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 NaClaq, KClaqand CaCl2aq, 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(MeCl2aq) = Σmmefor supercritical hydrothermal solutions is invalid.
ASJC Scopus subject areas