An X-ray absorption spectroscopy study of argutite solubility and aqueous Ge(IV) speciation in hydrothermal fluids to 500 °C and 400 bar

Dissolution of synthetic argutite (GeO2, tetragonal, rutile-like) in pure water, and local atomic structure around Ge(IV) in aqueous solution were characterized by in situ X-ray Absorption Fine Structure (XAFS) spectroscopy at the Ge K-edge using a new high temperature-high pressure cell. XAFS transmission and fluorescence spectra were collected from germanic acid aqueous solutions of 0.02-0.09 mol Ge/kg H2O at ambient pressure and temperature, and from a 0.04 mol Ge/kg H2O solution at 400 bar from 20 to 450 °C. Spectra at all temperatures and concentrations exhibit the 1st shell contribution from 4±0.3 oxygen atoms at 1.75±0.01 Å, consistent with a tetrahedral structure of the Ge(OH)4 complex which is a chemical analog of Si(OH)4 and is the dominant Ge aqueous species, in agreement with previous solubility and potentiometric measurements. The constancy of Ge(OH)4 structural parameters over the wide temperature and fluid-density range is consistent with the strong covalent character of the Ge-(OH) bonds. A dissolution experiment on argutite in pure H2O was carried out at 400 bar from 200 to 500 °C by monitoring the height of the Ge absorption edge of transmission spectra. The solubility of GeO2, derived from a classical X-ray absorption relation using the absorption height values and fluid density, increases with increasing temperature up to 400 °C but decreases at 450 and 500 °C, similar to that observed for quartz. The new solubility data were analyzed using both a density model and the revised HKF equation of state to better constrain Ge(OH)4 thermodynamic properties. Calculations using these models indicate that despite strong similarities in the structure and energetics of aqueous germanic acid and silica, differences in the thermodynamic properties of germanate and silicate dissolution reactions may account for the increase of the Ge/Si ratio observed in high-temperature natural fluids.