Abstract
We provide an experimental confirmation of the suggestion, based on thermodynamic simulations and extrapolations (Zhong et al. 2015), that Zn is transported in the form of chloride complexes in most acidic, shallow hydrothermal systems; while bisulfide complexes become increasingly important in deep, pH neutral to basic hydrothermal systems. We used in situ X‑ray absorption spectroscopy (XAS) diamond-anvil cell experiments to determine Zn(II) speciation in a 1 m NaHS + 0.2 m HCl solution in contact with sphalerite. XANES data indicate that Zn coordinates to oxy/hydroxyl/chloride ligands from room temperature up to and including 200 °C, and then at higher temperatures (≥300 °C) and pressures (>2 kbar) it changes to complexing with sulfur. Our data confirm that bisulfide complexes become increasingly important in neutral-alkaline solutions at high pressure and temperature, due to an increase in sulfur solubility and to favorable entropy contributions for bisulfide vs. chloride complexes.
Acknowledgments
We thank the APS (Sector 20) for beam time and Museum Victoria for supply‑ ing the sphalerite. We acknowledge travel funding provided by the ISAP, Australian Synchrotron, funded by the Australian Government. The AIMD calculations were supported by the Pawsey Supercomputing Centre, with funding from the Australian and Western Australian Governments. W.L. is a recipient of ARC Future Fellowship (FT130100510). We are grateful to Marion Louvel and two anonymous reviewers for helping us to improve this manuscript.
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