Niobium in hydrothermal systems related to alkali granites: Thermodynamic description of hydroxo and hydroxofluoride complexes

Abstract

Available experimental data on the solubility of Nb₂O₅ and the stability constants for particles of an aqueous solution in the Nb–O–H–F system were processed. As a result, a set of thermodynamic properties for 25°C and 1 bar was obtained, in addition to the equation parameters for the HKF model (Helgeson–Kirkham–Flowers) for hydroxo and hydroxofluoride niobium complexes. F^– ion is the most important factor governing the concentration of dissolved Nb: neutral hydroxo complex Nb(OH)₅(aq) is formed at a low HF concentration, whereas an increase in HF results in an increase in the first Nb(OH)₄F(aq) and second Nb(OH)₃F₂(aq) fluoride complexes. The Nb(OH)₅F^– oxofluoride anion determines oxide solubility in alkali F-bearing fluids. Neutralization of acidic fluoride solution can be the main factor leading to niobium deposition.