ENTHALPIES OF FORMATION OF PYRRHOTITE Fe1-0.125xS (0 ≤ X ≤ 1) SOLID SOLUTIONS

Binary iron sulfides are of major interest and importance in environmental, geological and planetary science. Oxidative high temperature oxide melt drop solution calorimetry in molten sodium molybdate (3Na₂O·4MoO₃) solvent at 975 K was applied to determine the energetics of formation of the binary iron monosulfide solid solution (pyrrhotite) (Fe_1-0.125xS, 0 ≤ x ≤ 1). High bubbling and flushing oxygen ensures rapid dissolution and complete oxidization of iron sulfides to dissolved iron cations (Fe³⁺) and sulfate species (SO₄^2-). The enthalpies of formation from elements are consistent with earlier data in the literature, available for a few compositions. Within the experimental errors, the enthalpies of formation of the solid solution from the end-members Fe_0.875S and FeS (ΔH_mix, kJ/mol) at 25 ^oC equal to zero. Under the assumption of random distribution of Fe vacancies, the Gibbs free energies of mixing of Fe_1-0.125xS (0 ≤ x ≤ 1) are estimated. Our calorimetric results at 25 ^oC support the two-sublattice model proposed by Waldner and Pelton (Waldner, P. and Pelton, A.D. (2005) Thermodynamic modeling of the Fe-S system. Journal of Phase Equilibria and Diffusion, 26(1), 23-38.). Since our calorimetric method is general and can be applied to various samples that have been characterized in detail prior to calorimetry, it shows special promise for multicomponent sulfides with a high degree of ionic substitution and non-stoichiometry, for example Fe-Ni-S and Fe-Zn-S.