AQUEOUS SPECIATION OF IRON AND MANGANESE CHLORIDES IN SUPERCRITICAL HYDROTHERMAL SOLUTIONS

Aqueous speciation of iron and manganese chlorides in supercritical hy drothermal solutions was determined by monitoring the effects of NaCl and KCl on the Ca/(Ca + Fe) and Ca/(Ca + Mn) ratios of aqueous chlorid e solutions in equilibrium with scheelite and ferberite and with schee lite and huebnerite, respectively. The experiments were carried out un der supercritical conditions of 400 and 600 degrees C, and 1 kbar. The ratios decrease significantly with increasing NaCl and KCl concentrat ions at both 400 degrees C and 600 degrees C. This is attributable to the formation of higher-order aqueous chloride complexes of Fe and Mn, presumably FeCl3aq- and MnCl3aq-. Based on the thermodynamic analysis of the experimental results using published dissociation constants fo r NaClaq, KClaq and CaCl2aq, logarithms of the formation constants for FeCl3aq- were obtained to be 0.2(NaCl) and 1.2(KCl) for 400 degrees C , and 1.9(NaCl) and 1.9(KCl) for 600 degrees C, and those for MnCl3aq- to be 0.4(NaCl) and 1.1(KCl) for 400 degrees C and 2.3(NaCl) and 2.3( KCl) for 600 degrees C. This means that the supposition of m(MeCl(2aq) ) = Sigma m(Me) for supercritical hydrothermal solutions is invalid.