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

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 logK = ?4.17(±0.55) + 4629(±464)/T(K) at 1 kbar, and logK = ?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.