Solubility and complexing of Ni in the system NiO-H2O-HCl

Abstract

The solubility of bunsenite (NiO) in Cl-bearing fluids in the range of 450°-700°C, 1-2 kbar was determined using the Ag + AgCl acid buffer technique. Based on the results of the experiments, it is concluded that the associated NiCl02 complex is the dominant Ni species in the fluid over the entire temperature-pressure range investigated. The temperature dependence of the equilibrium constant for the reaction NiO(s) + 2HCl0(aq) = NiCl02(aq) + H2O is given by log K = -4.17(±0.55) + 4629(±464)/T(K) at 1 kbar, and log K = -4.75(±0.91) + 5933(±756)/T(K) at 2 kbar. The calculated difference in standard state Gibbs free energy of formation between NiCl02 and 2HCl0 in kcal is G0(NiCl02) - 2G0(HCl0) = -20.77(±2.22) + 0.03264(±0.0026)T(K), at 1 kbar and G0(NiCl02) - 2G0(HCl0) = -25.01(±1.35) + 0.03264(±0.0016)T(K) at 2 kbar. Comparison of the solubilities of Ni end-member minerals with those of Ca, Mn, Fe, and Mg indicates that nickel minerals generally are the least soluble at a given temperature and pressure. The relatively low solubility of Ni end-member minerals, combined with the relatively low concentration of Ni in most rocks, should result in a quite low mobility of Ni in hydrothermal fluids. © 1984.