Aqueous speciation of iron and manganese chlorides in supercritical hydrothermal solutions

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Aqueous speciation of iron and manganese chlorides in supercritical hydrothermal solutions was determined by monitoring the effects of NaCl and KCl on the Ca/(Ca+Fe) and Ca/(Ca+Mn) ratios of aqueous chloride solutions in equilibrium with scheelite and ferberite and with scheelite and huebnerite, respectively. The experiments were carried out under supercritical conditions of 400 and 600°C, and 1 kbar. The ratios decrease significantly with increasing NaCl and KCl concentrations at both 400°C and 600°C. This is attributable to the formation of higher-order aqueous chloride complexes of Fe and Mn, presumably FeCl₃^-_aq and MnCl₃^-_aq. Based on the thermodynamic analysis of the experimental results using published dissociation constants for NaCl_aq, KCl_aq and CaCl_2aq, logarithms of the formation constants for FeCl₃^-_aq were obtained to be 0.2(NaCl) and 1.2(KCl) for 400°C, and 1.9(NaCl) and 1.9(KCl) for 600°C, and those for MnCl₃^-_aq to be 0.4(NaCl) and 1.1(KCl) for 400°C and 2.3(NaCl) and 2.3(KCl) for 600°C. This means that the supposition of m(MeCl_2aq)=Σm_Me for supercritical hydrothermal solutions is invalid.