An experimental study of cassiterite solubility in HCl-bearing water vapour at temperatures up to 350 °C. Implications for tin ore formation

The solubility of cassiterite was investigated experimentally in liquid-undersaturated HCl-bearing water vapour at temperatures of 300 to 350 °C and pressures up to 180 bar. Concentrations of tin varied between 0.1 and 32 ppb, depending on experimental conditions, and reached a maximum at 320 °C. These concentrations are at least four orders of magnitude higher than those reported for the water-free system. Positive correlation of the concentrations of tin with fH2O and fHCl indicate that cassiterite solubility is enhanced by solute–solvent interaction, and that this involves the formation of hydrated gaseous species of tin chloride having a tin–chlorine ratio of 1:2, and statistical hydration numbers of 2.9, 2.7, and 2.5 at 300, 320, and 350 °C, respectively. We therefore propose that the solubility of cassiterite in HCl-bearing water vapour is explained by the reaction: SnO 2 cassiterite + 2 · HCl gas + n · H 2 O vapour = SnOCl 2 · ( H 2 O ) n + 1 vapour ations based on this reaction and data for the solubility of tin in aqueous liquids indicate that, under all conditions, tin partitions preferentially into the latter phase. However, in high salinity, two-phase aqueous systems, where vapour pressures can be considerably higher than those in our experiments and chlorinity is not controlled by fHCl, but by NaCl content, tin solubility in the vapour phase may be appreciable.