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

Abstract

Dissolution of synthetic argutite (GeO 2 , 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 to 0.09 mol Ge/kg H 2 O at ambient pressure and temperature, and from a 0.04 mol Ge/kg H 2 O solution at 400 bar from 20 to 450°C. Spectra at all temperatures and concentrations exhibit the 1 st shell contribution from 4±0.3 oxygen atoms at 1.75±0.01Å, 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 argitite in pure H 2 O 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 GeO 2 , 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.

X-ray absorption spectroscopy study of solvation and ion-pairing in aqueous gallium bromide solutions at supercritical conditions

“…measured in the transmission mode [24]. This height is proportional to the density of the sample, the amount of dissolved absorbing species and the path length (l) of the X-ray beam inside the sample (constant, see right inset on Fig.…”

Silver in geological fluids from in situ X-ray absorption spectroscopy and first-principles molecular dynamics

“…A weighed amount of silver chloride, which is weakly soluble at ambient 170 temperature (< 0.001m), was pressed in pellets and loaded in the experimental cell. Total dissolved silver 171 concentration in solution was measured from the absorption edge height in transmission mode (Pokrovski et al, sample space probed by X-rays, were still observed for some experiments above 300°C (Table 1), it did not affect Silver in hydrothermal fluids 8 described in detail elsewhere (Pokrovski et al, 2005b(Pokrovski et al, , 2006 …”

An equation of state for predicting the thermodynamic properties and vapour–liquid partitioning of aqueous Ge(OH)4 in a wide range of water densities

“…The available GeO 2 solubility experiments and X-ray absorption spectroscopy studies of Ge-bearing aqueous solutions [4,7,8] unambiguously demonstrate that the uncharged tetra-hydroxide Ge(OH) 4 is by far the dominant species in aqueous solution from acidic to alkaline pH (1 < pH < 9) over a wide temperature, T, (298-723 K) and pressure, P, (0.1-50 MPa) range. The thermodynamic properties of Ge(OH) 4 (aq) derived from GeO 2 (s) solubility measurements using the HKF equation of state [8] are, however, valid only for dense aqueous fluids (pure water density >0.5 g cm À3 ) due to intrinsic limitations of the HKF model [10][11][12]. Other alternative models describing mineral solubilities such as the density model [8,13,14] applied to GeO 2 -water systems may be subjected to large errors beyond the T-P range covered by measurements because of an entirely empirical nature of the model coefficients.…”

“…In geochemistry, germanium is an important tracer of continental weathering and riverine flux [3,4], hydrothermal activity [4,5], and organic matter impact [6,7]. This is because in some geological contexts germanium behaves differently from silicon rendering the Ge/Si ratio in waters to be sensitive to the presence of clays and iron hydroxides, organic ligands, reduced sulphur, and temperature [8].…”

“…The thermodynamic properties of Ge(OH) 4 (aq) derived from GeO 2 (s) solubility measurements using the HKF equation of state [8] are, however, valid only for dense aqueous fluids (pure water density >0.5 g cm À3 ) due to intrinsic limitations of the HKF model [10][11][12]. Other alternative models describing mineral solubilities such as the density model [8,13,14] applied to GeO 2 -water systems may be subjected to large errors beyond the T-P range covered by measurements because of an entirely empirical nature of the model coefficients. In addition, very little is known about Ge hydroxide species identities, hydration structure, and stabilities in the low-density vapour phase; this lack hampers quantitative assessment of Ge volatility and vapour-liquid partitioning both in natural hydrothermalvolcanic systems and industrial processes.…”