Thermodynamic stability of pentlandite and violarite and new E(H)-pH diagrams for the iron-nickel sulphur aqueous system

Thermodynamic data for ideal stoichiometric pentlandite (Fe4.5Ni4.5S8) and violarite (FeNi2S4) have been re-evaluated from arguments concerning relative phase stability, in conjunction with known values for binary and ternary sulphides, The values Delta G(f)(degrees) Fe4.5Ni4.5S8 = -813 kJ mol(-1) and Delta G(f)(degrees) Fe1-xNi2+xS4 = -346 kJ mol(-1) (x approximate to 0) at 298 K were obtained. These values were used to calculate Nernst functions for the Fe-Ni-S aqueous system, which are displayed by a series of E(H)-pH diagrams. These diagrams reveal the co-existence of violarite and pentlandite in an aqueous system, which tallies with mineralogical observations. The value for violarite can plausibly account for the supergene nickel-enrichment process observed in nature.