Experimental measurement of the solubility of bismuth phases in water vapor from 220°C to 300°C: Implications for ore formation

Abstract

Abstract Preliminary measurements were carried out of the solubility of the O 2- buffering assemblage bismuth + bismite (Bi 2 O 3 ) in aqueous liquid–vapor and vapor-only systems at temperatures of 220, 250 and 300 °C. All experiments were carried out in Ti reaction vessels and were designed such that the Bi solids were contained in a silica tube that prevented contact with liquid water at any time during the experiment. Two blank (no Bi solids present) liquid–vapor experiments at 220 °C yielded Bi concentrations (±1 σ ) in the condensed liquid of 0.22 ± 0.02 mg/L, whereas the solubility measurements at this temperature yielded an average value of approximately 6 ± 9 mg/L, with replicate experiments ranging from 0.3 to 26 mg/L. Although the 6 mg/L value is associated with a considerable degree of uncertainty, the experiments do indicate transport of Bi through the vapor phase. Measured Bi concentrations in the condensed liquid at 250 °C were in the same range as those at 220 °C, whereas those at 300 °C were significantly lower (i.e., all below the blank value). Vapor-only experiments necessarily contained much smaller initial volumes of water, thereby making the results more susceptible to contamination. Single blank runs at 220 and 300 °C yielded Bi concentrations of 82 and 16 mg/L, respectively. Measured concentrations (±1 σ ) of Bi in the vapor-only solubility experiments at 220 °C were 235 ± 78 mg/L for an initial water volume of 0.5 mL, and at 300 °C were 56 ± 30 mg/L and 33 ± 21 for initial water volumes of 1 and 2 mL, respectively, suggesting strong preferential partitioning of Bi into the vapor. The results indicate a negative dependence of Bi solubility on temperature, but are inconclusive with respect to the dependence of Bi solubility on water density or fugacity. The experiments reported here suggest that significant Bi transport is possible in the vapor phase. Comparison of the liquid–vapor and vapor-only experiments further suggests that, at the conditions studied, the solubility of Bi in the vapor is significantly higher than that in the liquid phase (calculated distribution coefficients, i.e., C Vapor / C Liquid at 220 °C ranged from 81 to 168, where C is concentration in mg/L). The concentrations measured in the vapor-only experiments at 220 °C are approximately 11 orders of magnitude higher than those calculated from available thermodynamic data assuming no interaction between volatile Bi species and water molecules. This finding indicates that hydration of volatile Bi species probably occurs to a significant extent; similar to behavior observed previously in published vapor phase solubility studies of other metals. However, the interpretation of the results was complicated by the formation of Bi silicate phases and the lack of certainty that O 2 fugacity buffering was effective throughout the experiments.