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Cawo4-Fewo4-Mnwo4 Under Supercritical Condition

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Cawo4-Fewo4-Mnwo4 Under Supercritical Condition Geochemical Journal, Vol. 23, pp. 339 to 347, 1989 Experimental studies on ion exchange equilibria between minerals and aqueous chloride solution in the system CaWO4-FeWO4-MnWO4 under supercritical condition ETSUO UCHIDA, MASATAKA GIMA and NAOYA IMAI Department of Mineral Resources Engineering, School of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169, Japan (Received December 1, 1989; Accepted February 16, 1990) Ion exchange experiments in the system CaWO4-FeWO4-MnWO4-(Ca, Fe", Mn2+)C12-H20, cor responding to scheelite-ferberite-huebnerite series mineral as solid phase, were carried out at 400 and 600°C under 1000 kg/cm2. Ferberite and huebnerite form a continuous solid solution (wolframite ss), which may exhibit ther modynamically ideal behavior. Also, ferrous ion tends to concentrate preferably into wolframite ss than into aqueous chloride solution. This tendency becomes more pronounced with decreasing temperature. Scheelite and wolframite ss do not dissolve into each other essentially, but form a wide miscibility gap. The Ca /(Ca+Fe2++Mn2+) mole ratios of aqueous chloride solution coexisting with both minerals are about 0.65 at 400°C and 0.3 at 600°C. Temperature has a significant effect on the mole ratio, although a slight variation due to the compositional change of wolframite ss is recognized. The present experimental results cast a doubt on "wolframite geothermometer". series mineral, which belong to R"W04-type INTRODUCTION compound, represent the major source of Experimental studies of ion exchange tungsten in nature. However, such data for the equilibria between minerals and aqueous tungsten-bearing minerals have not been furnish chloride solution provide us with information on ed except for a preliminary work by Baumer et thermodynamic properties of minerals and al. (1985). In the present study, therefore, we car aqueous species, which are also important basic ried out ion exchange experiments in the system data for estimating the chemical compositions of CaWO4-FeWO4-MnWO4-(Ca, Fe e+, Mn2+ )C12 ore-forming fluids. This is partly the reason why H20 under supercritical condition. We deter experimental studies on ion exchange equilibria mined relative concentrations of calcium, fer have been carried out by many workers; e.g., rous, and divalent manganese ions in the Wyart and Sabatier(1956), Orville(1963), and aqueous chloride solution which coexists with Iiyama(1966) for alkali feldspars; Iiyama(1966), these solid phases in equilibrium. An attempt and Orville(1972) for plagioclase; Roux(1974) was also made to clarify the thermodynamic for nepheline, Schulien et al.(1970) and Bar properties of wolframite ss series. tholomew(1989) for olivine, Iiyama(1964), Schulien(1980), Pascal and Roux(1985), and ABBREVIATIONS, SYMBOLS AND CONSTANT Flux and Chatterjee(1986) for micas, Uchida(1982) for clinopyroxene, and Lehmann aq: in aqueous solution and Roux(1986) for spinel. fer: ferberite Scheelite and wolframite solid solution(ss) hue: huebnerite 339 340 E. Uchida et al. sch: scheelite periments. Wolframite ss is known to be stable wol: wolframite solid solution (ss) under these conditions of oxygen fugacities A G: Gibbs energy of reaction (Hsu, 1976). A G°: standard Gibbs energy of reaction After welding, the charged capsules were put A GEX: excess Gibbs energy of reaction into the pressure reactor of cold seal-type made Ga,EM: excess Gibbs energy of mixing for of Stellite-25 alloy. Pressure was measured by a phase a Heise gage (scale interval of 5 kg/cm2) with the Kd: distribution coefficient range of 0 to 2000 kg /cm', and was kept at 1000 m,: molality of component i kg/cm2 for every run of the present experiments. R: gas constant The uncertainty was within ± 10 kg/cm2. The T: temperature in Kelvin temperatures were measured with chromel W: interaction parameter in regular s olu alumel thermocouple, attached to the outside tion model wall of the reactor. In the present experiments, X7 a, mole fraction of component i in phase the temperatures of furnace were kept at 400°C a or 600°C, and controlled within ±5°C. Run Pa: chemical potential of component i in duration varied from 7 to 11 days at 400°C and phase a from 4 to 8 days at 600°C. The achievement of ?,a. ,ui standard chemical potential of compo equilibrium in the experiments was examined us nent i in phase a ing an internal consistency of Gibbs energy of a,EX: I excess chemical potential of component reaction as described later. i in phase a After the run, the charge was quenched by removing the reactor from the furnace and imme diately placing it in cold water. After the check EXPERIMENTAL PROCEDURES for leakage, gold capsules were opened and the A stoichimetric mixture of reagent-grade run products were washed away into a beaker calcium hydroxide and tungsten trioxide was with distilled water. Solid products were heated at 800°C for one day. The burned cake separated from aqueous chloride solution with thus obtained and ground was used as a starting membrane filter of 0.45,um pore size made by material for scheelite. Reagent-grade metallic Millipore Co. Solid phases were identified with a iron powder and metallic manganese powder polarization microscope, an X-ray diffrac were stoichiometrically mixed with tungsten tometer and a scanning electron microscope. In trioxide and used for ferberite and huebnerite, re order to avoid the effects of zoning and inclusion spectively. of starting material in synthesized minerals, Twenty to 50 milligrams of the starting chemical analysis on the grain surface was done materials and 10 to 50 milliliters of aqueous (Ca, using a JEOL 733 microprobe with EDS TN Fe", Mn2+)C12solution with 1 molar concentra 5400. In this study, surface equilibrium of tion* (1 mol/1) were sealed in gold capsules with minerals with aqueous chloride solution was a 3.0 mm outer diameter, a 0.15 mm wall assumed. The analytical precision of minerals is thickness, and a length of 30 to 35 mm. A few considered to be within ±2 mole%. The milligrams of anthracene were added as a reduc chemical composition of liquid phase was deter ing agent to keep iron and manganese ions in a mined by means of atomic absorption spec divalent state. The oxygen fugacity is considered trophotometry. to be controlled between FMQ and IM buffers or between FMQ and MW buffers during the ex *The difference between molarity(M) and molality(m) in aqueous chloride solution is not so large in this work. Accordingly, we assume in this study that molarity is equal to molality. Ion exchange equilibria between tungstates and chloride solution 341 other phase. The degree of crystallinity of these EXPERIMENTALRESULTS synthesized minerals is similar to the case of the The system CaW04-FeW04-(Ca, Fe2+)Cl2-H20 system CaWO4-FeWO4-(Ca, Fe 2+ )C12-H20 The experimental results of this system are The Ca /(Ca+Mn) mole ratio of the aqueous summarized in Fig. 1. Solid products consist of chloride solution coexisting with both scheelite scheelite and ferberite in every run. Under the and huebnerite is 0.644 at 400°C and 0.319 at present experimental conditions, scheelite and 600°C. These results are similar to those of the ferberite show extremely limited solubility in system CaWO4-FeWO4-(Ca, Fe 2+)C12-H2O. Ac each other phase. Both scheelite and ferberite cordingly, the thermodynamic behavior of formed at 600°C are well-crystallized and their ferberite is expected to be analogous to that of grain size reaches up to 20 ym (Fig. 2B). On the huebnerite. other hand, these minerals formed at 400°C show poor crystallinity and are accompanied by The system Ca WO4-FeWO4-(Fe2+, Mn2+)Cl2 fine particles of the same minerals (Fig. 2A). H20 The Ca/(Ca+Fe) mole ratio of the aqueous The experimental results of this system are chloride solution coexisting with both scheelite summarized diagrammatically in Fig. 4. Under and ferberite is 0.657 at 400°C and 0.381 at the present experimental conditions, ferberite 600°C, indicating that a temperature and huebnerite form a continuous solid solution dependence of the values is conspicuous. series; i.e., wolframite ss series. Under the polarization microscope, the synthesized The system CaWO4-Mn W04-(Ca, Mn2+)Cl2 wolframite ss is opaque near the end member H20 composition of ferberite, and with increasing The experimental results are summarized in huebnerite component, it becomes transparent Fig. 3. Solid products consist of scheelite and gradually with a colour of reddish brown. The huebnerite in every run. Scheelite and huebnerite crystallinity of wolframite ss is poor at 400°C also show extremely limited solubility in each (Fig. 2E). Ferrous ion tends to be concentrated into wolframite ss as compared with aqueous chloride solution at 600°C. This tendency 1.0 becomes more pronounced at 400°C. 0.8 The system CaWO4-FeWO4-Mn W04-(Ca, 00 Fe2+, Mn2+)CI2-H20 0 for 4000C sch 0 The experimental results of this system are 0.6 0 summarized diagrammatically in Figs. 5 and 6. U, Solid products consist of scheelite and 600°C wolframite in every run. 0.4 Figure 5 shows the relationship between Fe / (Fe + Mn) mole ratio of wolframite ss and 0.2 that of the coexisting aqueous chloride solution. The results are consistent with those obtained in the system FeWO4-MnWO4-(Fe2+, Mn2+ )C12 0 0.2 0.4 0.6 0.8 1.0 H20. Ca/(Ca+Fe) solid Figure 6 shows the relationship between Fig. 1. Ion exchange isotherms in the system Fe/(Fe+Mn) mole ratio of wolframite ss coex CaWO4-FeW04-(Ca, Fe2+)Cl2-H20 at 400°C(circle) isting with scheelite and Ca / (Ca + Fe - Mn) and 600°C(square) under 1000 kg/cm2.
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