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Tungsten (VI) speciation in hydrothermal solutions up to 400°c as revealed by in-situ Raman spectroscopy
Tungsten (VI) speciation in hydrothermal solutions is investigated through in-situ Raman spectroscopy coupled to the fused silica glass capillary technique at temperatures up to 400 °C. The effect of temperature, pH, chlorinity and carbonate speciation are evaluated in systems with highly soluble salts Na2WO4 and Na6W12O39. At all investigated temperatures, the tungstate ion WO42- (927 cm-1) is the only W species in solution at pH > 10. At a given pH, the presence of dissolved carbonates and chloride does not affect the tungsten speciation. Tungsten polymers reveal to be stable up to 400 °C under acidic to circum-neutral pH conditions and total tungsten concentration above 0.01 molkgH2O-1-. Among the three observed polymers, namely [W7O24]6- (paratungstate-A, ~ 960 cm-1), [W10O32]4- (tungstate-Y, ~ 970 cm-1), and α-[H2W12O40]6- (α-metatungstate, ~ 990 cm-1), only the hepta and dodeca-tungstate are stable at elevated temperature. Combined with revised literature data, these results allow the thermodynamic stability constants of these W polymers to be constrained, enabling quantitative predictions of their relative abundance at T up to 300 °C. These predictions suggest that W polymerization occurs under hydrothermal conditions even at low W concentration (down to 10-5 mol·kgH2O-1) under acidic conditions. These observations imply that the currently available geochemical models on W transport and deposition in deep and hot geological fluids need to be revised.
Keywords: Tungsten polymers; Polytungstates; Fused silica glass capillary technique; Ore deposits
Geochimica et Cosmochimica Acta (2021)
Online First (2021) DOI: 10.1016/j.gca.2021.11.004