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

Abstract

The molecular structure and stability of species formed by silver in aqueous saline solutions typical of hydrothermal settings were quantified using in situ X-ray absorption spectroscopy (XAS) measurements, quantum-chemical modeling of near-edge absorption spectra (XANES) and extended fine structure spectra (EXAFS), and first-principles molecular dynamics (FPMD). Results show that in nitrate-bearing acidic solutions to at least 200°C, silver speciation is dominated by the hydrated Ag+ cation surrounded by 4–6 water molecules in its nearest coordination shell with mean Ag–O distances of 2.32±0.02Å. In NaCl-bearing acidic aqueous solutions of total Cl concentration from 0.7 to 5.9mol/kg H2O (m) at temperatures from 200 to 450°C and pressures to 750bar, the dominant species are the di-chloride complex AgCl2− with Ag–Cl distances of 2.40±0.02Å and Cl–Ag–Cl angle of 160±10°, and the tri-chloride complex AgCl32− of a triangular structure and mean Ag–Cl distances of 2.60±0.05Å. With increasing temperature, the contribution of the tri-chloride species decreases from ∼50% of total dissolved Ag in the most concentrated solution (5.9m Cl) at 200°C to less than 10–20% at supercritical temperatures for all investigated solutions, so that AgCl2− becomes by far the dominant Ag-bearing species at conditions typical of hydrothermal–magmatic fluids. Both di- and tri-chloride species exhibit outer-sphere interactions with the solvent as shown by the detection, using FPMD modeling, of H2O, Cl−, and Na+ at distances of 3–4Å from the silver atom. The species fractions derived from XAS and FPMD analyses, and total AgCl(s) solubilities, measured in situ in this work from the absorption edge height of XAS spectra, are in accord with thermodynamic predictions using the stability constants of AgCl2− and AgCl32− from Akinfiev and Zotov (2001) and Zotov et al. (1995), respectively, which are based on extensive previous AgCl(s) solubility measurements. These data are thus recommended for chemical equilibrium calculations in mineral–fluid systems above 200°C. In contrast, our data disagree with SUPCRT-based datasets for Ag–Cl species, which predict large fractions of high-order chloride species, AgCl32− and AgCl43− in high-temperature saline fluids. Comparisons of the structural and stability data of Ag–Cl species derived in this study with those of their Au and Cu analogs suggest that molecular-level differences amongst the chloride complexes such as geometry, dipole moment, distances, and resulting outer-sphere interactions with the solvent may account, at least partly, for the observed partitioning of Au, Ag and Cu in vapor–brine and fluid–melt systems. In hydrothermal environments dominated by fluid–rock interactions, the contrasting affinity of these metals for sulfur ligands and the differences both in chemistry and stability of their main solid phases (Ag sulfides, Cu–Fe sulfides, and native Au) largely control the concentration and distribution of these metals in their economic deposits.