The monosulfid solid solution in the FeNiS system: relationship to the earth's core on the basis of experimental high-pressure investigations

Abstract

Experimental high-pressure results on phase stability, electrical conductivity and compression behavior up to 5 and 21 GPa respectively are used to calculate an isothermal equation of state for a monosulfid solid solution (MSS-composition) in the FeNiS system. The high-pressure relations in the range 1–8 GPa are very complex. A continuous electrical transition, from semiconducting to metallic, takes place at high pressures and temperatures and results in anomalous compression behavior at pressures in this region. No polymorphic transition from the NiAs-structure to another type could be observed; however, density increases by as much as 8.8%. Using compression values for pressure greater than 10 GPa, the bulk modulus, a zero-pressure density and a core density were calculated. Extrapolation for the conditions of the outer core yields a difference in the density of up to 20%, relative to seismological models. In a composition model with (Fe, Ni)+MSS, a MSS-content must be assumed to be in the range of 30–35 wt% at the core-mantle boundary (CMB) and 13–17 wt% at the inner-core boundary (ICB). That corresponds to a sulfur content of 10.8–13.3 wt% (CMB) and 4.9–6.5 wt% (ICB), respectively, the values increasing with increasing Ni content of the MSS-phase.