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), MeCl2aq tends to forms MeCl3-aq in the order of SrCl2aq= CaCl2aq = MgCl2aq ≪ NiCl2aq < FeCl2aq < CoCl2aq < MnCl2aq 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.
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