Phase relations of some tungstate minerals under hydrothermal conditions

The binary phase relations ofseveral naturally-occurring tungstats mins14ls, ofthe general form M2*WOae w€t€ investigated hydrothermally at fluid pressure of 1.0 kbar and temperatures of 400o-900oc. A complete solid solution, with tetragonal symmetry, between scheelite (M2* Ca) and Stolzite (Pb) forms above725"C. The solvus is alnost symmetrical with a crest near CaorPbrr. The observed linear variation ofthe 116 interplanar spacing, 1.687(2)A for Ca to 1.782Q)A for Pb, may be used to determine composition. Sanmartinite, (ZnWOa) forms a complete solid solution of monoclinic symmetry with both ferberite (FeWOa), and huebnerite (MnWOo), down to at least 400oC. The interplanar spacing of the 200 reflection of these solid solutions is also observed to vary linearly with composition. Solid solution is very limited between monoclinic and tetragonal tungstates. Less than 5 mole percent miscibility of one in the other is observed at temperatures up to 900oC. It appears that the solid solution among these tungstate minerals is affected strongly by both the availability and environment of the structural sites for specific divalent cations. Even where structural limitations perrnit extensive substitution, physicochemical characteristics of elements such as Zn ar.d Pb mav still restrict the occurrence of solid solutions in nature. Introduction Naturally-occurring tungstate minerals belong to two structural groups. One, including scheelite (CaWO.) and stolzite (PbWO4), crystallizes in the tetragonal system with space grotrp l4r/a: the other, including ferberite (FeWOo), huebnerite (MnWO.) and sanmartinite (ZnWO.), crystallirss in the monoclinic system with space group F2/c. The subsolidus phase relations between scheelite and many other tungstates with divalent cations, except Fe2*, were studied by Chang (1967) at temperatures from 550o to I150'C under anhydrous atmospheric conditions. He found a complete solid solution between CaWOo and PbWOo above 8l5oC with a broad miscibility gap below that temperature. According to Chang, the system CaWOo-MnWOo shows extremely low mutual solid solubility; that is, CaWOo takes up l0 mole percent MnWO. only at a temperature above I l00oc, whereas MnWOo takes up only 2.5 mole percent CaWOo at the same temperature. Yet Grubb (1967) reported that an almost complete solid solution series between wolframite, (Fe,Mn)WO., and scheelite exists at elevated temperatures, based on his chemical and X-ray diffraction 0m3-ffi 4X / 8 | / 0304-0298$02.00 studies on zoned crystals of ferberitic wolframite containing scheelite inclusions from Australia. Among the monoclinic tungstate minerals, a complete solid solution series between FeWO4 and MnWOo has long been known in nature, and Hsu (1976) experimentally demonstrated that the compositional variation of this series in terms of FelMn ratio can:rot be used to evaluate temperatures of ore formation. Chang (1968) showed that the solid solubility in the system MnWOo-ZnWOo is complete above 840oC, below which temperature a broad miscibility gap exists. On the other hand, Chernyshev et al. (1976) using hydrothermal techniques determined that no miscibility gaps are observed in the systems FeWOo-ZnWOo and MnWOo-ZnWOo at temperature down to 500'C at 1000 atm. PbWO4 in nature also occurs as the monoclinic mineral, raspite, which has a structure (space group F2,/a) different from other monoclinic tungstate minerals. Raspite is 6 percent denser than stolzite, using data of Fujita et al. (1977) for raspite and those of Plakhov et al. (1970) for stolzite. Raspite is reported to transform irreversibly to stolzite at 400oC (Shaw and Claringbull, 1955). Raspite has so far defied lab298 HSU: PHASE RELATIONS OF TUNGSTATE MINERALS oratory synthesis and its relation with stolzite remains unsolved (e.g., Chang, 1971). The purpose of this investigation was to resolve the above-mentioned conflicting conclusions and, in general, to determine as much as possible about tungstate minerals and mineralization by using conventional hydrothermal techniques and facilities at P-T ranges simulating those involved in natural processes. Experinental Conventional hydrothermal techniques were employed throughout the work. All the experiments were made in cold-seal pressure vessels (Tuttle, 1949). Details of the experimental apparatus and PZ calibrations are described elsewhere (Hsu, 1976). Three tungstate end members, CaWOo, PbWOo and ZnWOo were readily prepared by chemical precipitation of reagent grade NarWO4'2H2O solution, respectively, with reagent grade CaCl', Pb(NO,), and Zn(NOr)r' 6HrO solutions in stoichiometric proportions. After filtering, washing, and firing these precipitates at 700'C for 24 hours, mixtures of the three binary systems were prepared at l0 mole percent intervals by blending two respective end members. The other two end members FeWOo and MnWOo cannot be readily prepared by simple chemical precipitation. Mixtures of these binary systems were prepared by thoroughly mixing reagent grade tungstic acid with respective metal (electrolytic iron and manganese) powders and metal oxides (reagent grade CaO, PbO, and ZnO) stoichiometrically, also at l0 mole percent intervals. Additional mixtures at 5 mole percent intervals were prepared near two end members where necessary. Each run was sealed in a noble metal capsule with excess distilled water, brought to temperature, and quenched isobarically at a fluid pressure of 1.0 kbar. Whenever necessary, the same run was repeated several times at the same temperature for different periods to ensure equilibrium. Some homogeneous hightemperature products were run at lower temperatures to test whether exsolution had occurred (e.9., Run #5Ca5Pbl3). Condensed run products were examined with a petrographic microscope and a Norelco X-ray diffractometer equipped with a graphite monochromator and scintillation counter and using either CuKa or FeKa radiation. The interplanar spacings were obtained by choosing appropriate internal standards. Three oscillations at l/4" 20 per minute scan speed and l/2 n. per minute chart speed were made for each determination. Measurements were made to 0.005o 20 and results averaged. Phase Relations The subsolidus phase equilibria for nins of the ten possible binary systems were determined at Pr: 1.9 kbar and 7 : 400-900oC. These nine systems are Ca-Pb, Fe-Zn, Mn-Zn, Ca-Fe, Ca-Mn, Fe-Pb, Mn-Pb, Ca-Zn, and Pb-Zn. The first three involve end members of the same structure, the rest involve end members of differing structures. Phase relations for the remaining binary system, Fe-Mn, were reported previously (Hsu, 1976). The system CaWO oPbITO " A complete solid solution between scheelite and stolzite forms above 725"C. The solvus is alnost symmetrical with a crest near CaorPbrr. As is shown in Figure l, the position and shape of the solvus are quite diferent from those previously found (dotted tine) by Chang (1967) under anhydrous, one atmosphere conditions. The difference in the results of the two studies may be attributed principally to the slowness of diffusion and attainment of equilibrium in dry crystalline powders. Here in view of the nature of the starting material being used, hydrous conditions, and the sufficient duration of runs being adopted, equilibrium was considered to be attained. A reversal of reaction was demonstrated by Run #5Ca5Pbl3 in which a homogeneous phase formed at higher temperature underwent gnmiving, although a still longer run duration is needed as indicated by the measured interplanar spacings. The intervals between intermediate members plotted in Figure I are not l0 mole percent as intended, because a formula weight for CaWOo of 207.9276 instead of 287.9276 was used accidentally in preparing mixes. This mistake was discovered while plotting the drrc values of the two end members and nine intermediate members composition on the binary. Correct mole percent compositions are plotted. The intensity of the 116 reflection remains consistently high throughout the compositional range. The d,,u value varies linearly with composition as shown in Figure 2. Using clear natural quartz as an internal standard, with its 112 reflection calibrated against the 220 rcflection of Si(a : 4.43012L), the d,r" value was found to vary linearly from 1.687(2)A for scheelite to I.782Q)A for stolzite. The critical runs for this sytem are listed in Table l. Scheelite is the only tungstate mineral which consistently yields a blue to whitish fluorescent color unHSU: PHASE REI../ITIONS OF TUNGSTATE MINERALS