First-principles calculations of thermodynamic properties of Ni sulfides in the upper mantle

Nickel sulfides are common in nature, and have been found in mantle-related rocks and xenoliths. Their properties exhibit significant differences with silicates in density, conductivity, and elasticity, and may be responsible for the geophysical anomalies in the upper mantle. Whether there is a difference in thermodynamic properties between sulfides and silicates in the upper mantle is not yet known. In the present study, the thermodynamic properties of millerite (NiS), heazlewoodite (Ni3S2), and polydymite (Ni3S4)-violarite (FeNi2S4) series with Fe/(Fe + Ni) = 0, 1/6 and 2/6 are calculated using the first-principles methods together with the quasi-harmonic approximation. The thermodynamics properties of some sulfides, including equation of state, isothermal bulk modulus, thermal expansion coefficient, heat capacity, and entropy are consistent with the available previous calculated and experimental data. The effect of Fe content on the thermodynamic properties of polydymite are also investigated, which show that with the increase of substitution Fe for Ni, the bulk modulus increases while the coefficient of thermal expansion decreases. Our calculations show that the coefficient of thermal expansion of Ni sulfides are higher than that of olivine and pyroxenes, while the adiabatic bulk modulus of Ni sulfides are lower than that of forsterite.