly The minerals were separatedfrom impurities by coarsecrush- has been shown by Holland (1980) to be friction-free within ing and hand-picking.On examination in oils, both beryl samples experimental uncertainty. Experiments on Reaction 1, which were were found to have moderately abundant (0.010/oto 0.10/oby conducted between ll50 and 1450qC,used an assembly con- volume) fluid inclusions. The other phaseshave few inclusions sisting largely of pyrex glass.The pyrex assemblyhas an appre- with the exceptionof the chrysoberylwhich containssome oxide ciable friction requiring pressurecalibration. Consequently,re- inclusions (which may contribute to the observediron content). versals were done on the reaction anhydrous cordierite: sapphirine + quartz. This reaction has been reversedat 1250C between7000 7500 High-pressureexperiments and bars in an internallyheatedgasapparatus by R. C. Newton (pers. comm., 1980). For the calibration re- Piston-out piston-cylinder reversal experiments were per- versals, the cordierite and sapphirine + qluartz,were prepared in formed to determine the temperaturesas a function of pressure the manner of Newton (1972). Care was taken to ensure that the for Reactions I to 4: cordierite-sapphirine-quartzmix was completelydry before seal- ing the capsules.The "true" position of the reaction at other BerAlrSiuO,,: BeAlrOn + BerSiO4 temperaturescan be obtained from the 1250'C bracket and the + 5SiO, (water free) (1) Clausius-Clatpeyronslope (derived from the thermodynamic data : 20BeAlSiOo(OH) 3BerAlSi6O,s + 7BeAl,O4 presentedby Newton et al., 1974). Table 4 gives the results of + 2Be,SiOo+ loHro (2) the calibration experiments. These experiments and others at 4BeAlSiOo(OH)+ 2SiO, : BerAlrSiuO,, higher pressuresusing this samepressure assembly (Perkins and + BeAl,Oo + 2H2O (3) Barton, unpub.) indicated that the pressurecorrection is approx- -100/0. -100/o BerAlrSirO,, + 2AlrSiOs : 3BeAlO. imately A pressurecorrection of was applied to all experimentsusing the pyrex assembly. + 8SiO, (water saturated). (4) Tungsten-rheniumthermocouples were usedfor the high-tem- The startingmaterials (Tables 2 and 3) for eachofthese reactions peratureexperiments. Chromel-alumel thermocouples were used were mixed together in stoichiometric proportion with equal for the lower-temperatureexperiments. The position of the ther- massesof reactant and product assemblages.The mixes were mocouple was checkedon dissectionof the assemblyafter each ground under acetoneto an averageparticle size of 5 pm (deter- run, and any nrn in which the thermocouple was more than I mined by examination in oil). For Reaction 1, the beryl was mm from the capsulewas rejected.No pressurecorrection has dehydratedat 1400"Cfor I h. Although this is above the l-bar been applied to the temperature measurements.Uncertainty in stability limit of beryl (Ganguli, 1972), optical and X-ray ex- the pressuremeasurements is estimated to be about 200 bars. amination showedno evidenceof decomposition.After grinding, The salt-assemblyexperiments maintained constant pressureaf- the mixtures were loaded into 6-mmJong platinum capsules. ter an initial period ofexpansion that accompaniedthe attain- Approximately 2 mg of water were added to eachof the capsules ment of a thermal steady state. The pressurefor the pyrex-as- for the water-containingexperiments. The capsuleswere sealed sembly experiments often drifted considerably with time requiring by arc welding and folded in halfto fit into the pressureassembly. pressureadjustments over long periods. The estimatedprecision Before sealing, the capsulesfor Reaction I were crimped and of the temperaturemeasurements is +5'C. placed in an oven at 800'C for several hours to drive off any For all experiments,reaction direction wasdetermined by com- volatiles acquired during the mixing and loading steps. Devol- paring X-ray scansofthe run products with those of the starting BARTON: PHASE EQUILIBRIA IN THE BeO-AI,O,-SiO,-H,O SYSTEM 281

Table 3. Analysesof the berylsused in the experiments Table 4. Resultsfor beryl breakdown and pyrex-assembly calibration quebecl (N.M.N.H.#r23207) Run No. Iine Temp. Pressure (kbar) Results (h) ("c) noni nal corrected 65.74 64.2 si 02 Be3A125i60lB=BeAlZ04+ Be2Si04+ 55i02 dry, pyrexassembly Be0 t4.i3 n.o. '1285 Be-159 l3-0 5 8-6.2 5.40 weakbe At^0^ 18.03 17.9 Be-164 17.0 1285 6'5-7.? 6.20 strongch+ph+qz Fe^0^ 0.44 detemined as Feo Be-165 6.0 1350 5.8-6.2 5.40 moderatebe Be-l66 5.5 I 350 6.8-7 .2 6.30 strongch+ph+qz Fe0 0.25 0.75 Be-167 20.0 1225 7.3-7.7 6.75 noderatech+ph+qz Li 0.01 n.d, 4.0 l4l0 6.8-7.2 5'40 n0 reaction 20 Be-172 18.0 1200 6.8-7.2 6.30 weakbe Na20 0,t0 0.92 Be-175 Be-lg8 4-5 l4l0 6.4-6.8 5.95 verystrong ch+ph+qz K"0 0.21 Be-201 4.5 l4l0 5.6-5.9 5.l5 weakbe 0.04 Be-202 16.0 1250 6.2-6.6 5.75 weakbe Rb20 <0.05 Be-204 lB.0 1250 6.9-7.2 6.30 no reactron Cs20 0.l6 0.17 8.0 ]450 5.6-5.8 5.'l5 strong ch+ph+qz 't.522 Be-205 H.o l.0l Be-208 18,0 ll50 7.0-1,4 6.50 weakbe Be-234 48.0 ll50 7.1-8.0 7.05 strong ch+ph+qz Be-235 3.0 ]450 5.5-5.7 5.05 strongbe 99.9r BeAl Be25i04+55i02 water-saturated, sal t assemblv Be,AlrSiUOln= ZO4+ 1 Analysis by R.E. Stevens,provided by !1. Fleischer (U.S. Geo1ogical Be-137 24.0 825 ]0.0-10.9 complete be Survey). Be-138 20.0 825 15 - complete be 2 20 complete be Deteminedby loss on ignition. Be-l39 24.Q 825 Be-140 45.0 725 19.5-20'5 very strong be Cordierite = Sapphirine+ Quartzdry, pyrexassembly Be-215 10.0 1250 7.9-8.1 no reaction mixes. The anhydrouscharges were well sintered,homogeneous Be-211 ]0.0 1250 8.1-8.3 weaK co pellets that were quite difrcult to grind, whereas the hydrous Be-2lB 10.5 1250 7.7-7.9 weak sp+qz Re-221 4.0 |380 7.8-8.0 very strong cd chargeswere loose powders ofrather coarselyrecyrstallized phas- Re-227 4.0 I 380 8.I -8.3 strong sp+qz es. A reaction was consideredto have taken place if there was peak more than a 20o/ochange in the relative intensitiesofphases Another possible source of error is that the water content of peaks on one side of the reaction versusthe other. If any of the beryl may not have reachedequilibrium values during the ex- indicated inconsistent was an direction ofreaction, the run con- periments. Jochum et al. (1983) found that hydration and de- indicate sidered to no reaction. The relative intensities of the hydration in cordierite proceedsrapidly at temperaturesabove peaks for Reaction 4 were sensitive to the degreeofgrinding. It 400"C and pressuresabove 1 kbar (times on the order ofa few possible was to changethe apparent direction of the reaction hours or less).It therefore seemslikely that beryl achieved an from beryl + kyanite + quartz moderate to chrysoberyl with equilibrium water content during the experiments. Alkalies in grinding for experimentsnear equilibrium, As a result, only those beryl, however, inhibit dehydration (Ginzburg, 1955) and thus (peak experimentsfor which extent ofreaction was strong ratios possibly slowed hydration in theseexperiments, changedby more than a factor of two) to very strong (peaksof the reactant assemblagenearly gone) were used (determined by Silica-buffering experiments XRD after slight gnnding). For some runs, grain mounts were An extensive series of aqueous silica-buffering experiments made and optically examined, but they were used to determine were performed at I kbar on various assemblagesfrom the BeO- only the presenceor absenceofphases and not the direction of reaction. For the water-bearingexperiments, only those runs in which water was unambiguouslypresent on opening of the cap- = .lPh sule were used. 1Be (dry) r 1Ch + 5Qz Table 4 gives the results for Reaction I and for the cordierite --. Calculated (corr ) calibration reactions.The critical experimentsfor Reaction I are - Calculated (ideal) (t, :lel?.. plotted in Figure 2 along with the position of the equilibrium (! -o calculatedfrom the thermodynamic model presentedlater. Sev- .:( eral experimentsconducted on Reaction I (Table 4) in the pres- o enceof excesswater gave complete reaction to beryl in lessthan (t,o I d at 700 and 800"Cat pressuresup to 20 kbar. The uncertainty (D associatedwith the impurities in the beryl could havebeen avoid- G ed by using synthetic beryl; however, the easewith which com- plete yields of medium-grained beryl can be obtained by this method was not discovereduntil too late in the study. 4 Resultsfor Reactions2-4 aregiven in Table 5 and are plotted looo 1o5o 1100 1150 1200 1250 1300 1350 1400 1450 1500 in Figures3-5. These resultsagree reasonably well with those of Temperature (oC) Hsu (1983) and Franz and Morteani (1981). The slightly higher temperaturesfound in this study may be due to the alkali content Fig.2. The dry breakdown of beryl at high temperature.The of the beryl used in theseexperiments. Franz and Morteani and open rectanglesindicate reaction to chrysoberyl + phenakite + Hsu used synthetic beryl that presumably was alkali free. As quartz, tlre filled rectanglesindicate reaction to beryl, and the discussedin the Thermodynamics section, the alkali content of half-filled symbols indicate no reaction. The ideal line is calcu- beryl should have a significant efect on its stability. The effect lated for pure beryl; the corrected line takes into account the ofalkalies has been taken into account in the calculatedcurves impurities in the experimental beryl. See the text for details. for Figures 2-5. Abbreviations from Table 1. 282 BARTON:PHASE EQUILIBRIA IN THE BeO-AI,O,-SiO,-H,OSYSTEM

Table 5. Resultsfor high-pressure,water-saturated reactions 4Ev+zQz=1Be+1Ch+2W

RunNo. Tjme Temp. Pressu re Results (h) ("c) (bars ) o 48eArSi04(0H)+ 25i02 = Be3Al25i60tB+BeAl204+ 2H20 J Be-l 53 96 500 8000 moderate eu + qz Be-188 48 520 8u00 very strong be + ch Be-lgl 36 555 I 0000 moderate be + ch o 93 24 590 I 2000 very strong be + ch Be-l u) Be-195 24 580 I 2000 weak eu + qz o Be-l 99 48 545 I 0000 moderale eu + qz o Be-200 24 585 l 2000 n0 reac!10n (L Be-211 72 480 6000 weak eu + qz tse-213 96 510 8000 no reactron Be-216 72 490 6000 weak be + ch Be-224 21 640 I 5000 moderate eu + qz Be-228 20 650 15000 very strong be + ch 2o8eAl5i04(0H)= 3Be3Al2Si60tB+ 7BeAl204+ 2BeZSi04+ 2HZ0 300 350 400 450 500 550 600 650 700 750 800

Be-179 60 580 t0000 strong eu Temperature (oC) Be-183 36 590 r 0000 strong be + ch + ph Be-l85 48 550 8000 moderate be + ch + ph Be-ld6 24 610 I 20{.J0 strong eu Fig.4. Experimental results on the breakdown ofeuclase t Be-l 92 12 625 | 2000 moderate be + Ch + ph quartz. The ideal line is calculatedfor pure beryl; the corrected Be-l96 48 540 8000 n0 reactton Be-210 48 535 8000 weak eu line takes into account the impurities in the experimental beryl. Be-214 48 545 8000 no reacti on Be-223 20 665 I 5000 weak eu Seethe text for details. Abbreviations from Table l. Be-225 20 680 I 5000 weak be + ch + ph Be-230 72 500 6000 weak eu Be-23f 72 515 6000 weak be + ch + ph Be3Al25i60t8+ 2Al2si05(Kya)= 3BeAl204+ 85i02 (with excessrater) the charge in order to make opening the capsuleseasier at the

Be-262 24 775 I 5000 moderate be + kya end ofthe experiments.Platinum wasused because gold capsules Be-265 t20 800 I 5000 strong ch + qz tended to anneal themselvesto one another when run together. Be-269 20 850 r 6500 strong be + kya 20 850 I 7000 very strong ch + qz Chargesconsisted of 15-35 mg of silica solution or distilled water loaded using a microsyringe and l-2 mg of the solids that com- posedthe bufferingassemblages. After loading, the capsuleswere AlrO3-SiOr-HrOsystem and from the BeO-SiOr-HrO subsystem. crimped shut, the crimped endsclipped straight,and then sealed In theseexperiments, the solid-phaseassemblage bufers the con- by arc welding. At each stagethe capsuleswere weighed in order centration of aqueoussilica in solution. Given a measuredsilica to determine the massesof the reactantsand to detect losses.At concentrationand the assumptionthat aqueoussilica obeysHen- the end ofa run, any capsulethat differed by more than 0. I 5 mg ry's law over the concentrationrange ofinterest, the equilibrium from its prerun weight was rejected.Most capsulesvaried by less constant of the reaction (and hence the free energy) can be ob- than 0.10 mg. tained directly. This method has been used exrensivelyby J. J. The powders for the buffering assemblageswere prepared by Hemley and coworkers in studies of the MgO-SiOr-HrO and gnnding the reagentsto coarsepowders and then sieving and AlrO3-SiOr-HrO systems(Reed and Hemley, 1966; Hemley et washing with distilled water to concentratethe 30- to 150-pm al., 1977a,1977b, 1980). size fraction. Bromellite, however, was only coarsely crushed In this study the method has been modified somewhat from under acetonein a glove bag due to its toxicity. The beryl used that of Hemley et al. (1977a) to permit much smaller samples. in theseexperiments was not dehydrated. Capsuleswere made from 2-cm lengthsof 3-mm-diameter plat- Distilled waterand Ludox (ammonia-stabilizedcolloidal silica) inum tubing. The tubing was carefully annealedprior to loading were used to prepare starting silica solutions of various concen- trations. The experimentswere "reversed" by starting with silica concentrationson the high and low sidesofthe equilibrium value. Becauseaqueous silica is the most abundant-but not the only- 20Eu=3Be+7Ch+2Ph+tow speciesin such solutions, approaching equilibrium from both directions with respectto silica transferredthe greatestamount ofmass in both directions.ln other words, eventhough analogous o (u buffering reactions involving rhe solid phasesand the aqueous I beryllium and aluminum speciescan be written, the amount of C$ solution and reprecipitation necessaryto buffer their concentra- o tions is considerablysmaller than that neededto buffer the con- o eo o centration of silica. Therefore, by forcing the silica values to 0) > V< n < Thisstudy change substantially from either direction, rnuch more material < Hsu (1983) < Franz and Morteani (1981) is dissolved and precipitated than would be necessaryto butrer i"' < Seck and Okrusch (1972) the other speciesalone. These experiments should then be as - Calculated(ideal) closeto "true" reversalsas many conventionalphase-equilibrium 300 350 400 450 500 550 600 650 700 750 800 experimentsin which there are one or more degreesof compo- Temperature (oC) sitional freedom. After their preparation the capsules were loaded in groups of Fig. 3. Experimentalresults on the breakdownofeuclase. The four into standard cold-sealbombs. Running four capsulesto- ideal line is calculated for pure beryl; the corrected line takes into getherallowed multiple determinations of the sameequilibrium accountthe impurities in the experimentalberyl. Seethe text for while eliminating the effectof variations in temperatureand pres- details. Abbreviations from Table l. sure betweenruns. Internal chromel-alumel thermocoupleswere BARTON:PHASE EQUILIBRIA IN THE BeO-ALO'-SiO,-H,OSYSTEM 283

Table 6. Results for silica-bufering experiments 1Be + = + 2Als 3Ch 8Qz // Run No. T'ime ("C) log rHosioo Run no. Time ('C) lo9 tl4si04 days initial findl days initial final o > This Study 2Be0 + H4Si04 = Be2Si04 + 2H20 BeAl204 + Be2Si04 + 5H45i04 = (tr -- Calculated(corr.) ts Seck and oirusih /' Be-38a 14 600 -2.43 -1.86 Be-Al^Si.0,^+ 10H.0 . -2.30 -2.12 l< Mtstbl w/ And (id ) Be-56a 84 450 > Franz and Modeani Be-56b 84 450 -1.70 -2.10 Be-55a 83 450 -2.00 -1.72 -2.t5 -l.83 -1.47 -1.50 0) - MtstblExt (ideal) Be-60a 14 600 Be-7la 6 600 - Calculated(ideal) Be-60a 14 600 -2.15 -l.83 Be_71b 6 600 _- _l .49 f -2.15 -l.Bl @ -1.77 -1.82 Be-96b 28 380 o Be-60b 14 600 Be-96c 28 380 -1.65 -1.75 c) Be-67a 29 410 ^ -2.26 Be-67b 29 410 -1.77 -2.21 Be-95d 28 380 -1.65 -1.76 Be-70a 6 600 -1.77 -1.79 Be-106a 29 450 -2.15 -1.70 Be-70b 6 500 -- -1.85 Be-106b29 450 -2.15 -1.68 -1.65 -1.72 -2.15 -2,43 Be-106c 29 450 Be-76a 5l 350 Be-106d 29 450 -.l.65 -1.69 Be-76b 61 350 -- -2.43 Be-88c 28 450 -1.71 -2,09 Be-ll2c 14 600 -1.41 -1.48 28 450 ^ -2,06 Be-ll2d 14 600 -1.65 -1.47 4s0 500 550 600 650 700 750 950 Be-88d 800 850 900 Be-l30c 15 540 -2.15 -1.93 Be-123a 25 500 -1.47 -l .60 -1.47 -.].60 -1 -l Be-]23b 25 500 Temperature (oC) Be-l30d I 5 540 ,77 .92 Be-124a 25 500 -1.95 -1.59 + = + 4Be0 2H4Si04 Be45iZ07{oHZ3H20 Be-124b25 500 -1.95 -1.58 Fig. 5. Experimental results on the water-saturatedbreak- -1.55 -1 -2.00 -2.21 Be-127c 7 600 "47 Be-48a 41 350 Be-'127d 7 600 -1.47 -l,48 down ofberyl + aluminum silicate.The aluminum silicatephase Be-48b 41 350 -2.60 -2.3o -1.65 -1.59 -- -2.27 Be-l28c 33 450 (1982). Be-64a 42 350 Be-l3l c 14 540 -l .47 -l .55 relations are calculatedfrom the data in Robinson et al. Be-64b 42 350 -1.77 -2.14 All the other curvesare calculatedfrom the thermodynamic mod- Be-73a 60 350 -2.15 -2.21 Be-131d14 540 -1.65 -1.54 Be-l34c l0 650 -l .65 -l .57 Be-73b 60 350 -- -2.22 -l -1.54 el. The calculated(corrected) curve is for the alkali-containing -2.15 -t.96 Be-134d l0 650 .95 Be-83a 35 400 8e-134e l0 650 -1.65 -1.48 beryl used in this study. The solid curve that intersectsthe kya- Be-83b 36 400 -2.15 -1.92 Be-83c 36 400 -1.17 -l .98 3BeAl"0,,+ 8H,Si0"= nite-sillimanite reactionis for pure beryl and the stablealuminum -1 -l Be-84d 36 400 .77 .93 Be^A1.Si.0,.+ 2Al"Si0.+ 16H"0 silicate polymorph, whereasthe dotted and the dot-dash curves Be-lola 90 310 -2.15 -2,25 Be-l0lb 90 3t0 -2.15 -2,30 -2.00 -1.64 are for the metastable reactions with pure beryl appropriate to - _2.63 Be-54a 84 450 Be_l6lc 90 310 Be-59a 30 450 -1.77 -l .58 Be_lotd 90 310 - _2.70 -1.77 -1.59 the experimentsFranz and Morteani (1981)and Seckand Okrusch -1.77 -1,73 Be-88a 28 450 Be-108a 41 450 Be-l09b 39 450 -1.77 -l .56 (1972), respectively.Abbreviations from Table 1. Be-l08d 41 450 -l .65 -1,73 Be-l09d 39 540 -1,47 -1.55 Be-ll4a 106 310 -2.45 -2.50 -1.35 -1.39 -2.45 -2.49 Be-t3la 14 540 Be-ll4c 106 310 Be-l3lb 14 540 -1.65 -1.40 with midpoint Be-il4d 106 310 -2.45 -2.54 used the tip of the thermocouple at the of the Be-125a 97 3]0 -2.45 -2.53 4BeAlSi04(0H)+ 7H4Si04 t Be2Si04= capsules.The thermocoupleswere calibrated againsta standard Be-I25b 97 310 -2.45 -2.47 zBe-Al^Si-0,^+ 16H"0 thermocouple which was in turn calibrated against the melting 3Be^Si0.+ 2Al^Si0.+ 7H"Si0"= Be-84d 36 377 -2.15 -1.78 points ofNaCl and ice. Temperaturesare believedto be accurate 2Be^Al^Si-0,^+ l4H^0 Be-84b 36 377 -l .77 -l .78 Be-85a 36 4s0 -2.15 -1.75 to within +5'. Bourdon tube-type gaugeswere used to measure Be-80a 7 600 -1.47 -l .64 Be-85b 36 450 -1,77 -1,77 8e-80c 7 500 -- -1.66 Be-98a 85 385 -2.15 -1.75 the pressures.These gauges were intercalibrated with one another. -- -1.64 Be-80d 7 600 Be-98b 85 385 -1.65 -1.76 was performed a new gauge by Be-86a 28 450 -1.77 -l .84 -2.15 -1.76 Calibration also against certified -1.77 -1.81 Be-lo8a 41 450 Be-86b 28 450 Be-108b 41 450 -1.65 -1.72 the factory to !.2%. Pressuresare believedto be accuratewithin _- _1.77 Be_86c ZB 450 2BeAl.,0,+ Be,Si0,+ Be,Si0,+ 3H,Si0,= 470(40 bars) ofthe reported values. Be-86d 28 450 * -1,77 -1 + Be-89a 7 600 -1"47 .67 4BeAlSi04(0H)4H20 Run times varied from a few days at the highest temperatures Be-l09a 39 450 -1.77 -l .78 -1 -l Be-94a 86 380 -2.15 -1.86 Be-l09c 39 450 "47 .78 to over 3 mo at the lowest temperatures.The length of run was Be-94b 86 380 -2.15 -1.84 Be-130a l4 540 -1.77 -l -r.65 -1.77 determinedby experience.In general,the higher the temperature -.l.65 -1.68"69 Be-94c 86 380 Be-130b 14 540 Be-94d 86 380 -r.65 -1.75 and the smaller the mass of silica that neededto be transferred, Be-l2ld 52 400 -1.65 -t.72 Be-126d 52 400 -1.95 -1.79 the less the time that was required to reach equilibrium. Quenchingwas done at constantpressure by cooling the bomb as rapidly as possiblewith an air blast. A few runs were quenched by immersion of the bomb in water. In all casesthe temperature were examined optically for new phasesor for the complete de- wasbelow 300'C within 30 s ofthe start ofthe quenchand below composition of the starting phases.The high-temperaturelimits l00qC within 2 min. The capsuleswere then removed from the for experimentson euclaseand bertrandite were determined in bomb, weighed,washed, and analyzed. this manner, althoughit wasusually clear from the silica analyses Analysis consisted of first opening the capsulesin distilled that somethinganomalous had happened.No additional phases water and carefully washing to recover all the charge. The so- were observedin any of the experiments.Nothing was detected lutions were then diluted to 50 mL with distilled water and ana- that might correspondto the "hybrid" phaseof Franz and Mor- lyzed by using the molybdate blue method (Strickland and Par- teani (1981). None of the assemblagesused contained the bulk sons, 1972). Althoug*r the opening and dilution ofthe runs was composition of the "hybrid" phase;even if it is stableit may not done in glasscontainers, large variations in silica concentrations have been able to nucleate. between duplicate runs were detected only in a few analyseswhere Severalofthe reactionsstudied were metastablerelative to one NaOH solutions were usedin the washingstep or where analysis or more alternative assemblages;however, careful checking es- took place more than I d after opening. Such runs were rejected. tablished that in many casesit was possible to maintain the The estimated uncertainty in the molality of aqueous silica is metastableassemblage without nucleation of the stable phase(s) +0.05 log units. This compares with +0.02 log units for the throughout the course of the experiment. Table 6 presentsthe experimentsof Hemley et al. (1977a, 1977b, 1980)in which the results for these experiments. The results are plotted in Figures mass of solution was about 50 times larger. Better results could 6-8. have beenobtained using rapid-quenchbombs and the microan- Aqueous silica concentrationsmeasured for two butrers (ber- alytical techniquesdescribed by Frantz and Hare (1973). yl + chrysoberyl + andalusite, beryl + chrysoberyl + euclase) After many of the runs, grain mounts of the buffer assemblages were consistentlybelow the values predicted using all the other 284 BARTON:PHASE EQUILIBRIA IN THE BeO-Al,O,-SiOr-H,OSYSTEM

o

I E I o Y This Study (Ph/Br) o This Study (Bi/Br) a Ch + Ph/Eu I Hemleyet al (1977a,1980) Be/Eu + Ph > Hsu (reversal)(1983) I Hemley et al (1977a, Metastable '- Metastable ext s - Calculated - Calculated

370 380 400 410 420 430 Temperature (oC) Temperature (oC) Fig. 6. Silica-bufferingresults in the BeO-SiO,-H2O system Fig. 7. Silica-butreringresults for euclase-bearingassemblages at l-kbar water pressure.The l-kbar bracket of Hsu (1983) is at l-kbar water pressure.Abbreviations from Table 1. shown for comparison. Abbreviations from Table 1. results (Fig. 6). These two assemblages,which have the highest plexesin solution. Surfaceenergies should not have beena prob- silica concentrationsofthose studied,probably precipitatedsome lem becausethe reactantswere quite coarse and the reactions silica during quench,although no evidencewas seenin the grain could be tightly reversedat 350rc and above. If the pH in the mounts. Calculationsbased on the resultsofRimstidt and Barnes experimentswere strongly basic, then significant concentrations (1980)support this argument.The 540"Cexperiments on chryso- of polymeric silica speciescould have been present.BeO, how- beryl + andalusite + beryl were water quenched unlike the ever, is amphoteric and thus the solutions should have beennear others, and perhaps this procedure accounted for their closer neutral. Furthermore, the same effect should have occurred for approach to the calculated equilibrium. Air-quenched experi- the phenakite-bromellite assemblageand should not have changed mentsat 600'C scatteredwidely below the calculatedequilibrium the intersectionofthe two reactions.Interference ofBe-contain- value. The results for these two reactionswere not used in de- ing speciesin the analytical method is another possibility, but riving the thermodynamic properties. should be ruled out for the same reason as pH. There is a large discrepancyin the position of the reaction Thete seemsno obvious experimentalreason to prefer one set bertrandite : phenakite + water betweenthe reversedresults of ofbertrandite resultsover the other. Free energiesofbertrandite this study using silica buffering techniquesand the resultsofHsu are derived using both sets in the following section, and the (1983)using conventional reversal techniques (Fig. 6). Hsu found implications of eachare discussedin the CalculatedMineral Sta- that the reaction takesplace in the mid-300eCrange at pressures bilities section. from 500 to 3500 bars. In contrast, the results here require the temperature ofreaction to be less than 300"C, probably in the TtrnnuonvNAMrc MoDEL mid-200'C range.There is no obvious causefor this difference; other results from the two studies on the euclasebreakdown In order to make maximum use of the experimental reaction agreewell (Figs. 3, 7, 8). Both data setsare compatible data from this study and the experimental and calori- with the entropy and heat-capacitydata on the phases.Several metric data from the literature, and then to extrapolate possibilities exist. First, the results of Hsu may representmeta- theseresults to nature, it is necessaryto construct a model stable recyrstallization ofbertrandite from finely ground to coarser ofthe thermodynamicpropertiesofthe phasesin the BASH material at a rate much faster than the (stable)growth of phen- system. The derivation of such a model is discussedin akite. This is consistentwith the much better cleavagein ber. this section,along with a review of how it compareswith trandite that would tend to make it grind easierthan phenakite, earlier estimatesof the thermodynamic propertiesof min- but would require that the precipitation of bertrandite from so- lution be kinetically favored. Bertrandite spontaneouslydecom- eralsin the BASH system.The model is basedon a least- posesbetween 450 and 500'C; previous attempts to determine squares fit of published phase-equilibrium and calori- the reaction have either yielded bertrandite stability limits in this metric data and the data presentedhere. The method of range from synthesis-typeexperiments (Bukin, 1967; Ganguli Haas and Fisher (1976) was used. Becausewater has a and Saha,1967) or found the growh of bertrandite very difrcult significant effect on the stability of beryl, it is first nec- to achieve(this study;P. Renders,personal communication, 1984). essaryto discuss ways by which the energetic effect of Hsu's bracketsindicate a higher Clapeyron P-7 slope than pre- water can be taken into account. dicted by the entropy and volume data; however, becausehis bracketsare wide, a curve can be constructedthat is compatible Hydration of beryl with the calorimetry and all but one of his experiments.A steep€r Natural beryl contains up to severalweight percent water slope might indicate kinetic control of recrystallization which should be nearly independentof pressure(e.g., Wood and Wal- occurring in the channelsin the structure. Other volatile ther,1983). speciesas well as cations (principally alkalis and alkaline An alternative interpretation is that the silica concentrations earths)are found in the channels(Beus, 1966; Cern!, 1975; for bertrandite are too high. This might result from unusually Goldmanet al., 1978;Hawthorne and -ernf, I 977).Sim- high surface energieson the bertrandite or from unexpected com- ilar channelconstituents are common in cordierite (Gold- BARTON: PHASE EQUILIBRIA IN THE BeO-AI,O,-SiO,-H,O SYSTEM 285 man et al., 1977; Cohen et al., 1977), and their presence has a major effect on cordierite stability raising the high- pressure stability by several kilobars (Kurepin, 1979; Newton, 1972;Newton and Wood, 1979).Similar results for were by Burt (1978) and were, in beryl anticipated o' fact, found in this study. The results of several experi- ' 'l - -'" "'l--- ments on the hydrous and anhydrousbreakdown ofberyl E *f' (Table 4) demonstrate that the stability field of beryl is .. O Be+And/Ch greatly a Belch + Ph expanded by the addition of water. The volume Be/Ph + And (mtstbl) of reaction apparently chang,essign for severalimportant I Hemleyet al (1977a,1980) Metastable ext s reactions (seebelow). A number of experimental studies - Calculated have been made on the substitution of volatiles in cor- 590 620 dierite (e.g.,Mirwald and Schreyer,1977Mirwald et al., 380 410 440 470 500 530 Temperature (oC) 1979;Johannes and Schreyer,l98l), but none havebeen made on the substitution of volatiles or other speciesin Fig. 8. Silica-bufferingresults for high-temperatureassem- beryl. blagesin the BeO-AlrOr-SiOr-HrOsystem at l-kbar waterpres- The significant effect of channel water on the stability sure.See text for discussion.Abbreviations from Table 1. of beryl requires that hydration be taken into account in solute uncertainty in thesevalues, but varying any of the deriving a thermodynamic model for the BASH system parametersby more than 100/ogives a considerablyworse and in discussingthe stability relations of beryl in nature. fit to the experimental data, given the other assumptions Three hydration models were considered:(l) ideal solu- necessary.These values produce reasonableagreement tion betweenend-member anhydrous and hydrous beryls; with the cordierite hydration data of Mirwald and Schrey- (2) zeolitic water modeled using adsorption isotherms, er (1977), although they systematicallyunderestimate the and (3) a nonenergetic,volume-only interaction ("bottle water content at low pressures.Newer data on the water model"). of cordierite given by Mirwald et al. (1979) sug- Newton and Wood (1979)and Kurepin (1979)modeled contents gest that the variation of water with pressureand tem- water-containing cordierite as a solid solution between peraturemay be more complex than that predicted by the anhydrous cordierite and a hydrous cordierite of fixed Newton-Wood hydration model. water content. The mixing propertiesof this solution were Martignole and Sisi (1981) evaluated the activity of assumedto be ideal with the number of sitesavailable for anhydrous cordierite by Gibbs-Duhem integration of the mixing per mole the same as the number of water mol- activity over the pressureranges of interest. Their tech- eculesper mole of the hydrous cordierite end member. nique is similar to those used for evaluating adsorption This model was examined for beryl becauseof the struc- of water in zeolites.Although their calculationsare based tural similarity between beryl and cordierite. Reaction 5 the cordierite hydration data of Schreyerand Yoder is an analogoushydration reaction written for beryl, on (1964),which are probably lessaccurate than more recent BerAlrSiuO,.* nHrO: BerAlrSiuO,r-nHrO, (5) data, the calculationsbased on the adsorption model are in somewhatbetter agreementwith phaseequilibria than where z is the number of moles of water per mole of are calculationsbased on the Newton and Wood model. hydrousberyl. The equilibrium condition for this reaction Unfortunately, this technique cannot now be applied to is given by Equation 6 beryl becausethe required hydration measurementsare Pv not available. LIib - ZAS. + LV dP + nRT ln :;"H;B : 0, An alternative, simple model is to considerthe cavities )(eslnro (0) in the beryl (or cordierite) structure as "bottles" that can contain water, but with no energeticeffect: the difference where X^" and X"" are the mole fractions of anhydrous in Gibbs free energy between an anhydrous beryl and a and hydrous beryl and the values of n, A,Ho,ASF, and AZ beryl saturatedwith water would be -V",,(P - l), where need to be determined for beryl. For beryl, as for cor- 2""" is the efective volume of the "bottle." This model dierite, the volume of the anhydrous and hydrous phases predicts that the isohydrons (lines of constant water con- arethe samewithin uncertainty and thus A Z : 0. Because tent) parallel the isochores of water, which is generally no hydration data are available for beryl, the water model consistent with the cordierite experiments. In addition, is constrainedonly by the efect that varying its parameters the simplicity of this model greatly facilitates the appli- has on the internal agreementof the phaseequilibria and cation ofpHAszoor similar data-reductiontechniques be- solubility data. At an earlier stageof this study (Barton, causethis model is linearly related to the Gibbs free en- l98l), an iterative frt of this model with other (then avail- ergy, unlike the first two discussed.One consequenceof able)thermodynamic data gave n: 1.2,AH: -53 840 the bottle model is that the nominally water-freereactions J/mol, and A,S: -136.6 J/(K.mol). These values fall such as Reactions I and 7 within the stated uncertainty of the Newton and Wood values for cordierite. It would be difrcult to give an ab- BerAlrSiuO,,+ 2AlrSiOs:3BeAlrOo + 8SiO, (7) 286 BARTON: PHASE EQUILIBRIA IN THE BeO-ALO,-SiO,-H,O SYSTEM will be nearly free of curvature becausethe only change taking place in a system with an osmotic membrane is the addition of a constant volume (although there will permeableonly to the fluid. The total volume of reaction be minor curvature resulting from differencesin expan- will include Vu,but in calculatingthe Gibbs free energy sivity and compressibility). In contrast, the Newton and changefrom the standard state, the AZ,"uo"will be inte- Wood (1979)and Martignoleand Sisi(1981) models pre- gratedfrom I to P,oo,,whereas Z*" will only be integrated dict substantial curvature at low pressures. from I to Pou,o.Then, to the extent that 2"""is independent The cordierite hydration data of Mirwald et al. (1979) of pressure,the contribution to AG from the fluid will be are somewhat more consistent at low pressureswith the Vu,Po.^. ideal-solution model than with the bottle model. A con- The efect of water on the stability of beryl is consid- sistencytest is not appropriate for the adsorption method erably more pronounced than the effect of water on the becausethe adsorption isotherms are calculated directly stability ofcordierite. This is becausein cordierite-break- from the hydration data. The bottle model agreesreason- down reactions. an increase in aluminum coordination ably well with the hydrarion data of Mirwald et al. (1979) generallyleads to a large volume of reaction favoring the for cordierite at pressuresabove I kbar, excluding their stability of the high-pressurephases, whereas, in beryl- four highest-pressureexperiments below 600"C. The cal- breakdown reactions.there is no aluminum-coordination culated V*" for cordieriteis 16.2 + l.l cm3based on 30 changeand consequentlya smaller AZ.oro..This is illus- measurements(excluding the highest-and lowest-pressure trated by two reactions:the breakdown ofberyl to chryso- data). The value for all 42 measurementsgiven by Mir- beryl, phenakite,and quartz (Reaction I , discussedabove) wald et al. is 19.2 + 6.9 cm3.These values compare fa- and Reaction 7. Figure 9 showscalculated curyes for Re- vorably with 14.l cm3estimated here for beryl. The bottle action 7 for P"ro : Proor,0.5P.rr,and 0. Decreasingwater model, therefore, appearsto be reasonablefor both cor- pressurereduces the stability ofberyl * aluminum silicate dierite and beryl, at least to a first approximation if ac- by several hundred degreesat moderate pressures.This curate water contents are not neededat low pressures. decreasedstability results from an approximately 90' The simplicity of the bottle model is its greatadvantage; changein the Clapeyron slopeat any pressure.Differences however, there are some observationsthat are inconsis- in the predicted low-pressurephase equilibria might pro- tent with the simplistic interpretation of no energeticin- vide a way to test the differencebetween the alternative teraction. Spectroscopicinvestigations of water in cor- hydration models. dierite and beryl clearly show that there is an energetic pn-Lszo interaction between the water and the structure of the Application of mineral (e.g.,Aines and Rossman, 1984). Available evi- The program nHAS2o(Haas and Fisher, 1976) utilizes a dence,however, suggests that the energyofthis interaction weightedleast-squares method to enablesimultaneous es- is low (Langer and Schreyer, 1976; Aines and Rossman, timation and correlation of the thermodynamic properties 1984), and thus it should contribute little to the Gibbs ofrelated phases.Data that can be evaluatedby the pro- free energiesofthe hydrated phasesat moderate to high gram included heat capacities,heat contents, entropies, temperaturesand pressures.This contribution could be- volumes, compressibilities, expansivities, heats of reac- come important at low temperaturesand pressures.The tion, free energiesofreaction (e.g.,from phaseequilibria), somewhat poorer agreementof the calculated phase re- and EMF voltages.Other types of data can be incorpo- lations with natural assemblagescould be a consequence rated by meansof user-written subroutinesif they can be of this interaction (see following sections) as could the related to the underlying functions in a linear way. The differencesbetween the model and the cordierite hydra- BASH system is well suited for application of pnaszo. tion data. The molecular sum of alkalis and water com- Eighty-six data setscontaining more than 1000 indepen- monly exceedsl.0 per formula unit in alkali-rich natural dent observations were used. The data sets include all beryls (e.g., Beus, 1966) further demonstrating that the data types except EMF. The system is greatly overdeter- simplistic volume-for-volume replacement relationship mined for the sevenspecies ofinterest and therefore suit- hypothesized here cannot be correct in detail. Lacking able for a least-squaresfitting procedure. experimental constraints on the hydration of pure and Linear progmmming methods representan alternative alkali-bearing beryl, however, the development of more way to extract thermodynamic propertiesfrom a large set elaboratehydration models would be purely speculative. of experimentaldata (e.g.,Halbach and Chattet'ee, 1984). Beryl hydration is accommodated in thermodynamic These methods have the advantage of treating reversal calculationsby adding an additional water volume to the experiments as the inequalities that they are (in AG), volume of reaction. This formulation should hold for oth- whereasregression methods are better suited to data that er volatile (noninteracting or weakly interacting) com- have a regular distribution about the true value (e.9.,ca- ponentsin the beryl structure. It is clear that for volatile- lorimetric results, solubilities). In this study, only regres- saturatedsystems, the whole volume should be addedand sion techniques were used, although the ideal solution that for volatile-free systems,no cavity volume should be might combine a weighted linear programming method added. The caseof 0 < Puro ( P,o*rrequires discussion. for phase-equilibriumreversal data with a regressiontech- The effective volume for fluid-undersaturated cases is nique for other data. Reversal brackets were treated in- V-"Prr,id/P,oot.This can be seenby imagining a reaction dependentlywith each half-bracket included in the data BARTON:PHASE EQUILIBRIA IN THE BeO-ALO,-SiO,-H,OSYSTEM 287 set weighted by its P-I estimated accuracy.This results Be+2Als=3Ch+8Qz in a relatively even weight being applied throughout the Variable Water Pressure interval between the brackets (Demarest and Haselton, l98l) and thus help should minimize unjustified bias to- o ward the midpoints of the brackets in the final fit. The data of Robinson et al. (1982) for phasesin the j ASH system were acceptedas fixed values for the eval- o Hro-'total f uation of the thermodynamic properties of the beryllium o o phases.The datafor BeOgiven in Robie et al. (1978)were c) also used without modification. Free energiesof water at elevatedtemperatures and pressureswere calculatedfrom a program basedon Haar et al. (1979, l98l). All other parameterswere allowed to vary including the Gibbs free 25o 310 370 oto energyofaqueous silica at I kbar. r"*o"r"rrre (tc) The available heat-capacity,heat-content, and calori- Fig.9. Thelow-temperature stability limits of chrysoberyl+ metric-entropy data were fit first without employing any qwftz at water pressureequal to differentfractions of the total of the other data. The volumetric data also were fit in- pressure.See the text for details.Abbreviations from Table l. dependently.The results for the heat capacitiesand vol- umes were then fixed in the preliminary fitting of the phase-equilibrium and heat-of-reaction data. This pre- for bromellite. All the volumetric data were consistent vented the program from diverging. Following the prelim- with the final model within 0.2o/o,nearly all within 0.10/0. inary fit, all the regressedterms were allowed to vary to Heat-capacityand third-law entropy data. Hemingway obtain the final fit. A few data were rejectedin the process et al. (1986) determined the heat capacitiesand thirdlaw as it becameapparent that they were inconsistentwith the entropies of bertrandite, beryl, chrysoberyl (only high- bulk of the information. It is possible to justify these temperature heat capacity), euclase, and phenakite by exclusionsin all but one case.The data were all weighted combined adiabaticand differential scanningcalorimetry; in the regressionby the author-reported uncertainties. they discussedthe compositional corrections applied to the data for beryl and chrysoberyl. Furukawa and Saba Results and discussion (1966),Kelley (1939),and Kostryukov et al. (1977)mea- In this section, the derivation of the thermodynamic sured the low-temperature heat capacitiesand calculated propertiesand their agreementwith other thermodlnamic the thirdJaw entropiesof, respectively,chrysoberyl, phen- data arediscussed. The thermodynamic model reproduces akite, and behoite. High-temperature heat capacitiesfor most of the experimental results within stateduncertain- behoite were taken from the compilation of Stull and ties; calculatedphase relations basedon the model are in Prophet (1974). Ditmars and Douglas (1967) determined good agreement with observations on natural assem- the heat content of chrysoberyl from 273 to ll73 K. Be- blages.Comparison of the predictions of the model with causeall the heat-capacity measurementswere done at natural assemblages,however, is deferredto the next sec- 800 K or less,it proved necessaryto estimate values be- tron. tween 800 and 1500 K so that the fitted polynomials The thermodynamic propertiesfrom the final pHnszofit varied smoothly in that range. This was done using an are summarized in Table 7. The thermophysical prop- extrapolator method developed by G. R. Robinson, Jr. ertiesare summarizedin Table 8. nEcroN, a Fortran pro- (U.S. Geological Survey), and by oxide sum methods gram written by John Haas (U.S. GeologicalSurvey), was (Helgesonet al., 1978).All the heat-capacityand entropy used in conjunction with pxaszoto provide estimatesof data fell within two (author-reported)standard deviations the errors in the thermodynamic properties. Hemingway of the measurementsin the final fit. et al. (1986) have provided more complete tables for the Silica-buffering data. For the silica-bufferingreactions, thermochemical properties of the beryllium minerals. standard Gibbs free energiesof reaction were calculated Volumetric data. Thermal expansivitiesand compress- from the relation ibilities were incorporated in the model. Data for phases in theASH systemwere taken from Robinsonet al. (1982). LGr: -RZ ln fttk$iocfllzo Hazen et al. (1983, personalcommunication) determined volumes of bertrandite,beryl, bromellite, chrysoberyl,eu- * + bv*)dP, (8) clase,and phenakiteas functions ofpressureat room tem- J'{at*,,* perature. The compressibility of behoite was estimated from the data for bromellite. Expansivity data for beryl where nt"o*oo and n are the molality and stoichiometric were obtained from Schlenkeret al. (1977), for bromellite coefficient of aqueoussilica, ,Cro is the fugacity of water from Skinner (1966), and for chrysoberyl from Woolfrey at the temperature and pressureof the reaction, s is the (1973) and Cline (1979). Thermal expansivitiesfor be- stoichiometric coefficientof water. b is the stoichiometric hoite, euclase,and phenakitewere estimated from the data coefficientof beryl, and AZ*'* and V*" are, respectively, 288 BARTON: PHASE EQUILIBRIA IN THE BeO-AI'O'-SiO,-H,O SYSTEM

Table 7. Thermochemical properties of some phasesin the BeO-AlrOr-SiOr-HrO system

PhaseName Gzggi,tru" 5z9sx CP** VZsg Source c G S cpv J/mol J/K-mo1 J/K-mol J/bar

Andalusi te -2444564 91.49 510.336 -0 19180 i 66755 6.45249 -6117.205.156 1111 (456) (o 35) (0 016) Behoite -a27288 45.57 249.017 -0.02676 2 02574 -34a2.7 4 2 220 4446 (6734) (0.r8) (o 011) Bertrandite -4300625 172.11 825.336 -0.09966 3 66217 - 10570.3 9 150 3738 -4306375 9 (162e) \0.77) (0 001) Eeryl -8500360 346.73 I 625. 84 -0.42521 6.82544 0.12032 -20180.9 3738 (anhydrous) (3796) (4.73) (0.004) Bromellite -580078 13.77 69.936 0 00018 -0.67671 -635.740 0.8309 2222 chrysoberyl -2176161 66.25 362701 -0.08353 -0.06798 2.24819 -403369 3 444 3238 ( 1481) (0.30 ) (0 001) Corundum -1582326 50.94 238.320 -0.04127 0.04728 0.94196 -2561.61 2.560 1111 0iaspore -921432 35.34 155.894 -0.00340 a.29021 -IBtz 22 1 776 11r1 (403) (0.10) (0 017) Euclase -2370166 89 11 532.920 -0.15373 2-r9760 4.12232 -672030 4-702 3738 (1084) (0 40) (o 001) l{ater (ideal -228611 188.73 10.4831 0.02596 -1.31077 -4 46885 299.188 2479.0 1111 9as) H/Si0, (1 M, -1292370 345.09 4465.24 -2.1.5187 -72922.1 333- - - 1 Kbar) Kaolinite -3799611 205.15 749115 -0.13s42 1.49195 -8274.64 9 949 5111 (982) (0.47) (0.034 ) Kyanite -2445577 82.41 63451s -0.20041 1 91390 6 69583 -656I.91 4.421 (487) (0.30) (0.023) Phenakite -2028387 63.43 428.492 -0.09958 2.08263 1.98865 -567047 3.717 3738 (0.27 ) (0.001) Pyrophyllite -5268625 239.33 1094.40 -0 32990 4 01105 11 4631 - 13075.012 764 11I1 (807) 11.022) (0.015) -77 o Quartz -856306 41.s1 83.2575 0.02196 8.r10 2.259 1111 B Quartz -855777 37.68 58 9128 0.01039 2 295 1113 sil linanite -2442060 9s. sB 592.346 -4.?6236 2 7357A 9.43392 -742971 5.002 I111 (s0o) (0.33) (0 002) -3 Vdummy 1 412 (o 243)

*Errors given here are 20 for free energies of reaction from the oxjdes for all phasesexcept the oxjdes,

**cp = a + bT + c x 106T-2 + d x 10-5T2 + s1-0-5 lRobinron 2Robie 3This 4stull sHaas et al. (1982) et al. (1978) study and Prophet(1s7+ ) et a1 (1980) 6seitz THemingway 8Hazen 9-1"'s (-983) et al (1950) et al. (1985) et al. (in prep ) study,6rr"66n tl5u the volume changeof the solids in the reaction and the reactions to Gibbs free energiesof quartz-bearing reac- volume of the cavity. H4SiO4is used as the aqueoussilica tions. speciesbecause it is the one most commonly accepted A correction for solid solution in the experimentalberyl (e.g., Hemley et al., 1977a). The actual identity of the was applied to the free energiesofthe silica-bufferingre- speciesis not neededin the modeling aslong asthe activity actions. This correction has a trivial effect on the fit and coemcientofaqueous silica is constant over the concen- the calculated curves. Several sets of silica-buffering re- tration range of interest. This assumption is reasonable sults were not usedin the final fit. These include the data for neutral speciesin aqueoussolutions of low to moderate for the beryl + chrysoberyl + andalusite and beryl + concentrations.In this study the thermodynamic prop- chrysoberyl + euclaseexperiments discussedunder Ex- erties of aqueous silica were constrained by the experi- periments. Also eliminated were some experiments at mentally determined quartz solubilities of Hemley et al. 310"C on the bertrandite + bromellite buffer. Only those (1977a,1980). Their valuesare quite similar to the sta- experiments that started near the final silica value were tistical fit by Walther and Helgeson(1977) to much of the used. The other experiments,which started much farther earlier silica solubility work. Only the data of Hemley and away from equilibrium, did not converge sufrciently to others were used in the least-squaresfitting in order to provide good estimates of the equilibrium values. The maintain consistencywith the data of Robinson et al. final model agreeswell with all the silica-bufering results (1982) on the ASH system.Aqueous silica can then be exceptfor the two silica-rich reactions mentioned above. eliminated from all the reactionsby adding the Gibbs free Phase-equilibrium data. Free energiesof reaction for energyof Reaction 9 the high-pressureexperiments were extracted using the relation SiO, (quartz) + 2H2O: HoSiOo (9) thereby converting the free energiesofthe aqueoussilica AG?,,: -RZln fi,,o + T,'(A4d0" + bV*,) dP. (10) BARTON:PHASE EQUILIBRIA IN THE BeO-AI,O,-SiO,-H,OSYSTEM 289

The symbols are the same as for Equation 8. The Gibbs Table 8. Thermophysical properties of some phasesin the free energiesfor the beryl-bearing reactions done in this BeO-A1,Or-SiO2-H2Osystem study were correctedfor slight decreasesin the activity of Coefficients for VolumeEquation Phdse Name beryl due to solid solution in the starting materials and abcd for the portion of the cavity volume occupied by alkalis Andalusi te 50.0729 0.001737190.536253 -0.00002170 0.7700380 and thus unavailable to water. The activity correction Behoite 21.8476 0.00073054 0.3725r0 -0.00000890 Bertrandite 0.00301037 1.53490r -0.00008510 amountedto 1.8 (: 4ln 90.0311 J/(K.mol) c*.,) for the Brazilian Beryl 202.7476 0.00138506 1.529589 -0.00013287 -0.1797429 beryl and 0.5 J/(K. mol) for the Quebecberyl, Ideal mixing BromelI ite 8,2167 0.00027296 0.,]39211 -0.00000421-0.0541870 was assumedin the activity calculations,with the cations Chrysoberyl 33.6087 0.0a024292 -0. I 65870 0.8221744 (Table 2) mixing on sitesconsistent with the crystal-struc- corundum 25.2017 0.000655s2 -0.00000603 0.2011490 Diaspore 17.4058 0.00118889 -0.00003672 ture data (Hawthorne and Cemy,1977). The combination Euclase 46,2613 0.00r54872 0.795689 -0.00003360 ofthese two factors accountsfor the crossingat about 4 Kaolinite 98.4196 0.00356979 -0.00040442 kbar ofthe calculated "ideal" and "corrected" curves in Kyanite 42.3174 0.00124871 0.229753 -0.00000804 1.4324300 Figures 3 and 4. This crossingimplies that the reversed Phenakite 36.6712 0.00t22702 0.631233 -0.00002525-0.09784r0 Pyrophyllite 126.8190 0.001i3097 0.815952 -0.00000725 beryl-bearingreactions determined in this study are slight- 0 Quartz 22.1819 0.00135725 -0.00004270 ly metastablerelative to the reactionsin the pwe system. g Quartz 23.5794 -0.00004731 Decreasingthe entropies and the volumes of these reac- Si I 1iinani te 49, I 338 0.0009357r 0.423863 -0.000026r4 0.4486300 -0.00000936-0.0138458 tions (i.e., treating the beryl as if it were pure) degrades Vdummy 14.2420 0.00010422 0. I 20800 the fit ofthe data and predicts unrealistically low values for V*".It is the prediction of an unrealistically low cavity volume that is the main reason for including the com- positional correction. This correction, however, is prob- ably well within the total uncertainties, and thus it is werenot heavily weightedin the refinement.These results retainedeven though it leadsto what must be an imperfect and Hsu's (1983) bertrandite experiments were the only interpretation of the true phaserelations. data that could not be made consistentwith the model or The value of V. was allowed to vary with only loose justifiably eliminated from consideration. " constraintsapplied (the value was estimated for the pur- Severalfactors probably contribute to discrepanciesbe- posesof the refinement as 20 + l0 cm3).This value was tween the thermodynamic model and the phase-equilib- chosen becauseit is consistent with the range of water rium data. The most important is almost certainly the contents observed in natural beryls and cordierites and inadequacyof the simple beryl hydration model to fully the amount found to enter cordierite in experiments.The account for the energeticcontribution ofwater and other thermal expansionand compressibility of y.," were con- volatiles. This is illustrated by the relatively poor fit of strained to equal those of beryl. The final estimate (14.1 the model to the reversalsfor Reaction 4 (Fig. 3). Reaction cm3)is consistentwith maximum water contentsof about 4 is nominally independentof water and has only a small one mole water per mole of beryl, somewhat less than A Z.or;a"and AS; therefore the calculated position of the that found for cordierite and that predicted for beryl using equilibrium is sensitive to Y*, The calculated slope for the Newton and Wood model (Barton, 1981). Reaction 4 could be brought into agreementwith the re- The resultsagree reasonably well with the data of Franz sults of this study by changingAV by +0.4 J/bar. Errors and Morteani (1981)and Hsu (1983)on euclase-bearing of this magnitude could result from either uncertainty in reactionsthat were used in the fit. Becausethe "hybrid" Vu or in the volumes of the phases(the total volume for phase was not observed in this study, direct comment one side of the reaction is about 29 J/bar). An additional cannot be made on its stability. The phaseappears to be energycontribution from the interaction of water with the metastablefor several reasons.The phase has not been beryl framework is another possibility, as suggestedin reported from nature although appropriate bulk compo- cordierite by the higher-than-predicted (bottle model) water sitions exist. Also, the field of occurrencefor the hybrid contentsat low pressuresand the ordering found by spec- phase shown by Franz and Morteani cannot be a stable troscopic studies. field even considering only their other experimental re- The phase-equilibriumexperiments of Hsu (1983) on sults. the reaction bertrandite : phenakite + water and the sil- Franz and Morteani (198l) and Seckand Okrusch(1972) ica-bufering results reported here are inconsistent. Pos- did experiments on Reaction 4. Their results, taken to- sible reasonsfor this were discussedunder Experiments. gether, indicate that this reaction under water-saturated The valuesin Table 7 representindividual fits to two sets conditions has a low negative slope (Fig. 3). A negative of data using the calorimetric entropy and heat capacities. slope in the kyanite field is incompatible with the results The Gibbs free energy of bertrandite given in Table 7 is of this study (Fig. 3) and with the calculated LV. The 1000 kJ more negative than the least-squaresfit. This accuracyof the other determinations is not clear-Seck result is still well within the experimentaluncertainty and and Okrusch may not have reversedtheir reaction; in this gives more reasonablebreakdown temperatures (about study the direction of reaction was found to be difficult l5.C higher). to determine by X-ray ditrraction methods-hence, they On the basis of sl,nthesisexperiments, Ganguli (1972) 290 BARTON: PHASE EQUILIBRIA IN THE BeO-AI'O,-SiO,-H,O SYSTEM suggestedthat at I bar, beryl becomesunstable relative bertrandite, chrysoberyl, euclase,and phenakite, where to chrysoberyl + phenakite + cristobalite between 1300 the numbers in parenthesesare one standard deviation and 1400'C.The equilibrium temperaturepredicted from reportedby Kiselevaet al. (1984).Ifthe uncertaintiesin the data in Table 7 and Robinson et al. (1982) is about Table 7 are included, then the two sets fall within about 970'C. The largedifference may be becauseGanguli's syn- two standard deviations of each other. The differences thesis experiments did not approach equilibrium. Gan- are,however, still somewhatlarger than expected,and full guli's results were not used in the data reduction. interpretation must await publication of the experimental Heat-of-solution and other data. Free energiesofreac- results by Kiseleva and others. tions involving phenakite are presentedby Schuiling et The free energyofbehoite was calculatedfrom the av- al. (1976) and by Bambergerand Baes (1972). Schuiling erageof the three heats of formation given by Stull and et al. (1976)did a seriesofhalf-reversals at 1000 K in- Prophet(1974) and the entropiesfrom Robie et al. (1978) volving phenakite, bromellite, and several other oxides and Kostryukov et al. (1977). and orthosilicates.They estimated that the free energyof The free eneryiesand entropiesestimated in this study reaction for bromellite + quartz: phenakite was be- are quite similar to those estimated in an earlier version tween - 14.3I and - 6.48 kJ. This range agreeswell with (Barton, 1981). Only the entropy of euclasedifers by an the value of -7.97 kJ (1000 K) calculatedfrom the ther- unexpectedlylarge amount, being l0 J/K'mol lower in modynamic model. Bambergerand Baesused silica glass this study. The difference in the estimated entropy for as the referencematerial for silica, and they were not able euclaseleads to significant differencesin the positions of to achieveequilibrium in their beryllia-rich experiments. some euclase-bearingequilibria. As a result their results are diftcult to compare directly, but their calculated Gibbs free energy of formation is Cll,cur,.q,rnD MTNERALSTABILITIES - 1967.4 kJ, 6 I kJ larger than the value derived here.The The fitted thermodynamic properties can be used to Bambergerand Baesvalue was not usedin the evaluation, calculate beryllium-mineral stabilities in the BASH sys- whereasthe data ofSchuiling and others were used. tem and, in conjunction with data from other sources, Holm and Kleppa (1966) and Kiseleva and Shuriga equilibria in more complex systems.In this section, sev- (1981)measured the heatsof solutionofbromellite, phen- eral calculatedphase diagrams are presented,discussed in akite, and qtartz and, derived enthalpies of reaction for terms of their agreementwith natural assemblages,and bromellite + quartz: phenakite of -19.7(2.5) and compared with phase relations proposed for the BASH -25.7(4.4)kJ at 968 K, respectively.The Afln., calculated systemby Burt (1978),Franz and Morteani (1981),and in this study is - I 3.8 kJ, slightly greaterthan two standard Hsu (1983). Figures 10, ll, and 12 show, respectively, deviations larger.Although there is no obvious reasonfor pressure-temperature,log 4"n$oo-temperature,and log this discrepancy,the sourceprobably lies in the solution 4Ar2oj-temperatureprojections of the phaserelations. calorimetry (O. J. Kleppa, pers. comm.). The more neg- The overall pressure-temperaturestabilities for the stoi- ative valuesof Holm and Kleppa ( I 966) and Kiseleva and chiometric beryllium minerals are well defined (Fig. l0). Shuriga(1981) are not consistentwith the occurrenceof With decreasingtemperature, beryl * aluminum silicate bromellite with talc in metasomatizedultramafic rocks replaceschrysoberyl + qvartz under water-saturatedcon- (Klementyeva,I 969)-the calorimetric valuespredict that ditions. In the 300-400oC range, beryl + aluminum sil- phenakite * periclaseor brucite would be stableinstead. icate assemblagesare replaced by euclase-containingas- The phenakite heat-of-solution measurementswere not semblages.About 100'lower, pure beryl + water reacts included in the final data reduction. to euclase-or bertrandite-bearingassemblages and phen- Calorimetric enthalpies for the reaction bromellite + akite + water goesto bertrandite. In general,the uncer- corundum : chrysoberyl(Holm and Kleppa, I 966; Robie tainty in the reactions increaseswith decreasingtemper- et al., I 978) usedin the data reduction agreewith the final ature mainly becausethe hydration model is likely to be value within two standarderrors. This result is important less adequate.A related problem is the increaseduncer- in demonstrating the overall consistencyof the thermo- tainty associatedwith extrapolationof the Gibbs free ener- dynamic model in that the Gibbs free energies of the giesto temperatureslower than those of the experiments. beryllium phasesare tied to that of corundum through Overall. +30"C seemsa conservativeestimate of the un- both andalusite and chrysoberyl within experimental error. certainty in the reactionsthat are not directly constrained Kiseleva et al. (1984) determined heats of formation by experiments.Because the uncertaintiesare highly cor- for bertrandite, beryl, chrysoberyl,euclase, and phenakite related,the overall topology of the phasediagrams should by high-temperature oxide-melt solution calorimetry. not vary greatly. Their enthalpieswere not used in the pnnszofit because Although the overall sequencesand temperatures are only derived parametersare given in their abstract.I. A. well defined, some details of the topology remain uncer- Kiseleva (pers. comm., 1984) indicated that their oxide tain. The uncertaintiesmainly involve reactionsthat mark dala are consistentwith those of Robie et al. ( I 978) per- the high-temperaturelimit of euclase+ quartz. There are mitting a comparison of their results to this study. many possible confrgurations becausethe pertinent re- - -6.5(3.8), rri'iBf""u' f/t'i5"*"t"'": 16.7(13.4),31.4(12.1), actions involve the aluminum silicatesand fall within the ll.4(4.6), and ll.3(4.2) kJ/mol for, respectively,beryl, samerange where aluminum silicate phasetransitions oc- BARTON:PHASE EQUILIBRIA IN THE BeO-AI'O,-SiO,-H,OSYSTEM 291

P(water)=P(total)

-1

o (!

! 9-1 o I U) U) O)r o (L -2

2oo 260 320 680 74o 800 "'tJ;;"':t.."uu,o't,u'o

Fig. 10. Calculatedwater-satuated phase relations in theBeO- Al2O3-SiO2-HrOsystem. Abbreviations from Table l. Activity Beryl = O.1 (B) cur. For example,the invariant point at about 3500 bars o ,^ and 400"C among the phasesberyl, euclase,kyanite, py- - rophyllite, qtarlz, and water could be several kilobars o) higher in pressureor so much lower in pressureas to be O -19 metastable. The log activity-temperature relations (Figs. ll, 12) presentuseful alternative projections ofthe BASH phase relations at I kbar. These are calculated from the ther- : - -25 modynamic model using the relation AG'r,, RT In a,, 2OO 24O where a, is the activity of the speciesof interest. The uncertainties in these diagrams are probably <0. 15 log units. Figure 12 is a nonconventionalprojection that deserves -1 3 some comment. The activity of alumina is rarely consid- ered an independent variable in petrology, perhaps be- cause of the relative immobility of aluminum in most o-1 6 I geologic environments. The activities of other compo- F nents -1 such as silica, the alkalis, alkaline earths,or hydro- o g gen ion are used instead. Most complex equilibria in- volving aluminosilicate phases(feldspars, micas, chlorites, clays, etc.) can, however, be written with the activity of alumina as an independent variable. The advantage of doing this is that the stability limits of one set of minerals 240 2aO 320 360 400 440 480 520 560 600 (in this case,beryllium minerals in the BASH system)can Temperature (oC) be shown with a wide variety of completely different re- Fig. 11. Log nt*.,oo-temperature projection of phase rela- actions. For example, even though only the K-feldspar- tions at I kbar. (A) Calculatedphase relations for the pure system. muscovite buffer is shown on Figure I l, many other al- (B) Calculatedphase relations for ao*,:0.1. Note the large ex- umina bufferscould be shown including serpentine-clino- pansionofthe beryl fields at the expenseofthe others.The curves chlore-talc, albite-paragonite, topaz-quartz, and anor- are the sameas in (A) exceptwhere labeled. (q Path consistent thite-zoisite-quartz, eachof which is of interest in certain vdth the sequenceofassemblages seen in the Lost River, Alaska, beryllium deposits. beryllium deposits.The curves are the same as in (A). Seetext Solid solution in beryl should have a substantial effect for discussion.Abbreviations from Table l. on the phase relations. Figures llB and l2B show the calculatedphase relations for the activity of beryl : 0.1. Activities of this magnitude are reasonablefor natural alkali beryls assuming ideal mixing on the appropriate Compatibility with natural assemblages crystallo$aphic sites.Alkali beryl commonly forms at late A critical test of the calculated phase relations is how stagesin pegmatite replacementzones replacing earlier, well they agree with observed beryllium-mineral para- purerberyl and persisting to lowertemperatures than would geneses.A large number of occurrencesof two or three pure beryl (Beus,1966). minerals in the BASH system have been described (see, 292 BARTON: PHASE EQUILIBRIA IN THE BeO-Al,O,-SiOr-H,O SYSTEM

Table 9. Proposednatural assemblagesin the BeO-AlrOr- SiOr-HrO system

Assemblages 0ccurrence References

o Ch+Ph+Be pegmatites 0krusch(l971) Ch+Di greisens, pegmatites (1968), (1967) (g Sainsbury Appotlonov Ch+Qz+sit pegmatites Heinrich and Buchi(1969), Stanek o (1978), Franzand llorteani (1984) Ch+Qz+Be pegnati tes Heinrichand Buchi (1969) Eu+ Di (?) grersens Sainsbury(1968) Co+Be pegmatites Grazianiand Di ciulio ( 1979) Ka+Be greisens, pegnatites Kayode(197')), Kerr (1946) Cerny(1968) Ka+Be+Bt pegmatites Gallagherand Hawkes(1966) Temperature (oC) Ka+Qz+Bt volcanogenic depos'i ts Levinson(1962), Lindsey (1977) Ka+Qz+Eu grersens 0lsen (1971) pegmatites, grejsens GaiI agher and Hawkes( I 966) , ActivityBeryl=0.1 Kalyuzhnayaand Kalyuzhni(1963) Be+Bt+Eu peqnatites Strand (1953), Konarova(,l974) Qz+Bt+Eu pegmatites, greisens llartensotr(1960), Hawley(1969) Qz+Bt+Be pegmatites,greisens manyreferences Qz+Ph+Be pegmatites,greisens Adans(1954), Kupriyanova(1970) Be+Ph+Eu pegnatt tes Kalyuzhnayaand Kalyuzhni(1963)

small or nonexistentfields of stability, at leastfor activities of beryl near unity. These associations,discussed here, include corundum + beryl, kaolinite + beryl and/or + bertrandite, and beryl + bertrandite. No pyrophyllite-beryllium-mineral assemblageshave beendescribed, which is unfortunate becausepyrophyllite is a key phase in the predicted equilibria in the 300"C range.The activities ofalkalis, alkaline earths,or fluorine are apparently high enough in the environments where o the beryllium minerals form to replace pyrophyllite by (t micas, feldspars,or topaz. o) o Corundum * beryl. Corundum crystals have been re- ported as inclusions in a beryl crystal from Brazil (Gra- ziani and Di Giulio, 1979). Becauseidentification was made solely on the basisof electron microprobe analyses, which would not detect beryllium, and becausesome of the "corundum" was surroundedby quartz (Graziani and 200 250 300 350 400 450 500 550 600 650 700 it plausible that the mineral may be Temperature (.C) Guidi, 1979) seems chrysoberyl. If it is corundum, then the associationwith Fig. 12. log 4^ror-temperature projection of BASH phase quartz would indicate a disequilibrium assemblage.As relations at I kbar. (A) Calcuiated phase relations for the pure can be seenfrom Figures I lA and I lB, only low activities system. AIso shown is the curve for the reaction muscovite : of beryl (.0.01) can be compatiblewith corundum. K-feldspar * alumina + water. (B) Calculatedphase relations for Kaolinite * bertrandite. Kaolinite * bertrandite has : (A) a*r, 0. I . The curvesare the sameas in exceptwhere labeled. beenreported from volcanogenicberyllium deposits(Lev- Note the effect of the expansion of the beryl stability fields. (C) inson, 1962; Lindsey, 1979).This assemblageis stable Projection of the stability fields of euclase(horizontally ruled) qtartz pressures and euclase+ quartz (vertically ruled) with dbvr : l. The curves relative to euclaseI only at moderate are the same as in (A). Note that euclaseis incompatible with and low temperatures.Cogenetic silica in these deposits K-feldspar. Seetext for further discussion.Abbreviations from is representedby a poorly crystalline polymorph, indi- Table l. cating an unusually high silica activity at the time of for- mation. Hydrothermal K-feldspar is presentwith the ka- for example, Beus, 1966). Table 9 summarizes key, well- olinite, also indicating metastability (with respect to described associations of two or more minerals in the muscovite). Silica activities much higher than quartz sol- BASH system. The observed and calculated phase rela- ubility are well known in geothermal waters and can sig- tions correspond well overall, but several associations have nificantly displacesome equilibria such as the hydrolysis BARTON: PHASE EQUILIBRIA IN THE BeO-Al,OrSiO,-H,O SYSTEM 293

of K-feldspar (Meyer and Hemley, 1967). The pertinent Comparisonwith alternative topologies reaction here is 4BeAlSiO.(OH) + 2SiO, * 3HrO: BeoSirOr(OH),+ 2AlrSirO,(OHL. Silica activities greater Burt (1978)proposed a detailed,general model for phase than that of quartz will drive the reaction to the right. relationsin the BASH systembased on multisystemsanal- Calculations based on the thermodynamic data indicate ysis and on the then-existingphase-equilibrium, mineral- that a supersaturationof +0.6 (this study) or *0.2 (Hsu, association,and mineral-volume data. Although there are 1983) log units will stabilize the right-hand side at I bar similarities between Burt's topology and the one calcu- and 100.C. Theseconcentrations are below that of amor- lated here, they difer substantially. The principal differ- phous silica at the same conditions, lending credenceto encesarise from two sources:interpretations ofthe P-?" the model. The abundant fluoride in these deposits may extent of the assemblagechrysoberyl + quartz and of the alsohelp stabilizebertrandite (Hsu, 1983). P-Zextent of euclasestability at low temperatures.In the Bertrandite * beryl. Bertrandite very commonly re- absenceof reported natural assemblagesof beryl with py- places beryl, usually with a sheet silicate taking up the rophyllite or aluminum silicate, Burt omitted them from aluminum. The bertrandite + beryl assemblage,however, his multisystem. This omission led to the erroneouscon- has only a small stability field in the simple system (Fig. clusion that chrysoberyl * quartz is stableto temperatures l0). There are several possible explanations for this: a below the stability limits of the anhydrousaluminum sil- higher-temperaturebreakdown of bertrandite than pro- icatesplus water; thus, the replacementof chrysoberylin posedhere; an underestimateof the stability of pure beryl quartz-containingsystems would be by a reaction such as at low temperatures;or the stabilization of beryl by solid chrysoberyl + quartz: beryl + kaolinite. In contrast, the solution. If the decomposition of bertrandite is close to experimentalresults on this reaction (Fig. 3) indicate that the conditions indicated by Hsu (1983), the I 00" increase chrysoberyl + q\artz breaks down to give beryl + alu- over Figure l0 would dramatically broaden the field of minum silicate at temperatures substantially above the coexistenceof bertrandite and beryl. Two lines of evi- stability ofkaolinite and pyrophyllite and probably above dence, however, support the lower-temperature break- the stability limit of euclase. down proposedin this study. In an extensivefluid-inclu- As noted above, pyrophyllite-a key mineral in the cal- sion study ofbertrandite and phenakite from one deposit, culated phaserelations-has not been reported with be- Kosals et al. (1973) found that the bertrandite formed ryllium minerals. The association chrysoberyl + eu- below 290'C and the phenakite above 200"C. Their esti- clase + quartz would support Burt's model, as would any mate of the total pressurewas 700 bars. Breakdown of hydrous aluminum silicate * chrysoberyl * quartz, but bertrandite above 300'C would also precludea stablefield suchassemblages have not been reported. In contrast, the for the observed assemblagemuscovite + phenakite + associationberyl + andalusiteor sillimanite has been re- quartz (Franz et al., pers. comm.; Stager, 1960; cf. Fig. ported from severalplaces, although the two alwaysseem t2A). to be in a reactionrelationship (Heinrich and Buchi, 1969; As discussedabove (Thermodynamics), the beryl hy- Franz and Morteani, 1984). dration model is most likely to be inadequateat low tem- The paucity of the lower-temperaturecoexisting pairs peraturesand pressruesleading to an underestimateofthe beryl plus andalusite or sillimanite, which might be used stability of beryl. Thus it is possible that beryl * ber- to argue for their incompatibility, can be explained by a trandite could be much more extensive even with the bulk-compositioneffect. Because pegmatites generally have lower-temperature bertrandite-breakdown reaction. Ex- abundant K-feldspar, aluminum silicate assemblages pansion of the beryl-stability field by solid solution, how- would not be expectedin pegmatitesfor temperaturesless ever, seems the most likely interpretation. Reasonable than those of the reaction KAlr[AlSirO,o](OH), + SiO, : decreasesin the activity of beryl greatly expand the beryl- KAlSi3Or + AlrSiOs + HrO. At higher pressures,this re- stability field and hence the region of coexistencewith action is replaced by melting reactions (e.g., Huang and bertrandite (Figs. I I, l2). Wyllie, 1974).Franzand Morteani (1984)have proposed Kaolinite * beryl. Beryl most commonly alters to clays the formation of chrysoberyl + quartz by related melting or micasplus otherminerals. Cernf (1968),Kayode (1971), reactions. Other evidence given by Franz and Morteani and Kerr (1946) documented the alteration of beryl to (1984) and Heinrich and Buchi (1969) suggeststhat kaolinite. The calculatedphase relations indicate that this chrysoberyl forms during a high-grade, probably prograde associationis not stable at high activities of beryl (Figs. event. Chrysoberyl * quartz assemblagesare not associ- I lA, l2A), but that it doesbecome stable with reasonable ated with later hydrous alteration even though they may decreasesin the activity of beryl (Figs. llB, l2B). This contain crystals of substantial size. This suggestsforma- interpretation is partly supportedby the fact that the peg- tion from high-temperaturefluids such as melts. Chryso- matitic berylsdiscussed by Ken (1946)and eern! (1968) beryl + quartz hasbeen described by Franz and Morteani tend to be alkali-rich and thus should have activities sig- (1981)as reactingto beryl * muscovite.Although this is nificantly lessthan l. The associationkaolinite + beryl + not within the BASH system, it does indicate that chry- bertrandite (Gallagherand Hawkes, 1966) can be ration- soberyl * quartz is not highly stable as indicated by the alized by silica supersaturation(Burt, 1978) only at low narrownessof the muscovite + quartz field at high tem- activities of beryl. peratures(Fig. l2). 294 BARTON:PHASE EQUILIBRIA IN THE BeO-A1'O,-SiO,-H,OSYSTEM

Many alternative assemblagesto chrysoberyl * quartz P, S, and F. Additional feldspar-mica and many other are known. An important possibility is euclase+ dia- reactions could be calculated using a suitable data base spore. Sainsbury (1968) described euclase* white mica (e.g.,Robie et al., 1978). The topologiesof the phase veinlets cutting diaspore * chrysoberyl replacement zones relations in more complex systemshave been discussed at Lost River, Alaska. It is possible, but not clear from by many authors,notably Burt (1975a, 1975b, 1980),Ku- his descriptions,that euclasemay also coexist with dia- priyanova(1982), and Beusand Dikov (1967). spore.This associationsupports the thermodynamic model that predicts a broad stability range for this pair (Fig. I l) pnrnolocrc precluding the assemblagechrysoberyl + quartz at inter- Solrn APPLTCATIoNS mediate temperatures.Other reported associations,how- A comprehensivediscussion ofthe paragenesesofbe- ever, do not prohibit chrysoberyl + qvartz at least at the ryllium minerals in this systemand related,more complex higher temperatures.The apparent lack of appropriate systemsis beyond the scopeof this study. Instead, some natural bulk compositions precludesan unambiguousan- generalobservations on the paragenesesofberyllium min- swer to this question. erals will be presentedin terms of the predictions of the The other significant distinction between Burt's topol- thermodynamic model. The calculated phase relations ogy and that calculatedhere is in his inferenceofthe low- constrain not only the pressuresand temperaturesof for- temperature breakdown of euclaseto bertrandite * ka- mation of beryllium-mineral assemblages,but they also olinite assemblages.The thermodynamic model, how- place useful constraintson the activities of silica and alu- ever, suggeststhat bertrandite and kaolinite can coexist mina. With few exceptions,discrete beryllium minerals only under conditions of unusually high silica activity, are found only in alkalic felsic igneousrocks and related consistentwith the natural occurrences. metasomatic rocks. The distribution of beryllium min- Hsu (1983)reproduced a portion ofBurt's topology and erals is thoroughly discussedby Vlasov (1967, Vols. II indicated where the reactions that he determined fit inl and III) and Beus (1966) and will not be reviewed here. however,neither his experimentaldata nor any others are compatible with the stableexistence of the invariant point Limitations on pressureand temperature [Qz] shown in Hsu's Figure 6. The variation in beryllium mineral assemblagesfrom The BASH phaserelations proposedby Franz and Mor- pegmatitesto greisensto volcanogenicdeposits (Table 9) teani (1981) on the basis of their experimental results is consistentwith a trend ofgenerally decreasingpressure share some featureswith Figure l0 at intermediate and and temperature. The calculated phase relations in the high temperatures.The similarity results from the large quaternary system agree with observed assemblagesas stability field for beryl + alurninum silicates (both hy- discussedin the last section.The paucity ofuniquely high- drous and anhydrous) shown by Franz and Morteani in or low-pressureassemblages precludes the estimation of contrast to Burt (1978). There are, however, many differ- pressure.The phaserelations provide reasonabletemper- encesin the details of the topology that result from the ature constraints,although solid solution in beryl should use by Franz and Morteani of Chatterjee's(1976) phase causelarge changes in the positions of beryl-bearingequi- relations for the ASH system. libria. Franz and Morteani infer phaserelations at lower tem- Only euclase+ phenakite + qaartz or beryllium min- peratures by extrapolation of new reactions that result erals with kyanite have nontrivial minimum pressuresof from the intersection of their experimentally determined formation (ca. 0.5 and 2.5 kbar, respectively,but highly reactions.These extrapolations indicate a minimum pres- uncertain for the former). Neither of these assemblages sure for euclase+ quartz of about 2 kbar and lead to a has been reported. The fields of the low-pressureassem- low-temperature topology similar to that of Burt (1978). blagesbertrandite + beryl, pyrophyllite * beryl, and an- Such graphical extrapolationsare uncertain, and the pre- dalusite + other phasesare similarly uncertain. dictions conflict with the thermodynamic extrapolations BASH phaseequilibria provide usefultemperature con- done in this study on the basis of a similar, but far more straints. The assemblagechrysoberyl + quartz indicates extensiveset of experimental results. amphibolite- to granulite-gradeconditions, except under conditions ofreduced volatile pressure.Euclase- and ber- Phase equilibria in more complex systems trandite-bearing assemblagesrestrict maximum temper- Most beryllium minerals occur in rocks with more aturesto middle and lower greenschistfacies, respectively. components than those in the BASH system. The most More preciseestimates might be made for particular as- important of these additional componentsare the Na2O, semblages,if solid solution in beryl is accountedfor. Be- trlrO, and CaO that form feldspars, micas, and clays. causemost beryllium minerals form in dynamic environ- Feldspar-micaequilibria are representedin Figure 12 by ments where temperaturesand possibly pressureschange the reactionKAlSi3Os + HrO + (AlrO3): KAlr[AlSirO'o]- substantiallyduring petrogenesis,the sequenceof assem- (OH)r, where alumina is enclosedin parenthesesto em- blagesand hence the I(-P) paths are generallyofgreater phasizethat in generalit is not representedby corundum. interest than specific temperature or pressureestimates. Other componentsimportant in equilibria involving be- For example, consider the formation of pegmatites.The ryllium-aluminum silicates include Li, Mg, Mn, Fe, Zn, small size ofpegrnatites requiresthat they cool quickly to BARTON:PHASE EQUILIBRIA IN THE BeO-AI,O,-SiO,-H,OSYSTEM 295 the temperature of their surroundings, but subsequently "crossingJine"(desilicated) pegnatites (Vlasov, I 96 7, Vol. their conditions changequite slowly along with the host III) offer excellentexamples. Apocarbonate greisens form rocks. If the pegmatitic fluids are lost fairly rapidly, the by addition of fluorine, alumina, alkalis, silica, and trace fluid-dominated crystallization processeswill reflect tem, elementsfrom granitesto nearby carbonaterocks. In the peratures only down to that of ths host at the time of Lost River, Alaska,beryllium depositsdescribed by Sains- formation. Therefore, the beryllium minerals (and many bury (1968), the earliest mineralization is replacementof others) might be used to constrain the cooling path and limestone by fine-graineddiaspore, fluorite, and "curdy" placean upper bound on the temperatureofthe host rocks chrysoberyl.This stageis cutby euclase-(bertrandite-)mica- at the time of emplacement. fluorite veins that are in turn followed by muscovite-fluo- Equilibria in more complex systemshave not beencal- rite-beryl-quartz veins. This sequence can be simply culated here with the exception of the K-feldspar-mus- interpreted as the consequenceof the cooling and pro- covite reaction in Figure 12. Franz et al. (unpub. ms.) gressivereaction of fluids derived from associatedgre- discussedthe formation of beryl from phenakitein Alpine isenized granite. Figure llC shows this path superim- prograde regional metamorphism by several reactions, posed on the l-kbar aqueous-silicaphase relations. The among them 2KAlr[AlSi3Oro](OH),+ 3BerSiO4+ Lost River assemblagesare consistentwith a path across 9SiOr: 2BerAlrSiuO,.+ 2KAlSi3Os+ 2HrO. At 6 kbar, the diagram from high to low temperatureand from low this reaction should take place at about 420C, which is to high silica activities (the activity ofquartz is unity on 80-130'below the peaktemperatures estimated from oth- the quartz saturation line). At the highest temperatures er reactions in the same rocks (Franz et al., unpublished and low silica activities, chrysoberyl + diasporeis stable; manuscript), but consistentwith the mineral associations with decreasingtemperature, chrysoberyl should react to in pegmatites where beryl + K-feldspar is much more give euclaseand diaspore. Further decreasein tempera- common than phenakite * muscovite. ture results in the decomposition of euclaseto give py- rophyllite * beryl and eventually to precipitation ofquartz. Limitations on activities of components Alkalies in the fluids at Lost River were apparently con- In most occurrences,textural evidence suggeststhat only centrated enough to give micas in place of the pure alu- a singleberyllium mineral is stablein a given assemblage. minous phasesat lower temperatures.The large under- If more than one is present,textures commonly indicate saturationin silica is likely due to the retrogradesolubility a reaction relation. As a component present in but one of quartz at the moderatetemperatures and low pressures phasein a given assemblage,BeO could be ignored in the of formation of the deposit (Dobson, 1982; Walther and considerationof equilibria in a rock, much as other trace Helgeson, 1977). This would causeinitially quartz-satu- componentscan be neglected(e.g., Miyashiro, 1975, p. rated fluids coming from the granite to go through aq\afiz- 125).A trace component (such as zirconia) that is almost undersaturatedregion leading to silica-deficient assem- always present in the same mineral (zircon, in granites) blages.Progressive cooling would lead to the restoration has a limited utility as an indicator of the intensive vari- of a progradesilica solubility and eventually a concomi- ables(e.g., I, P, chemical potentials) during petrogenesis. tant increasein the silica content of the precipitated min- In contrast, the several minerals in the BASH system, erals. which share all but one component with major phases, At higher pressures,the solubility ofquartz is prograde yield constraintson the activities ofthe major components at all temperatures.Therefore, similar apocarbonategrei- (Al2O3,SiOr, and HrO). This is particularly important in sensformed at higher pressuresshould show more sili- metasomaticrocks where there are many degreesof free- cious assemblages.This appears to be the case at Mc- dom and where estimatesof activities are neededin order Cullough Butte, Eureka County, Nevada, where beryl and to make petrogeneticinterpretations. Comparison of the late bertrandite are the predominant beryllium minerals total set of buffers to the beryllium-free buffers in Figures (Barton, 1982a).At Mccullough Butte, the vein assem- I I and I 2 illustrates the superior resolution possiblewith blagesare mostly quartz-saturatedwith the exception of beryllium minerals present.This useofthe beryllium min- a quartz-free muscovite-dominated associationthat in turn eralsin metasomaticrocks is analogousto the evaluation is followed by quartz-rich muscovite-fluorite veins. of the activities of major components in igneous rocks Crossing-line(desilicated) pegmatites commonly show (e.9.,Carmichael et al., 1974). zoning from beryl (+ quartz) in the centersto chrysoberyl The activity of water is the most difrcult of the three and/or phenakite in outer (silica-poor) parts (Beus, 1966; to evaluate directly. Only chrysoberyl + quartz can be Vlasov, 1967). This sequenceis compatible with a path easily interpreted with water pressureas an independent from high to low silica activities at relatively high tem- variable; however,because other volatiles should stabilize peratures(Fig. I l). In a few localities where beryllium- beryl in the same way that water does, there will still be rich solutions reacted with ultramafic rocks, the silica ambiguities in interpretation. activity was reducedbeneath the phenakite-bromellitere- Log a"n",on.Beryllium minerals are good indicators of action producing bromellite (in highly magnesianassem- silica activities (Fig. I l) and hencecan be usedto interpret blages,Klementyeva,19691' Smirnov, 1977).At constant the conditions and processesof formation. Beryllium- temperature, the presence of talc + antigorite buffers mineral paragenesesin "apocarbonate" greisens and aqueoussilica just below the value for bromellite * phen- 296 BARTON: PHASE EQUILIBRIA IN THE BeO-Al,OrSiO,-H,O SYSTEM akite (Fig. 6, Hemley et al., 1977b) consistent with the plain why euclaseis usually found in rocks deficient in occurrenceof bromellite with talc (Klementyeva, 1969). the alkalies relative to alumina, such as hydrolytically al- greisens). Log an ror.Beryllium-mineral paragenesescan also pro- tered rocks (e.g.,aluminosilicate Although ex- vide useful constraintson the activity of alumina. As dis- pansion of the stability field of beryl with solid solution cussedabove, alumina is generally a fixed component, may also play a role in diminishing the stability field of that is, other components (alkalies, hydrogen ion, silica, euclase,the high H* to total alkali ratio (Reaction llc) and halogens)are generally more mobile and thus used would work againstthis. Consistentwith this, most greisen in conventional projections of the phase equilibria. The beryls are more nearly stoichiometric than late-stagepeg- advantage,however, of treating the activity of alumina as matitic beryls that formed at similar temperatures(Beus, an independentvariable is that it allows direct comparison 1966). of a variety of beryllium-mineral equilibria on the same The topology of Figure 12 illustrates the reason that diagram without having to draw a separate diagram for muscovite * bertrandite + quartz is the most common eachnew set of additional components.For example,the assemblageto replace beryl. Beryl + K-feldspar cooling activity of alumina is related to conventional hydrolysis with fluid will react accordingly to give muscovite + reactions(and hence hydrogen ion and alkali concentra- phenakite or bertrandite at temperaturesbelow 250'C at tions) by reactions such as the following: I kbar. Becausethe interval for phenakite is quite small, the predominant product will be bertrandite. In fact, the KAlSirOr+HrO+(AlrO3) only product will be bertrandite if the stability field of : KALIAISi3O'ol(OH), (11a) beryl is expandedslightly by solid solution. 3KAlSi3O8+ 2H+ Becausethe chemical potentials of the alkalis vary in- : 2K* + 6SiO, + KAI,[AISi3O'J(OH[ (l lb) verselywith that of alumina in quartz * mica or feldspar 2KAISi3OE+ 2H* assemblages,the variations described here may equally : 2K* + H,O + 6SiO, + (Al,O3). (l lc) well be thought of as due to variations in alkalinity (with alumina as an inert component). The absenceof chryso- As can be seenfrom Reaction I lc, which is the diference beryl and the relative abundance of bertrandite, phen- of I la and I lb, the activity of alumina is inversely related akite, and the alkali-beryllium silicates in peralkaline to (a*.)'?and proportional to (a"-)'?.Similar relationships granitesand syenites(cf., Beus, 1966; Vlasov, 1967, Yol. exist for other hydrolysis reactions as well as for many II) reflect the low intrinsic chemical potentials of alumina other kinds ofreactions (e.g.,fluorination reactions). in these rocks. In peralkaline rocks, alumina-undersatu- An immediate consequenceof examining such a dia- rated minerals such as acmite and riebeckite require that gram is an explanation for the comparative rarity of eu- the alumina activities be significantly below that of the clase in spite of the central role that it plays in the cal- K-feldspar-muscovite buffer (Reaction I l). Alumina- culated phaseequilibria in the BASH system. There are buffering reactionsinvolving peralkaline minerals will be two related reasonsfor its rarity. First, as can be seenin below the stability fields of euclaseand chrysoberyl and, Figure l2C, euclaseand qtartz are stabletogether only at especiallyin the absenceofquartz, will be below the sta- relatively low temperatures.Because most beryllium-rich bility field of beryl, thus rationalizing the observedabun- assemblagesare quartz saturated (most pegmatites and dance of the aluminum-free beryllium silicates in peral- greisens),the quartz + euclase reaction representsthe kaline rocks. most common upper limit. Still, euclase+ quartz is stable to higher temperaturesthan bertrandite, yet bertrandite is much more common. The secondreason for the rarity SuPrlrlnY of euclaseis made clear by Figure 12. Euclaserequires a The thermodynamic properties of behoite, bertrandite, relatively high chemical potential of alumina, above that beryl, chrysoberyl,euclase, and phenakitehave beeneval- at which K-feldspar is stable.In many greisensand nearly uated from a combination of new phase-equilibrium, ca- all pegmatitesthe alumina contents are buffered by re- lorimetric, and volumetric measurementsalong with data actions such as I I between the white micas, feldspars, from the literature. Of 86 data setscontaining over 1000 topaz, and quarlz. Muscovite, K-feldspar, and quartz fix observations,only one set does not fit the model or have the chemical potential of alumina in the beryl field over good reasonsfor being discarded.The resulting internally most of the temperature range (Fig. l2). Topaz-quartz consistentthermodynamic model is compatible with nat- (-muscovite) assemblageswhich begin at the andalusite- ural assemblagesin the BASH system and provides a quartz line and extend downward do likewise, although suitablebasis for the prediction ofequilibria in more com- at high fluorine fugacities,they may buffer p^ro, to values plex systems. within the quartz + phenakite field. The reaction Beryllium minerals in the BASH systemprovide limits 2Be,AlrSi.O,, + 4HF : 3BezSiO+ + 2AlrSiO4F, + on the temperaturesand activities of silica and alumina 7SiO, + 2HrO has been proposedby Burt (1975b)as a during formation. Beryl has a broad stability range, con- natural fluorine buffer; such assemblages,while uncom- sistent with its natural abundance.The stability of beryl mon, are known in pegmatites, miarolitic cavities, and is significantly enhanced by incorporation of volatile some greisens(Barton, 1982b). These relationships ex- speciesand alkalis. The effect ofvolatile incorporation is BARTON: PHASE EQUILIBRIA IN THE BeO-ALO'-SiO,-H,O SYSTEM 297 modeled as a simple volumetric interaction. This model RlrnnrNcrs works well for beryl and appearsreasonable for cordierite, although it resultsin a somewhatpoorer fit than the other, Adams, J.W. (1953) Beryllium deposits of the Mount Antero more elaboratemodels proposed for cordierite. Solution region,Chaffee County, Colorado.U.S. GeologicalSurvey Bul- letin 982-D,95-119. of nonvolatile speciesin beryl is also inferred to have a Aines, R.D.F., and Rossman,G.R. (1984) The high temperature major efect on equilibria involving beryl. behavior of water and carbon dioxide in cordierite and beryl. Occurrencesof the BASH minerals provide broad con- American Mineralogist, 69, 319-327. straints on temperature,but no independentpressure con- Appolonov, V.N. (1967) Morphological feature of chrysoberyl from . Zapistn Vsesoyunogo Mineralogichekogo Ob- straints. Chrysoberyl + quartz is restricted to high tem- Sargodon ( shchestva,96, 329-332. peratures except when Po",o P,oo,. Euclase-bearing Bamberger,C.E., and Baes, C.F., Jr. (1972) Equilibria of SiF. assemblagesreplace aluminous beryl-bearingassemblages with SiOr, BerSiOo,and BeO in molten LirBeFo. American in the 300-500oCrange. Stoichiometric beryl breaksdown Ceramic SocietyJournal, 55, 564-568. in the presenceofwater about 100"C lower, although al- Barton, M.D. (1981)The thermodynamicpropertiesoftopaz and systemBeO-ALO,-SiOr-H,O. Ph.D. the- persist someminerals in the kali-rich beryls will to lower temperatures. The sis, University of Chicago,Chicago, Illinois. resultsof this study indicate that bertrandite and behoite - (1982a) Some aspectsofthe geologyand mineralogy of are stableonly below 260"Cin agreementwith the limited the fluorine-rich skarn at McCullough Butte, Eureka County, natural data, although the results of Hsu (1983) suggest Nevada. CarnegieInstitution of Washington Year Book 81, that bertrandite is stable to 350.C. 324-328. - (1982b) The thermodynamic properties of topaz solid Becauseberyllium minerals most commonly occur sin- solutions and some petrologic applications. American Min- gly in metasomatic rocks, perhapstheir most useful pet- eralogist,67, 9 56-974. rologic attribute is as indicators ofthe activities ofsilica Beus,A.A. (1966) Geochemistry of beryllium and genetictypes and alumina. For example,the beryllium minerals permit ofberyllium deposits.W. H. Freemanand Company, SanFran- a much finer clsco. discrimination of the activities of silica in Beus,A.A., and Dikov, Yu. P. (1967)The geochemistryof be- greisens apocarbonate and desilicated pegmatites than ryllium in the processesof endogeneticmineralization. Nedra, would be possiblefrom the phasesof the major elements Moscow. alone. Similarly, the beryllium minerals provide addi- Bukin, G.V. (1967) Crystallization conditions of the phenakite- (experimental Akademi- tional constraintson the activities of alumina in greisens. bertrandite-quartz association data). ya Nauk SSSRDoklady, 176,664-667. Euclase,for example, is stable only at elevated alumina Burnham, C.W. (1962) Lattice constantrefinement. Carnegie In- activities, indicating environments where the alkalis have stitution of Washington Year Book 61, 132-135. unusually low activities either as a consequenceof acid Burt, D.M. (1975a) Beryllium mineral stabilities in CaO-BeO- metasomatism(as in aluminosilicate greisens),or unusu- SiOr-PrOr-FrO-' and the breakdown of beryl. Economic Ge- ology,70, 1279-1292. ally low initial alkali contents and high aluminum mo- - (1975b)Natural fluorine butrersin the systemBeO-AlrOr- bility (as in apocarbonategreisens). SiOr-FrO-,. (abs.)EOS American GeophysicalUnion Trans- Additional studies that might help resolve problems actions,56,467. found here are an improved determination of Reaction - (1976) Generation of high HF fugacities in miarolitic pegmatites granites: possiblerole Geological 4, more data on equilibria below 400t (solubility exper- and The oftopaz. Societyof America Abstracts with Programs,8, 798. iments might work best),direct beryl hydration measure- - (1978) Multisystems analysis of beryllium mineral sta- ments, evaluation of the effect of solid solution on beryl bilities: The system BeO-A1'Or-SiOr-HrO.American Miner- stability (perhaps by displacement of Reaction 1), and alogist,63,664-676. - investigation of the occurrenceof critical natural assem- (1980) The stability of danalite, FeoBer(SiOo)rS.Ameri- blages such quartz can Mineralogist,65, 355-361 as chrysoberyl + * euclase or eu- - (1982) Minerals of beryllium. In P. Cernf, Ed., Granitic clase * andalusite. pegmatitesin science and industry, 135-148. Mineralogical Association ofCanada Short Course Handbook 8. AcxNowlnncMENTS Carmichael, I.S.E., Turner, F.J., and Verhoogen, J. (197q Ig- petrology. New York. It is a pleasureto acknowledgehelpful reviews by D. M. Burt, - neous McGraw-Hill, Cernf, P. (1968)Beryllumwandlungen ia Pegnatiten-Verlaufund W. A. Dollase,W. G. Ernst,and R. Ewing paper, C. on this and Produkte. Neues Jahrbuch liir Mineralogie Abhandlungen, 108, by J. R. Goldsmith,P. B. Moore,R. C. Newton,and P. J. Wyllie I 66-l 80. on an earlierversion. Many other peoplecontributed helpful - (1975) Alkali variations in pegmatitic beryl and their discussionsor assistanceon variousaspects of this projectin- petrogeneticimplications. NeuesJahrbuch fi,ir Mineralogie Ab- cludingP. B. Barton,Jr., J. L. Haas,Jr., H. T. Haselton,J. J. handlungen, 123, 198-212. Hemley,D. M. Jenkins,W. Moloznik, D. PerkinsIII, G. R. Chatterjee, N.D. (1976) Margarite stability and compatibility Robinson,Jr., P. Toulmin III, and W. J. Ullman. R. C. Aller relations in the system CaO-AlrOr-SiOr-HrO as a pressure- providedthe facilitiesfor andhelp with the silicaanalyses. The temperatureindicator. American Mineralogist, 61, 699-709. (1979) experimentalwork wassupported by NSF grantEAR78-13675 Cline, C.F., Morris, R.C., Dutoit, M., and Harget,P.J. Physical properties of BeAlrOo single crystals. Materials Sci- to J. R. Goldsmith.Support for the thermodynamicmodeling enceJournal, 14, 941-944. was obtainedfrom EAR84-08388to M. D. Barton.While a Cohen,J.P., Ross, F.K., and Gibbs, G.V. (1977)An X-ray and student,I wassupported by a NationalScience Foundation Grad- neutron ditrraction study ofhydrous low cordierite. American uateFellowship and by a McCormickFellowship from the Uni- Mineralogist, 62, 67-7 8. versityof Chicago. Demarest, H.H., Jr., and Haselton, H.T. (1981) Error analysis 298 BARTON:PHASE EQUILIBRIA IN THE BeO-AI,O,-SiO,-H,OSYSTEM

for bracketed phase equilibrium data. Geochimica et Cos- Hawthorne,F.C., and Cernf, P. (1977)Thealkali-metalpositions mochimicaActa. 45. 217-224. in Cs-Li beryl. Canadian Mineralogist, 15, 4 14-421. Ditmars, D.A., and Douglas, T.B. (1967) Relative enthalpy of Hazen, R.M., Finger, L.W., and Barton, M.D. (1983) High-pres- beryllium 1:1 aluminate, BeO.AlrO, from 298 to 1173 K. sure structuresand compressibilitiesof bertrandite,beryl, and Thermodynamic propertiesfrom273 to 2150 K. National Bu- euclase.Carnegie Institution of Washington Year Book 82, reau ofStandards Journal ofResearch, 71A, 89-95. 36l -363. Dobson, D.C. (1982) Geology and alteration of the Lost River Heinrich, E.W., and Buchi, S.H. (1969) Beryl-chrysoberyl-silli- tin-tungsten-fluorinedeposit, Alaska. Economic Geology, 77, manite paragenesisin pegmatites. Indian Mineralogist, 10, I 033-l 052. l-'7. Fleischer,M. (1980) Glossary of mineral species1980. Miner- Helgeson,H.C., Delany, J.H., Nesbitt, H.W., and Bird, D.K. alogical Record, Tucson, Arizona. (1978)Summary and critique ofthe thermodynamic properties Frantz, J.D., and Hare, P.E. (1973) The analysisof silica at sub- ofrock-forming minerals.American Journal of Science,278-4. microgram levelsusing a flow-cell colorimetric technique.Car- Hemingway,B.S., Barton, M.D., Robie, R.A., and Haselton,H.T., negie Institution of Washington Year Book 72, 704-706. Jr. (1986) The heat capacitiesand thermodynamic functions Franz,G., and Morteani,G. (1981)The systemBeO-AlrO3-SiOr- for beryl, BerAlrSiuO,., phenakite, BerSiOo, euclase, BeAl- HrO: Hydrothermal investigation of the stability of beryl and SiO.(OH), bertrandite, BeoSi,O'(OH)r, and chrysoberyl, euclasein the range from I to 6 kb and 400 to 800f. Neues BeAlrO.. American Mineralogist, 7 l, 557-568. Jahrbuch ftir Mineralogie Abhandlungen, 140, 273-299. Hemley, J.J., Montoya, J.W., Christ, C.L., and Hostetler, P.B. - (1984) The formation of chrysoberylin metamorphosed (1977a\ Mineral equilibria in the system MgO-SiOz-HrO: L pegmatites.Journal of Petrology, 25, 27-52. Talc-chrysotile-forsterite-brucite-stabilityrelations. American Furukawa, G.T., and Saba,W.G. (1966) Heat capacityand ther- Journal of Science,27 7, 322-351. modynamic properties of beryllium aluminate (chrysoberyl), Hemley,J.J., Montoya, J.W., Shaw,D.R., and Luce,R.W. (1977b) BeO.AlrOr, from 16 to 380K. National Bureau of Standards Mineral equilibria in the MgO-SiOr-H,O system:II. Talc-an- Journalof Research,69A, 13-18. tigorite-forsrerite-anthopyllite-enstatite stability relations and Gallagher, M.J., and Hawkes, J.R. (1966) Beryllium minerals some geologicimplications in the system. American Journal from Rhodesiaand Uganda. Geological Survey of Great Brit- of Science,277 , 353-383. ain Bulletin. 25. 59-75. Hemley, J.J., Montoya, J.W., Marinenko, J.W., and Luce, R.W. 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(1955) Chemical composition of beryl. Trudy AlrO3-SiO2-HrOwith comments on the effect of HF. Geolog- Mineralogichekogo Muzeya Akademiya Nauk SSSR, 7 (not ical Society of China Memoir 5, 33-46. seen;cited in Beus,1966). Huang, W.L., and Wyllie, P.J. (1974) Melting relations of mus- Goldman,D.S., Rossman, G.R., andDollase,W.A. (1977)Chan- covite with quartz and sanidine in the KrO-AlrO3-SiOr-H2O nel constituents in cordierite. American Mineralogist, 62, ll44- system to 30 kilobars and an outline of paragonite melting 1t57. relations. American Journal of Science,27 4, 378-39 5. Goldman,D.S., Rossman, G.R., and Parkin,K.M. (1978)Chan- Jochum,C., Mirwald, P.W., Maresch,W., and Schreyer,W. (1983) nel constituentsin beryl. Physicsand Chemistry of Minerals, The kinetics of HrO exchangebetween cordierite and fluid 3,225-237. during retrogression.Fortschritte der Mineralogie, 61, 103- Grazian| G., and Di Giulio, V. (1979) Growth of an aquamarine l 05. crystal from Brazil. Neues Jahrbuch fiir Mineralogie Monat- Johannes,W., and Schreyer,W. (1981) Experimental introduc- shefte.1979. l0l-108. tion of CO, and HrO into Mg-cordierite. American Journal of Graziani, G., and Guidi, G. (1979) Mineralogical study of a star- Science,281,299-317. beryl and its inclusions.Neues Jahrbuch liir Mineralogie Mon- Kalyuzhnaya,K.M., and Kalyuzhnyi, V.A. (1963) The paragen- atshefte. 1979. 86-92. esisofaccessory beryl, phenakite,and euclasein topaz-morion Haar, L., Gallagher, J., and Kell, J.S. (1979) Thermodynamic (smoky quartz) pegmatites.Mineralogichekogo Sbomik (L'vov), propertiesfor fluid water. Water and steam-Their properties r7.r35-t47. and current industrial applications.Proceedings of the 9th In- Kayode, A.A. (1971) A kaolinite greisenfrom Dorowa-Babuje, ternational Conferenceon the PropertiesofSteam, 69-82. northern Nigeria. Economic Geology, 66, 8l l-812. - (1981) The anatomy of the thermodynamic surface of Kelley, K.K. (1939) The low temperatureheat capacitiesof BeO water: The formulation and comparisonswith data. Proceed- (bromellite) and BerSiOo(phenacite). American Ceramic So- ingsofthe 8th Symposiumon ThermophysicalProperties,298. ciety Journal, 61, 1217-1219. Haas, J.L., Jr., and Fisher, J.R. (1976) Simultaneousevaluation Kerr, P.F. (1946) Kaolinite after beryl from Alto do Giz,Brazil. and correlation of thermodynamic data. American Journal of American Mineralogist, 3 l, 435-442. Science,276,525-545. Kiseleva, I.A., and Shuriga,T.N. (1981) New data on the ther- Haas, J.L., Hemingway,B.S., and Robinson, G.L., Jr. (1980) modynamic properties of phenakite. Akademiya Nauk SSSR Tablesfor the thermodynamic propertiesof phasesin the CaO- Doklady, 266, 3lO-313. AlrO3-SiO2-H2O.U.S. GeologicalSurvey Open-File Report 8G Kiseleva, 1.A., Ogorodova, L.P., and Kupriyanova, LI. (1984) 908. Mineral equilibira in the system [BeO-Al,Or-SiO,-H,O]. In- Halbach,H., and Chattef ee,N.D. 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