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The geochemistry of antimony in hydrothermal solutions

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posted on 2021-11-16, 03:34 authored by Olsen, Nellie J.

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.


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Te Herenga Waka—Victoria University of Wellington

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Degree Grantor

Te Herenga Waka—Victoria University of Wellington

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Degree Name

Doctor of Philosophy

ANZSRC Type Of Activity code

970104 Expanding Knowledge in the Earth Sciences

Victoria University of Wellington Item Type

Awarded Doctoral Thesis



Victoria University of Wellington School

School of Geography, Environment and Earth Sciences


Seward, Terry; Mountain, Bruce