The geochemistry of antimony in hydrothermal solutions

In this thesis, 30°C stibnite solubility experiments, ambient temperature X-ray absorption spectroscopic measurements of antimony in solution, and high temperature (70 to 400°C) stibnite solubility experiments were carried out in order to determine the aqueous antimony species present in equilibrium with stibnite in hydrosulfide solutions from pH = 3.5 to 12 and reduced sulfur concentrations from 0.001 to 0.1 mol kg⁻¹. Both ambient and elevated temperature solubility studies were conducted using a flow-through apparatus containing a column of stibnite grains though which solutions were pumped. Above 100°C, solubility experiments were conducted at slightly above saturated water vapour pressure to pressures of 300 bar.  At 30°C, the stibnite solubility curve was best reproduced by a scheme of five species: Sb₂S₄²⁻, HSb₂S₄⁻, H₂Sb₂S₅²⁻, H₃SbS₂O, and Sb(OH)₃. At higher temperatures (≥ 70 °C), stibnite solubility at the conditions of the experiments was due to the following four species: Sb₂S₄²⁻, HSb₂S₄⁻, H₃SbS₂O, and Sb(OH)₃. Equilibrium constants were determined for the following five heterogeneous solubility reactions for the temperature ranges listed:  [Please consult the thesis for details.]  Stibnite solubility was independent of pressure at ≤ 350°C. At ~ 400°C, the solubility of stibnite was strongly dependent on pressure and decreased from Sbtotal = 0.015 to 0.0003 mol kg⁻¹ (~2000 to 40 ppm) with a pressure decrease from 300 to 160 bars.  The Sb K-edge X-ray absorption spectroscopic (XAS) measurements of antimony in alkaline (pH = 10. 9 to 12) hydrosulfide solutions gave average first shell coordination environments that were consistent with the speciation model derived from solubility experiments for strongly alkaline solutions (i.e., Sb₂S₄²⁻ and Sb(OH)₃). XAS data enable the elimination of a speciation model involving only monomeric antimony complexes at strongly alkaline pH.  Antimony speciation in near neutral to strongly alkaline pH’s is dominated by dimeric antimony-sulfide complexes at 30°C and sulfide concentrations > 0.001 mol kg⁻¹. With increasing temperature, antimony speciation becomes increasingly dominated by Sb(OH)₃. For hydrothermal solutions with sulfide concentrations between 0.0001 and 0.01 mol kg⁻¹, antimony-sulfide complexes are predominant at < 100°C, whereas antimonous acid, Sb(OH)₃, is the main aqueous species at contributing to stibnite solubility at > 200°C with the speciation in the intervening temperature range being dependent on the pH and sulfide concentration of the solution. For higher sulfide concentrations (i.e., ~ 0.1 mol kg⁻¹), HSb₂S₄⁻ and Sb₂S₄²⁻ control stibnite solubility to higher temperatures.