THERMODYNAMIC STABILITY OF PENTLANDITE AND VIOLARITE AND NEW E(H)-PH DIAGRAMS FOR THE IRON-NICKEL SULFUR AQUEOUS SYSTEM

Thermodynamic data for ideal stoichiometric pentlandite (Fe4.5Ni4.5S8) and violarite (FeNi2S4) have been re-evaluated from arguments concern ing relative phase stability, in conjunction with known values for bin ary 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 use d 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 t he co-existence of violarite and pentlandite in an aqueous system, whi ch tallies with mineralogical observations. The value for violarite ca n plausibly account for the supergene nickel-enrichment process observ ed in nature.