Thermodynamic mixing properties of pyrrhotine, Fe _xS

Published data from pyrrhotine-vapour equilibrium experiments are used to make inferences concerning the high-temperature thermodynamic 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 extremely 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 thermodynamics 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 considerable amount of published experimental data on the dependence of the activity of sulphur on the non-stoichiometry of pyrrhotine (Burgrnann et al., 1968; Rau, 1976; following the classic work of Toulmin and Barton, 1964). The phase relationships in Fe-S are summarized by Craig and Scott (1974) and Vaughan and Craig (1978). Pyrrhotine is non-stoichiometric having excess sulphur compared with FeS. The nonstoichiometry in pyrrhotine in the assemblage pyrrhotine+pyri te ranges from X ~ 0.099 at 300 ~ to X ~ 0.179 at 730 ~ the upper stability limit of pyrite at low pressure (e.g. Toulmin and Barton, 1964). Pyrrhotine which is in equilibrium with metallic iron is stoichiometric FeS within experimental error in this temperature range (e.g. Burgmann et al., 1968). This asymmetry in the t~) Copyright the Mineralogical Society non-stoichiometry with respect to the composition FeS must reflect the radically different energetics involved in producing the defects to make Fe-excess and S-excess pyrrhotine. There is little doubt that the defects responsible for the non-stoichiometry in pyrrhotine are vacancies on the Fe site. As the stoichiometric composition is approached, a compensating defect having the potential to produce Fe-excess pyrrhotines becomes important. It has been suggested that these defects are interstitial Fe atoms (Libowitz, 1972), as in wiistite, or Fe atoms on the S site (Rau, 1976). At temperatures above about 300 ~ the only stable pyrrhotine has a hexagonal NiAs structure in which the vacancies and Fe atoms on the Fe site are more or less randomly distributed. Nevertheless it might be expected that vacancies would show a preference for avoiding vacancy nearest neighbours. Below 300 ~ various pyrrhotine superstructures appear (Kissin and Scott, 1982). These superstructures involve various optimal orderings of the vacancies, avoiding vacancy near neighbours. For example, 4C (monoclinic) pyrrhotine occurs at or near the Fe7Sa composition, and the vacancy ordering is well-understood for this composition, (e.g. Vaughan and Craig, 1978, 46-7). Thermodynamics of pyrrhotine Equilibrium between pyrrhotine and a vapour can be described using: 89 #s2,v = /~S,po (1) where #ij is the chemical potential of end-member i in phase j. Thus the first requirement is to formulate the chemical potential of sulphur in pyrrhotine for different models of the energetics of the phase. Compositions away from stoichiometric FeS (say X > 0.025). Given that the defects in pyrrhotine with compositions away from stoichiometric FeS are Fe site vacancies, the formulation of the thermodynamics requires a decision on whether the species