An XAS study of molybdenum speciation in hydrothermal chloride solutions from 25-385°C and 600 bar

Molybdenum(VI) is expected to be the main form of Mo in hydrothermal fluids up to magmatic hydrothermal conditions. We conducted an in situ X-ray absorption spectroscopic study of Mo(VI) speciation in H2O-HCl-NaCl brines up to 385 degrees C at a pressure of 600 bar. The EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near-Edge Structure) data supported by ab-initio XANES calculations confirm that tetrahedral complexes containing the molybdate anion (MoO42-) predominate in basic to near neutral solutions with chloride concentration up to 5.5 m. The geometry of this species changes little over the investigated P-T range, and there is no complexation between MoO42- and chloride. In highly acidic solutions, Mo(VI) speciation is dominated by distorted octahedral oxo-chloro complexes MoOmCln6-2m-n; the number of chloride ligands increases with increasing temperature, to a maximum of similar to 5 (e. g., MoOCl5-) at 340 degrees C in 6.21 m HCl. These data suggest that Mo speciation is dominated by tetrahedral molybdate species (e. g., HMoO4- or MoO42-) in the basic to near neutral solution phase of natural hydrothermal fluid, and chloride complexes will only play a significant role in highly acidic fluid under our experimental P-T conditions. (C) 2012 Elsevier Ltd. All rights reserved.