Thermodynamic mixing properties of pyrrhotine, Fe _xS

ABSTRACT. Published data from pyrrhotine-vapour equilibrium experiments are used to make inferences concerning the high-temperature th rmodynamic mixing properties of pyrrhotine. Darken's quadratic formalism provides the justification for plotting activity of sulphur data against ( l-X) 2, where X is used in Fel_x S to express the composition of non-stoichiometric pyrrhotine. Such plots show that the thermodynamic behaviour of pyrrhotine appears to be different on either side of X = 0.125, the FevS s composition. PYRRHOTINE occurs widely in rocks. Early hopes that the composition of pyrrhotine in the common assemblage pyrrhotine +pyrite might be used geothermometrically foundered when it was discovered that pyrrhotine re-equilibrated xtremely readily on cooling (e.g. Barton and Skinner, 1979). Nevertheless, the thermodynamic properties of high-temperature, hexagonal pyrrhotine are of interest because they are required for the description of phase relations in several important systems, notably Fe-Zn-S and Fe-Ni-S. The thermo-dynamics of pyrrhotine is also of intrinsic interest because it involves understanding the formation of, and interactions between defects in a crystal structure. Pyrrhotine is a good phase in which to study defect equilibria because there is a con-siderable amount of published experimental data on the dependence of the activity of sulphur on the non-stoichiometry of pyrrhotine (Burgrnann et al.