AQUEOUS SPECIATION OF MAGNESIUM, STRONTIUM, NICKEL AND COBALT CHLORIDES IN HYDROTHERMAL SOLUTIONS AT 600-DEGREES-C AND 1 KBAR

The effect of NaCl on ion exchange equilibria in the system CaWO4-MeWO (4)-CaCl2-MeCl(2)-H2O (Me: Mg, Sr, Ni and Co) was experimentally monit ored at 600 degrees C and 1 kbar. The Ca/(Ca + Me) ratio of the aqueou s chloride solution in equilibrium with both CaWO4 and MeWO(4) was con stant for Mg and Sr in spite of the addition of NaCl, whereas the rati o 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 CoCl3aq-, whereas both Mg2 + and Sr2+ may not. The thermodynamic analysis of the experimental res ults gives 1.5 and 2.1 in logarithm units for the formation constants for NiCl3 aq and CoCl3aq- respectively. By combining the results of Uc hida et al. (1995), MeCl(2aq2aq) tends to forms MeCl(3 aq) in the orde r of SrCl2aq CaCl2aq = MgCl2aq much less than NiCl2aq < FeCl2aq < CoCl 2aq < MnCl2aq at 600 degrees C and 1 kbar. Based on ligand field theor y, the above order may suggest that the transition metal cations exist as tetrahedral chloro-complexes with low-spin state in the supercriti cal hydrothermal solutions.