Modelling the structure and properties of Iron Sulfide minerals

Scientific problem

The Fe-S minerals are common in igneous, sedimentary and metamorphic environments, where the stable phases are pyrite (FeS2) and pyrrhotite (Fe1-xS). Breakdown of these minerals is responsible for the generation of acid rock drainage (ARD) and is thus of major environmental concern. The non-stoichiometric pyrrhotite phases have variable physical and magnetic properties and can display different dissolution behaviour depending on the ordering of Fe vacancies.

The structure and magenetic properties of troilite (FeS) have been investigated using density functional theory. Comparison of the unit cell and bulk modulus obtained with and without spin interpolation showed that inclusion of antiferromagnetic ordering of the Fe atoms is required to accurately simulate this system.

Scientific results

Work performed during the first two years of the project focussed on obtaining an accurate description of the structure and properties of troilite (FeS). Simulations were carried out using the DFT code VASP, with the GGA PW91 functional and the spin interpolation for the exchange-correlation energy of Vosko et al. The results for nonmagnetic calculations give a c/a ration that is in error by 9% from that determined by experiment. This error is rediced to less that 1% when the effects of anitiferromagnetic ordering are included. Furthermore, these magnetic interactions may be responsible for the volume collapse observed experimentally on the high pressure transition of Fe-S to a MnP trype structure.

Credits

This work was carried out by Stephen Wells, Kate Wright and Richard Catlow (Royal Institution).