Liu, W.Borg, S.Testemale, D.Etschmann, B.Hazemann, J.Brugger, J.2012-04-022012-04-022011Geochimica et Cosmochimica Acta, 2011; 75(5):1227-12480016-70371872-9533http://hdl.handle.net/2440/70227Aqueous Co(II) chloride complexes play a crucial role in cobalt transport and deposition in ore-forming hydrothermal systems, ore processing plants, and in the corrosion of special Co-bearing alloys. Reactive transport modelling of cobalt in hydrothermal fluids relies on the availability of thermodynamic properties for Co complexes over a wide range of temperature, pressure and salinity. Synchrotron X-ray absorption spectroscopy was used to determine the speciation of cobalt(II) in 0-6m chloride solutions at temperatures between 35 and 440°C at a constant pressure of 600bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35°C, while tetrahedral species become increasingly important with increasing temperature. Ab initio XANES calculations and EXAFS analyses suggest that in high temperature solutions the main species at high salinity (Cl:Co>>2) is CoCl<inf>4</inf> ^2-, while a lower order tetrahedral complex, most likely CoCl<inf>2</inf>(H<inf>2</inf>O)<inf>2(aq)</inf>, predominates at low salinity (Cl:Co ratios ~2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H<inf>2</inf>O)<inf>6</inf> ²⁺ (^octCo-O=2.075(19)Å), tetrahedral CoCl<inf>4</inf> ^2- (^tetCo-Cl=2.252(19)Å) and tetrahedral CoCl<inf>2</inf>(H<inf>2</inf>O)<inf>2(aq)</inf> (^tetCo-O=2.038(54)Å and ^tetCo-Cl=2.210(56)Å). An analysis of the Co(II) speciation in sodium bromide solutions shows a similar trend, with tetrahedral bromide complexes becoming predominant at higher temperature/salinity than in the chloride system. EXAFS analysis confirms that the limiting complex at high bromide concentration at high temperature is CoBr<inf>4</inf> ^2-. Finally, XANES spectra were used to derive the thermodynamic properties for the CoCl<inf>4</inf> ^2- and CoCl<inf>2</inf>(H<inf>2</inf>O)<inf>2(aq)</inf> complexes, enabling thermodynamic modelling of cobalt transport in hydrothermal fluids. Solubility calculations show that tetrahedral CoCl<inf>4</inf> ^2- is responsible for transport of cobalt in hydrothermal solutions with moderate chloride concentration (~2m NaCl) at temperatures of 250°C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids. © 2010 Elsevier Ltd.enCopyright © 2010 Elsevier Ltd. All rights reserved.Journal article002011554610.1016/j.gca.2010.12.0020002872892000052-s2.0-7955148807026378