Solubility of the Pyrochlore Supergroup of Minerals in Supercritical Aqueous Fluoride Solutions

A review of experimental data on the solubility of niobium and tantalum oxides and oxyfluorides in fluoride solutions is performed. Using experimental data on the solubility of pyrochlore (CaNa)Nb₂O₆F and microlite (CaNa)Ta₂O₆F the thermodynamic properties of these minerals for 300–800°С have been calculated. Thermodynamic properties of albite, andalusite, muscovite, paragonite, and pyrophyllite for the temperature range 300–550°С have been clarified. Thermodynamic calculations modeling the influence of solution composition and mineral aluminosilicate associations on the solubility of pyrochlore and microlite in the supercritical region of physicochemical parameters have been carried out. Calculations show that the solubility of the considered ore minerals is very low and the removal of niobium and tantalum by metamorphosed solutions is not possible. The mechanisms of HF accumulation are proposed, which may play an important role in niobium and tantalum dissolution, recrystallization and replacement of ore mineral phases at limited distances.