Solubility of Pyrochlore-Supergroup Minerals in Supercritical Aqueous Fluoride Solutions

Experimental data on the solubility of niobium and tantalum oxides and oxyfluorides in fluoride solutions were reviewed. Experimental solubility data for pyrochlore (CaNa)Nb2O6F and microlite (CaNa)Ta2O6F were used to calculate their thermodynamic properties at 300–800°C. The thermodynamic properties of albite, andalusite, muscovite, paragonite, and pyrophyllite were refined in the temperature range 300–550°C. The influence of solution composition and aluminosilicate mineral association on the solubility of pyrochlore and microlite in the supercritical region of physicochemical parameters was modeled by thermodynamic calculations. The calculations showed that the solubility of the minerals is very low, and niobium and tantalum cannot be removed by metamorphosed solutions. Mechanisms were proposed for HF accumulation, which could play an important role in niobium and tantalum dissolution, as well as recrystallization and replacement of ore mineral phases over limited distances.