Solubility of stibnite in hydrogen sulfide solutions, speciation, and equilibrium constants, from 25 to 350°C
Solubility of stibnite (Sb 2S 3) was measured in aqueous hydrogen sulfide solutions as a function of pH and total free sulfur (TFS) concentrations at 25, 90, 200, 275, and 350°C and at saturated vapor pressures. At 25 and 90°C and TFS ≈ 0.01 molal solubility is controlled by the thioantimonite complexes H 2Sb 2S o4, HSb 2S -4, Sb 2S 2-4 (and polymers of the general composition H 2-xSb 2nS x-3n+1). At higher temperatures the hydroxothioantimonite complex Sb 2S 2(OH) 02 (or polymers of composition Sb 2nS 3n-1(OH) 02) becomes dominant. Polymerization due to condensation reactions yields long chains made up of trigonal-pyramidal SbS 3-groups (and SbS 2OH-groups). Equilibrium constants were derived for the dimers. The transition from thioantimonites to the hydroxothioantimonite species at approximately 120°C is endothermic and is entirely driven by a gain in entropy. Stibnite solubility calculated for some geothermal fluids indicate that these fluids are undersaturated in Sb if stibnite is the solid equilibrium phase. At high temperatures (>100°C) precipitation of stibnite from ore fluids can occur in response to conductive cooling, while at low temperatures, where thioantimonites dominate, acidification of the fluid ( e.g. due to oxidation of H 2S) is the more likely mechanism. Precipitation of stibnite from fluids containing hydroxothioantimonite consumes H 2S and may thus trigger precipitation of other metals carried as sulfide complexes, e.g. Au(HS) -2.