In order to obtain the stepwise formation constants of tri-chloro complexes of Pb2+ and Sn2+, experiments on the effect of NaCl concentration on the ion exchange equilibria in the systems PbS - ZnS - PbCl2 - ZnCl2 - H2O and CaWO4 - SnWO4 - CaCl2 - SnCl2 - H2O were carried out. The experiments were conducted at 500°, 600°, 700° and 800°C (800°C is only for Sn2+), 1 kbar and also at 600°C, 0.5 kbar using 2 m aqueous chloride solutions. In all the experiments, the Σm(Pb)/(Σm(Pb)+Σm(Zn)) ratio of the aqueous chloride solutions in equilibrium with both PbS and ZnS and the Σm(Ca)/Σm(Ca)+Σm(Sn)) ratio of the aqueous chloride solutions in equilibrium with both CaWO4 and SnWO4 decrease with the increase of NaCl concentration. The experimental results were analyzed thermodynamically using the previously published dissociation constants for NaClaq, CaCl2aq and ZnCl3-aq. As a result, the stepwise formation constants (log K) for PbCl3-aq (PbCl2aq + Cl-aq = PbCl3-aq) were obtained to be -0.1 at 500°C and 1 kbar, 0.9 at 600°C and 1 kbar, 1.4 at 700°C and 1 kbar, and 1.3 at 600°C and 0.5 kbar. The stepwise formation constants (log K) for SnCl3-aq (SnCl2aq + Cl-aq = SnCl3-aq) were obtained to be 0.5 at 500°C and 1 kbar, 1.4 at 600°C and 1 kbar, 2.0 at 700°C and 1 kbar, 2.6 at 800°C and 1 kbar, and 1.9 at 600°C and 0.5 kbar. The stepwise formation constants (log K) of tri-chloro complexes of Pb2+, Sn2+, Fe2+, Mn2+, Co2+, Ni2+, Zn2+ and Cd2+ vary with sixfold coordinated ionic radius showing a parabola-shaped curve with a maximum between Mn2+ and Cd2+. This may suggest that the best ionic radius for tri-chloro complex exists between Mn2+ and Cd2+.
ASJC Scopus subject areas
- Geochemistry and Petrology