Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 131, Issue -, Pages 196-212Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2014.01.033
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Funding
- Australian Research Council (ARC) [DP0878903]
- Minerals Down Under Flagship
- NERC [NE/I02349X/1]
- University of Adelaide
- CSIRO Minerals Down Under Flagship
- Australian Research Council [DP0878903] Funding Source: Australian Research Council
- Natural Environment Research Council [NE/I02349X/1] Funding Source: researchfish
- NERC [NE/I02349X/1] Funding Source: UKRI
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Low-density supercritical fluids are suspected of being able to transport metals, but it is unclear what the speciation/complexation would be in such conditions. In this work, we used ab initio molecular dynamics simulations to investigate the complexation, ion association and hydration of Cu+ and Au+ in NaCl brines as a function of solution density, from ambient to supercritical conditions (to 1000 degrees C, 5000 bar). Cu(I) and Au(I) form distorted linear complexes with two chloride ligands (i.e., CuCl2- and AuCl2-) in subcritical chloride brines. We have discovered that these charged complexes remain in high density supercritical fluids even at high temperature; however, with decreasing density, these complexes become progressively neutralized by ion association with Na+ to form low-charge (NanCuCl2)(n-1) and (NanAuCl2)(n-1) complexes. In these species, the Na+ ion is very weakly bonded in the outer coordination sphere, resulting in highly disordered structures and fast (few pico-seconds) exchange among coordinated and solvent Na+ ions. Thermodynamic models to predict the solubility of metals in low-density magmatic or metamorphic fluids must account for these species. In addition, we found that the number of water molecules (i.e., the hydration number) surrounding the Cu+, Au+, Na+ and Cl- ions decreases linearly when fluid density decreases; this supports empirical thermodynamic models that correlate the stability constants of complexation reactions with solvent density. The traditional Born-model description explains the ion association as resulting from the decreased dielectric constant of the solvent. However at a molecular level, the increased ion association results from the increase in translational entropy associated with ion dehydration. (C) 2014 Elsevier Ltd. All rights reserved.
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