AmericanMineralogist, Volume6I , pages40941 3, 1976 Equilibria andorigin of mineralsin the systemAI2OB-AIPOr-H2O Wlr-ltnuS. Wtse eNp SpnNcsR E. Loul Departmentof GeologicalSciences, Uniuersity of California Santa Barbara, Califurnia 93106 Abstract Usingstandard hydrothermal techniques, the phase relations of threeAl-phosphate miner- als-berlinite, AIPOo;augelite, AIrPO{(OH)3; and trolleite, AI.(PO4)3(OH)r-were determined in the temperatureand pressurerange of400' to 600"Cand 500to 3000bars. The univariant line, definingthe upperstability limit of trolleite(trolleite = augelite* berlinite),can be ex- pressedby I('C) : 0.0373P(bars)+ 350.7.The univariant curve defining the upper stability of augelite(augelite : berlinite * corundum * vapor) is expressedby log P(bars) = -r4,938/T ("K\ + 22.48. Thesecurves intersect at an invariantpoint near520oC and 4500bars. The additionof diasporeleads to anotherinvariant point at approximately550'C and 12 kilobars.The completeP-T diagramindicates that the stability field of trolleite is greatly extendedby pressure,whereas the stability of augeliteabruptly terminates near 520oC,regardless of the pressure. Calculations,based on the breakdownof augelite,indicate that excessquartz reduces the stabilityof augeliteby about35o. From the calculated free energy of augelite(AG : -671.5 + 1.5kcal at 25'C) it is concludedthat Al-phosphateminerals in the presenceof phosphoric acid are far more stablethan any Al-silicate.This meansthat if Al-silicateminerals have formed by hydrolysisof micasand feldsparsand if the hydrolyzingfluid containsany of an HsPOncomponent, Al-phosphate minerals will also form. Introduction mine and others in the western U.S. forms by high Al-phosphateminerals, most notablylazulite, oc- temperature hydrolysis of meta-volcanic, sericitic hydrolysis processes cur with Al-silicate-quartzmasses in severallocalities schists.This is an extensionof (1964), Eugster in the world, for exampleHorrsjdberg and Vdstan6, discussed by Hemley and Jones (1970), (1971), Wintsch (1975).If the Sweden;Graves Mountain, Georgia; and White Gresens and are accompaniedby Mountains,California. Three Al-phosphateminer- H+ ions in the hydrolizing fluids and feldspars are als-berlinite, AIPOo; augelite,AlrPO4(OH)r; and Cl- anions only, the muscovite (pyrophyllite, andalusite,or trolleite, A11(POn)s(OH)r-werefirst found with convertedto Al-silicates are presentin the hematite,kyanite, pyrophyllite, and quartzat Vista- kyanite). However, if other anions fluoride leads n6; however,no studyof the mineralsin situ hasever fluid, new phasesappear. For example, to alunite; beenmade (Geijer, 1963). Augelite and trolleite occur to the crystallization of topaz; sulfate, and phos- in well-exposedoutcrops at the Mono County An- borate, to tourmaline or dumortierite; dalusiteMine, White Mountains,California (Gross phate, to lazulite or other Al-phosphates. phosphate and Parwell,1968). The replacementtextures with In order to evaluate the hypothesized phase in the system quartz,andalusite, and muscovite indicate these min- reactions,sub-solidus relations This paper presents eralshave a metasomaticorigin. AlrOa-AlPOr-HrO were studied. that the stabil- Wise (1975) suggestedthat the assemblagethe resultsof that study, which shows place fairly narrow quartz*Al-silicate*Al-phosphate*rutileof this ity fields of augeliteand trolleite limits on the P-T range in which the phosphate- bearing assemblageof the White Mountain deposit I Presentaddress: Department of GeologicalSciences, Virginia the PolytechnicInstitute and State University,Blacksburg, Virginia was formed. Extrapolation of the data allows 24061. conclusion that Al-phosphates in the presenceof 409 4ta W. S. IYISE AND S. E, LOH phosphoric acid are far more stable than any Al- The precipitateheated at 600'C for l2 hours yielded silicate. berlinite. Reagent-gradeAl(OH), was used in some runs, as was alpha-AlrOr, prepared by heating The system AlrOs-AlPOn-HrO Alcls.6HrO at 1000'C for 20 hours. A charge was approximately 30 mg of mix or various Preuious work synthetic phaseswith l0 to 15 mg HrO. The phases that appear in the system The physicalproperties of the syntheticphases are AlrOs-P2Ob-HrO at temperaturesabove 300oC are presentedin the appendix. shown in Figure L Diaspore-corundum equilibria along the AlrOa-H20 join have been recentlystudied Stability of augelite and tolleite (1972). by Haas Berlinitewas found to be isostruc- Two univariant curves for the reactionsthat limit tural with quartz by Huttenlocher (1935), and the the stability of augeliteand trolleite are: transitions to polymorphs analogous to the silica trolleite: augelite* 2 (l) polymorphs were determined by Gruner (1945) and berlinite Beck (1949). Augelite does not appear to have been augelite: berlinite* /z corundumI lVzHzO (2\ previousfy synthesized,but Sclar et al. (1963) re- ported the synthesisand propertiesof trolleite. Runs defining the P-T positions of thesecurves are listed in Table 1, and plotted on E xpe riment al pro cedure Figure 2. Augelite was easily prepared, and the breakdown Berlinite, augelite,and trolleite wereall synthesized reaction was reversed.Reactions normally went to in sealed gold t/z tubes at water pressuresof to 3 completion in two weeks,except when the temper- kilobars in standard, externally heated, cold-seal ature was within 5'C of the phase boundary. Trol- pressure vessels.Tempe ratures were regulated to leite generallynucleated around ,clustersof berlinite, within a3o, and pressuresto within *10 bars.Indi- preventing the reaction from going to completion. viduaf runs completely crystallized,and yielded ap- Nevertheless,the direction of the reaction was easily proximately 30 mg of fine-grainedcrystalline aggre- determined,even for runs closeto phaseboundaries. gates,which were identifiedby X-ray diffraction and optical methods. Discussionof results Starting materialswere mixes of Al(OH), or Al2O3 Since the two reactionsinvestigated involve three with AlPOo. The AIPO. was prepared by dissolving components(Al2Oa, AlPO4, and HrO) and five phases reagent-grade Al wire in diluted (l: l) phosphoric (corundum, berlinite, trolleite, augelite,and vapor), acid, which produces a precipitate of AlpOo.2HrO. there should be two more univariant curves. augelite: 7: trolleite* /: corundum* HrO (3) trolleite: 3 berlinite* 7: corundum-t lVzHzO (4\ Tlrle l. Experimentaldata usedto define univariant curves R@ p(kb) ';Lltffi) producrsr Nuber !("c) "::]il:H. lrB I+82(2) J-/2 tto \82(2) r/2 l+22 B+ C rr-3 A+B 37t(1) A+B+T 1214 tr1-1 39r(r ) 590 A+B r09 A !91(t) t!) 110 B+C \9r( r) 5V> B+ C 53 B+C \21(3) \98 T+B ?2 trl-1 )r29Q) \j9 B+A 73 B+C 5ol+(31 183 B+C ,o A 505(1 ) 2 1?q A o) A 509(5) 2 \96 B+C 106 tra-a 462(r) 3 332 r(+ B) Al2 7\ trl-1 155(3) 3 287 B+A 83 50r(2) 3 rbo A(+ B) 89 ,zo(2) 3 304 B+C Flc. l. Phasesin the system AIrOs-prOb-HrO above 300oC The experimentalphases, labelled with letters.are A : augelite, = = .uindaT = B berlinite,C corundum, D : diaspore,T = trolleite, and abbteviations : A augelite, B = betTinite. C cot@dum. ? = ttoTTeite, and til.-l = bet-Zinite , V : vapor, composed mostly of water. allog) j. EQUILIBRIA AND ORIGIN OF MINERALS IN THE SYSTEM AI|OA.AIPON.H2O 4ll curvesare consistentwith a Schreinemakersanalysis of the univariant curves around the invariant point. The completediagram (right portion of Fig. 3) shows that trolleite is stabilizedwith high pressure,which supports the observationsof Sclar el a/. (1963).The stability of augelite,however, is abruptly terminated near 525oC regardlessof the pressure. Below about 400'C diaspore will appear in phase a aao o ttE o ll I assemblages.This additional phaserequires an addi- tional invariant point, at which diaspore, trolleite, 6 GI augelite, corundum, and vapor are in equilibrium. I o- The univariant curvesemanating from this point are: = rrl diaspore /r corundum* VzH"O (5) I I : I 3 augelite trolleite* 2 diaspore+ 2H2O (6) / ao 6 diaspore* trolleite= 3 augelite* 2 corundum (7) I augelite = 7: trolleite * % corundum * HrO (3) 400 500 Since the P-T curve for reaction 5 has been deter- (1972) and the curve for reaction (3) Tcmp. (cC) mined by Haas is reasonablyfixed by the discussionabove, the inter- Frc. 2. P-T plot of the experimental data, defining the stability limits of trolleite and augelite. Closed circles indicate trolleite is stable; open circles, augelite * berlinite are stable. Closed squares indicate augelite is stable, open squaresindicate berlinite * corun- dum * water vapor (augelite bulk composition) are stable. The experimentally determined curves can be pro- jectedto the invariant point. Reaction(1) involvesno vapor phase, and therefore appears as a nearly straight line in terms of P and Z. The equation for this lineis I('C) : 0.0373P(bars) * 350.7.A plot of reaction(2) in terms of log P and l/T ("K) yieldsthe straight line, log P (bars) = -14,938/T('K) + 22.48. Projection of the two linesgives an intersectionnear 4t 520'C and 4500 bars. I to For reactions(1) and (2), LH was calculatedfrom o the Clapeyron equation, dP/dT : AH/TAV. The f o- slope for reaction(l) was assumedto be linear to the invariant point, whereasthe slope for the other reac- tion was calculatedby differentiatingthe log P - 1/T expression.Values for AV are from 25oC, densitiesof the solid phasesand specificvolumes of water from Burnham et al. (1969). The values obtained, for 520oC, are AIl0(l) : Il.7 kcal and AHo(2) : 83.0 kcal. Calculation of A/{ for reactions(3) and (4) from AH (3) : 2AA,H (2) - h AH (t) : 51.4kcal and Frc. 3. P-T diagram illustrating the two invariant points and LH (4): Afl (l) + AH (2) :94.7 kcal univariant curvesin the systemAlrO.-AlPOn-HrO Mineral sym- bols are the same as in Fig. l. The heaviercurves are experimen- allowed the calculationof curve slopes(Fig. 3). Both tallv determined(D : C * V is from Haas, 1972). 4t2 W. S. WISE AND S. E. LOH sectionof the two curveswill determinethe invariant point (Fig. 3). This intersectionoccurs at such a high pressureas to be geologicallyunimportant.