Enthalpies of formation of pyrrhotite Fe (sub 1-0.125x) S (0< or =x< or =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 (3Na (sub 2) O.4MoO (sub 3) ) solvent at 975 K was applied to determine the energetics of formation of the binary iron monosulfide solid solution (pyrrhotite) (Fe (sub 1-0.125x) S, 0< or =x< or =1). 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 (sub 0.875) S and FeS (Delta H (sub mix) , kJ/mol) at 25 degrees C equal to zero. Under the assumption of random distribution of Fe vacancies, the Gibbs free energies of mixing of Fe (sub 1-0.125x) S (0< or =x< or =1) are estimated. Our data support the two-sublattice model proposed by Waldner and Pelton (2005).