Aqueous speciation of iron and manganese chlorides in supercritical hydrothermal solutions

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 KC1 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 ^- _aqand MnCl₃ ^- _aq. Based on the thermodynamic analysis of the experimental results using published dissociation constants for NaCl_aq, KCl_aqand CaCl_2aq, logarithms of the formation constants for FeCl₃ ^- _aqwere 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 ^- _aqto be 0.4(NaCI) 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) = Σmmefor supercritical hydrothermal solutions is invalid.