Thermochemistry of sulfide liquids. II. Associated solution model for sulfide liquids in the system O-S-Fe

We present a new formulation to describe the thermodynamics of liquids in the system O-S-Fe. The model is based on an associated regular solution formulation. According to this model, liquids in the O-S-Fe ternary are made up of an equilibrium solution of the six melt species S, Fe, FeO, FeO_1.5, FeS and FeOS. The model presented here represents oxygen and sulfur fugacities as well as phase equilibria with stoichiometric solid phases better than models from the literature on O-Fe and S-Fe binaries. Furthermore, this model represents a substantial improvement on the model of Kress (1997), which is the only other thermodynamic model available in the ternary system. Asymmetric regular solution parameters are required along the FeO join in order to reproduce experimental data with the chosen list of species. Symmetric regular solution parameters are required along the Fe-S binary. Mixing between any of the species considered and FeOS close to ideal. The associated solution model presented here will serve as a more solid foundation for future models in O-S-Fe- Ni-Cu liquids. Efficient and robust strategies for calculating equilibrium speciation and estimating model parameters are presented. Received: 15 June 1999 / Accepted: 5 February 2000