Amertcan Mineralogist, Volume 66, pages 298-308, l98l Phaserelations of sometungstate minerals under hydrothermalconditions L. C. Hsu Nevada Bureau of Mines/Geologlt and Department of Geological Sciences, Mackay School of Mines University of Nevada, Reno, Nevada 89557 Abstract The binary phaserelations ofseveral naturally-occurringtungstats mins14ls, ofthe general form M2*WOaew€t€ investigatedhydrothermally at fluid pressureof 1.0 kbar and temper- aturesof 400o-900oc.A completesolid solution, with tetragonalsymmetry, between scheel- ite (M2* - Ca) and Stolzite(Pb) forms above725"C.The solvusis alnost symmetricalwith a crest near CaorPbrr.The observedlinear variation ofthe 116interplanar spacing, 1.687(2)A for Ca to 1.782Q)Afor Pb, may be used to determinecomposition. Sanmartinite, (ZnWOa) forms a complete solid solution of monoclinic symmetry with both ferberite (FeWOa), and huebnerite(MnWOo), down to at least400oC. The interplanar spacingof the 200reflection of these solid solutions is also observedto vary linearly with composition. Solid solution is very limited betweenmonoclinic and tetragonaltungstates. Less than 5 mole percentmiscibility of one in the other is observedat temperaturesup to 900oC. It appearsthat the solid solution among thesetungstate minerals is affectedstrongly by both the availability and environment of the structural sitesfor specific divalent cations. Even where structural limitations perrnit extensivesubstitution, physicochemical characteristics of elements such as Zn ar.d Pb mav still restrict the occurrence of solid solutions in nature. Introduction studieson zonedcrystals of ferberitic wolframite con- Naturally-occurring tungstate minerals belong to taining scheelite inclusions from Australia. two structural groups. One, including scheelite Among the monoclinic tungstate minerals, a com- (CaWO.) and stolzite (PbWO4), crystallizes in the plete solid solution series between FeWO4 and tetragonal systemwith space grotrpl4r/a: the other, MnWOo has long been known in nature, and Hsu including ferberite (FeWOo), huebnerite (MnWO.) (1976)experimentally demonstrated that the compo- and sanmartinite(ZnWO.), crystallirss in the mono- sitional variation of this seriesin terms of FelMn ra- clinic system with space group F2/c. tio can:rot be used to evaluate temperaturesof ore The subsolidusphase relations between scheelite formation. Chang (1968)showed that the solid solu- and many other tungstateswith divalent cations,ex- bility in the system MnWOo-ZnWOo is complete cept Fe2*,were studied by Chang (1967) at temper- above 840oC,below which temperaturea broad mis- aturesfrom 550oto I150'C under anhydrousatmo- cibility gap exists.On the other hand, Chernyshevet spheric conditions. He found a complete solid al. (1976)using hydrothermal techniquesdetermined solution betweenCaWOo and PbWOo above 8l5oC that no miscibility gaps are observedin the systems with a broad miscibility gap below that temperature. FeWOo-ZnWOo and MnWOo-ZnWOo at temper- According to Chang, the system CaWOo-MnWOo ature down to 500'C at 1000atm. shows extremely low mutual solid solubility; that is, PbWO4 in nature also occurs as the monoclinic CaWOo takes up l0 mole percent MnWO. only at a mineral, raspite, which has a structure (spacegroup temperature above I l00oc, whereasMnWOo takes F2,/a) different from other monoclinic tungstate up only 2.5 mole percentCaWOo at the sametemper- minerals.Raspite is 6 percentdenser than stolzite,us- ature. Yet Grubb (1967) reported that an almost ing data of Fujita et al. (1977)for raspiteand thoseof complete solid solution series between wolframite, Plakhov et al. (1970)for stolzite.Raspite is reported (Fe,Mn)WO., and scheeliteexists at elevatedtemper- to transform irreversibly to stolzite at 400oC (Shaw atures, based on his chemical and X-ray diffraction and Claringbull, 1955).Raspite has so far defied lab- 0m3-ffi4X / 8| / 0304-0298$02.00 298 HSU: PHASE RELATIONS OF TUNGSTATE MINERALS oratory synthesis and its relation with stolzite re- for eachdetermination. Measurements were made to mains unsolved(e.g., Chang, 1971). 0.005o20 and resultsaveraged. The purposeof this investigationwas to resolvethe PhaseRelations above-mentionedconflicting conclusions and, in gen- eral, to determine as much as possible about tung- The subsolidusphase equilibria for nins of the ten state minerals and mineralization by using conven- possiblebinary systemswere determinedat Pr: 1.9 tional hydrothermal techniques and facilities at P-T kbar and 7 : 400-900oC. These nine systemsare ranges simulating those involved in natural pro- Ca-Pb, Fe-Zn, Mn-Zn, Ca-Fe, Ca-Mn, Fe-Pb, cesses. Mn-Pb, Ca-Zn, and Pb-Zn. The first three involve end membersof the samestructure, the rest involve Experinental end members of differing structures. Phase relations Conventional hydrothermal techniques were em- for the remaining binary system, Fe-Mn, were re- ployed throughout the work. All the experiments ported previously (Hsu, 1976). were made in cold-seal pressure vessels (Tuttle, The systemCaWO o- PbITO 1949).Details of the experimental apparatus and P- " Z calibrations are describedelsewhere (Hsu, 1976). A complete solid solution between scheelite and Three tungstate end members, CaWOo, PbWOo stolzite forms above 725"C. The solvus is alnost and ZnWOo were readily preparedby chemical pre- symmetrical with a crest near CaorPbrr.As is shown cipitation of reagentgrade NarWO4'2H2O solution, in Figure l, the position and shapeof the solvusare respectively,with reagentgrade CaCl', Pb(NO,), and quite diferent from those previously found (dotted Zn(NOr)r' 6HrO solutions in stoichiometricpropor- tine) by Chang (1967) under anhydrous, one atmo- tions. After filtering, washing, and firing these precip- sphere conditions. The difference in the results of the itates at 700'C for 24 hours, mixtures of the three bi- two studies may be attributed principally to the slow- nary systemswere prepared at l0 mole percent ness of diffusion and attainment of equilibrium in intervals by blending two respectiveend members. dry crystalline powders. Here in view of the nature of The other two end members FeWOo and MnWOo the starting material being used, hydrous conditions, cannot be readily prepared by simple chemical pre- and the sufficient duration of runs being adopted, cipitation. Mixtures of thesebinary systemswere pre- equilibrium was consideredto be attained.A reversal pared by thoroughly mixing reagent grade tungstic of reaction was demonstratedby Run #5Ca5Pbl3 in acid with respectivemetal (electrolytic iron and man- which a homogeneousphase formed at higher tem- ganese)powders and metal oxides (reagent grade peratureunderwent gnmiving, although a still longer CaO, PbO, and ZnO) stoichiometrically,also at l0 run duration is needed as indicated by the measured mole percent intervals. Additional mixtures at 5 mole interplanar spacings. percent intervals were prepared near two end mem- The intervals between intermediate members bers where necessary. plotted in Figure I are not l0 mole percent as in- Each run was sealedin a noble metal capsule with tended, because a formula weight for CaWOo of excessdistilled water, brought to temperature,and 207.9276instead of 287.9276was used accidentally in quenchedisobarically at a fluid pressureof 1.0 kbar. preparing mixes. This mistake was discovered while Whenevernecessary, the samerun was repeatedsev- plotting the drrcvalues of the two end membersand eral times at the same temperature for different peri- nine intermediate members composition on the bi- ods to ensureequilibrium. Somehomogeneous high- nary. Correct mole percentcompositions are plotted. temperature products were run at lower temperatures The intensity of the 116reflection remains consis- to test whether exsolution had occurred (e.9., Run tently high throughout the compositional range. The #5Ca5Pbl3). d,,u value varies linearly with compositionas shown Condensed run products were examined with a in Figure 2. Using clear natural quartz as an internal petrographic microscopeand a Norelco X-ray dif- standard,with its 112reflection calibrated against the fractometer equipped with a graphite mono- 220 rcflectionof Si(a : 4.43012L),the d,r" value was chromator and scintillation counter and using either found to vary linearly from 1.687(2)Afor scheeliteto CuKa or FeKa radiation. The interplanar spacings I.782Q)A for stolzite.The critical runs for this sytem were obtained by choosing appropriate internal stan- are listed in Table l. dards. Three oscillationsat l/4" 20 per minute scan Scheelite is the only tungstate mineral which con- speedand l/2 n. per minute chart speedwere made sistently yields a blue to whitish fluorescent color un- HSU: PHASE REI../ITIONS OF TUNGSTATE MINERALS Pf "lKb o oo o o o o o oo o o o o A o a o o A o o o A A. o A A o \ \ 0 40 60 roo CoWOo Pbw04 (c) Mole "/" PbWO4 (P) Figure l. T-X diagram at P1 : 1.9 kbar for the Ca-Pb system. Solid circles: singte solid solution; opea triangles: two solid solutions Dotted line is the solvus after Chang (1967). A r.soo T = 75O"C P1=LOKb 40 60 80 too Pbwo4 Molc 96 PbWO4 (P) Figure 2. Variation of d,,6 with compositionin the Ca-Pb systcm. HSU: PHASE REI'IITIONS OF TUNGSTATE MINERALS 301 Table l. Run data for the Ca-Pb system der short-wave ultraviolet Ught. With 2 mole peroent CaMoOo in solid solution, scheelite quickly changes Duration Run Product Run No. hrerial T"C (days) arrOi* its fluorescent color to yellow and remains essentially solid solution series 9Ca1Pb2 Tungsrete nlx 900 2 single solid sofn. so througftout