Abstract
Solubilities of AgCl in aqueous Zn(II)-HCl solutions have been measured from 100 to 350°C at the saturated vapour pressure. From these measurements, the cumulative equilibrium formation constants of chlorozinc(II) complexes have been evaluated using a nonlinear least squares procedure. At 100°C. all the species ZnCl 2−n n ( n = 0 to 4) exist, but as temperature rises, ZnCl − 3 concentrations decrease and its stability field vanishes above 200°C. Chlorozinc(II) complex formation is characterized by large positive enthalpy and entropy changes, reflecting the predominantly electrostatic interaction between the acceptor (Zn) and donor (Cl) species. Zinc transport is evaluated in two contrasting geothermal fluids and it is found that Zn chloride complexing is important in low pH, S-deficient hydrothermal solutions (Bacon-Manito, Philippines) of moderate to high salinity but not necessarily in near neutral to alkaline solutions (Broadlands, New Zealand). Sphalerite solubilities in chloride solutions have been calculated using the formation constants obtained in the present study.
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