Niobium and Tantalum in Hydrothermal Fluids: Thermodynamic description of Hydroxide and Hydroxofluoride Complexes

Abstract Available experimental data on the solubility of β -Nb2O5 and β -Ta2O5 in the Nb–O–H–F and Ta–O–H–F systems were critically evaluated, and the thermodynamic properties of hydroxide and hydroxofluoride complexes were estimated in a wide range of temperatures (0 - 600 °C) and pressures (1 – 3000 bar). In near neutral pH solutions, the solubility of the oxides is governed by uncharged aqueous hydroxides, Nb(OH)5 and Ta(OH)5, and is rather low even at high temperatures (1 – 9 ppm for Nb and 0.1 – 0.9 ppm for Ta at 600 °C, 1000 bar). In acid HF bearing solutions, the solubility is governed by Nb(OH)2F3 and Ta(OH)2F3 complexes and increases sharply with increasing HF molality: at mHF = 0.1 mol·kg–1, 600 °C and 1000 bar, the Nb and Ta concentrations reach 80 and 30 ppm respectively. In alkaline fluoride bearing solutions, the HNbO3F– and HTaO3F– - oxyfluoride anions control the solubility of the oxides. The temperature and pressure dependences of the oxides solubility governed by these fluoride complexes are rather weak; therefore, the neutralization of acid fluoride solution and the loss of fluoride ligand during fluid evolution can be the main factors of Nb and Ta deposition.