FORMATION OF COMPOUNDS WITH ALUNITE-LIKE STRUCTURE IN THE BiO—AlO—FeO—PO—HO SYSTEM UNDER HYDROTHERMAL CONDITIONS

Under hydrothermal conditions in the BiO—AlO—FeO—PO—HO system, variable-composition compounds Bi(AlFe)(PO)(OH) with an alunite-like structure were obtained. Based on experimental data on the miscibility limits of components in the (1 — x)BiAl(PO)(OH)—xBiFe(PO)(OH) system, the parameters of the subregular solution model (Q = 6.395, Q = 8.987 kJ/mol) were determined, and spinodal and binodal decomposition curves of solid solutions with an alunite-like structure were calculated. Thermodynamic calculations showed that individual BiAl(PO)(OH) and BiFe(PO)(OH) can form at temperatures above 122 and 170°C, respectively, which is consistent with the experimental data obtained in this study, indicating the absence of compounds with alunite-like structure at 160°C in the range 0.8 < x ⩽ 1.