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.