The low-temperature crystallization of (Fe,Mn)WO 4 (wolframite), (Zn,Fe)WO 4 (sanmartinite) and (Zn,Mn)WO 4 solid solutions under hydrothermal conditions

Abstract

Data concerning the hydrothermal synthesis of the solid-solution series (Fe,Mn)WO 4 (wolframite), (Zn,Mn)WO 4 and (Zn,Fe)WO 4 (sanmartinite) in the temperature interval 200–450°C at pressures of 0.5·10 8–1.0·10 8 Pa are reported. The chemical composition of the mixed crystals was investigated by electron probe microanalysis and X-ray diffraction. At a temperature of 250°C complete miscibility of FeWO 4 and MnWO 4 obtains. At lesser temperatures (200°C) the Fe-rich phases exhibit pronounced compositional variations. The kinetics of the crystallization in this heterogeneous interval was examined more closely at 200°C. Variation of the pH-value between 4 and 9 does not influence the mixed-crystal proportions in the temperature range considered here. In the system ZnWO 4/FeWO 4, a complete series of solid solution forms down to 200°C. On the contrary, for Zn/Mn-tungstates at 210°C only a heterogeneous product could be identified for initial solutions containing 50 mole% ZnWO 4. The solubility of ZnWO 4 was calculated and compared with available data of ferberite and hübnerite. The absence of Zn 2+ in natural wolframites is discussed in consideration of miscibilities and thermochemical data.