Skip to Main content Skip to Navigation
Journal articles

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 nearedge 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 ± 0.02A ˚ . In NaCl-bearing acidic aqueous solutions of total Cl concentration from 0.7 to 5.9 mol/kg H2O (m) at temperatures from 200 to 450 C and pressures to 750 bar, the dominant species are the di-chloride complex AgCl2 with Ag-Cl distances of 2.40 ± 0.02A ˚ and Cl-Ag-Cl angle of 160 ± 10 , and the tri-chloride complex AgCl3 2 of a triangular structure and mean Ag-Cl distances of 2.60 ± 0.05A ˚ . 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 3-4A ˚ 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 AgCl3 2 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, AgCl3 2 and AgCl4 3 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 outersphere 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.
Document type :
Journal articles
Complete list of metadatas

Cited literature [121 references]  Display  Hide  Download

https://hal.archives-ouvertes.fr/hal-00794819
Contributor : Danielle Thomas <>
Submitted on : Saturday, September 12, 2020 - 9:57:15 AM
Last modification on : Tuesday, September 22, 2020 - 3:45:48 AM

File

Pokrovski_GCA_2013.pdf
Files produced by the author(s)

Identifiers

Citation

Gleb S. Pokrovski, Jacques Roux, Guillaume Ferlat, Romain Jonchiere, Ari P. Seitsonen, et al.. Silver in geological fluids from in situ X-ray absorption spectroscopy and first-principles molecular dynamics. Geochimica et Cosmochimica Acta, Elsevier, 2013, 106, pp.501-523. ⟨10.1016/j.gca.2012.12.012⟩. ⟨hal-00794819⟩

Share

Metrics

Record views

965

Files downloads

21