Thermodynamic activity of WO 3 in wolframite

The article discusses the structure of wolframite revealed by chemical analysis of the geological samples from Russian occurrences described in B.F. Barabanov’s monograph “Mineralogy of wolframite occurrences of Transbaikalia”, 1975. The other source is “Review of the Bulgarian Geological Society”, 1992. After the literature analyses the new facts were established, in particular, that the ratio of moles of WO 3 in wolframites to the sum of moles of FeO and MnO is constant and equal to 1.0. Moreover, the changes in the composition of wolframite are determined by the ratio of FeO/MnO at a constant value of WO 3. Linear variation of crystal lattice parameters of wolframite at the entire range of compositions from FeWO 4 to MnWO 4 proves the assumption that wolframites are solid solutions of the isomorphic range FeWO 4 - MnWO 4 without intermediate phases of the general formula (Fe, Mn)WO 4. This conclusion is confirmed by the results of the research made on aluminothermic reduction of wolframite. It was experimentally proved that WO 3 always reduces first from wolframites but at a different temperature depending on the composition of the solid solution. Low-temperature aluminothermic reduction of wolframite occurs when the WO 3 vapor interacts with the molten aluminum. The thermodynamic activity of WO 3 in wolframites was determined by the Langmuir method, when the authors compared not the WO 3 vapor pressure of the test sample of wolframite and standard (pure WO 3) but the mass of evaporated wolframite and standard. This increased the reliability and accuracy of the experiment. Experimental error is ± 0.02 %. The novelty of the research is that it was for the first time that the activity of WO 3 in wolframite was determined in the whole range of the isomorphic composition FeWO 4 - MnWO 4. Electron microscope JEOL JSM-6460 LV and derivatograph Q-1500D were used to provide high accuracy of the experimental results.