Solubility and complexing of Ni in the system NiO-H 2O-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 NiCl 0 2 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) + 2 HCl 0( aq) = NiCl 0 2( aq) + H 2 O 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 NiCl 0 2 and 2HCl 0 in kcal is G 0( NiCl 0 2) − 2 G 0( HCl 0) = −20.77(±2.22) + 0.03264(±0.0026) T( K), at 1 kbar and G 0( NiCl 0 2) − 2 G 0( HCl 0) = −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.