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

Preliminary measurements were carried out of the solubility of the O2-buffering assemblage bismuth + bismite (Bi2O3) 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 (±1σ) 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 (±1σ) 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.