Data on the solubility of Re metal in subcritical hydrothermal solutions presented in Xiong and Wood (2001) have been reinterpreted assuming that the following reaction controlled dissolved Re concentrations: ReO2(s) + 2H2O(l) ↔ Re(OH)4(aq). The formation of ReO2(s) on the surface of Re metal at the conditions of the previous experiments was demonstrated conclusively in a new experiment using X-ray photoelectron spectroscopy (XPS). The temperature dependence (200°–25°C) of the equilibrium constant for the above reaction at − 4700 ± 760 infinite dilution can be expressed as

\(\mathit{K}\ =\ \frac{{-}4700\ {\pm}\ 760}{\mathit{T}}\ +\ 6.2\ {\pm}\ 2.0(2{\sigma})\)
. These new equilibrium constants predict that metallic rhenium, ReO2(s), and ReS2(s) have much lower solubilities than those implied in Xiong and Wood (2001). Based upon the new equilibrium constants, the free energy of formation, the enthalpy of formation, and entropy at the reference state (298.15 K and 1 bar) for the neutral rhenium species, Re(OH)4(aq) (also alternatively formulated as ReO(OH)2(aq)), are derived in this study. Based on derived equilibrium constants at 25°C for dissolution of ReS2(s) as Re(OH)4(aq), we predict that rhenium can be enriched in the reduced zone in supergene environments in porphyry copper-molybdenum deposits.

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