Aqueous speciation of magnesium, strontium, nickel and cobalt chlorides in hydrothermal solutions at 600°C and 1 kbar

The effect of NaCl on ion exchange equilibria in the system CaWO₄-MeWO₄-CaCl₂-MeCl₂-H ₂O (Me: Mg, Sr, Ni and Co) was experimentally monitored at 600°C and 1 kbar. The Ca/(Ca + Me) ratio of the aqueous chloride solution in equilibrium with both CaWO₄ and MeWO₄ was constant for Mg and Sr in spite of the addition of NaCl, whereas the ratio decreased significantly for Ni and Co with the increase of NaCl. The experimental results indicate that both Ni²⁺ and Co²⁺ do form higher-order chloro-complexes such as NiCl₃^-_aq and CoCl₃^-_aq, whereas both Mg²⁺ and Sr²⁺ may not. The thermodynamic analysis of the experimental results gives 1.5 and 2.1 in logarithm units for the formation constants for NiCl₃^-_aq and CoCl₃^-_aq, respectively. By combining the results of Uchida et al. (1995), MeCl_2aq tends to forms MeCl₃^-_aq in the order of SrCl_2aq= CaCl_2aq = MgCl_2aq ≪ NiCl_2aq < FeCl_2aq < CoCl_2aq < MnCl_2aq at 600°C and 1 kbar. Based on ligand field theory, the above order may suggest that the transition metal cations exist as tetrahedral chloro-complexes with low-spin state in the supercritical hydrothermal solutions.