First-principles calculations of the thermodynamic mixing properties of arsenic incorporation into pyrite and marcasite

Abstract

The thermodynamic mixing properties of As into pyrite and marcasite have been investigated using first-principles and Monte Carlo calculations in order to understand the incorporation of this important metalloid into solid solution. Using quantum-mechanical methods to account for spin and electron transfer processes typical of sulfide minerals, the total energies of different As-S configurations were calculated at the atomic scale, and the resulting As-S interactions were incorporated into Monte Carlo simulations. Enthalpies, configurational entropies and Gibbs free energies of mixing show that two-phase mixtures of FeS2 (pyrite or marcasite) and FeAsS (arsenopyrite) are energetically more favorable than the solid solution Fe(S,AS)(2) (arsenian pyrite or marcasite) for a wide range of geologically relevant temperatures. Although miscibility gaps dominate both solid solution series, the solubility of As is favored for X-As < 0.05 in iron disulfides. Consequently, pyrite and marcasite can host up to similar to 6 wt.% of As in solid solution before unmixing into (pyrite or marcasite)+arsenopyrite. This finding is in agreement with previously published HRTEM observations of As-rich pyrites (> 6 wt.% As) that document the presence of randomly distributed domains of pyrite+ arsenopyrite at the nanoscale. According to the calculations, stable and metastable varieties of arsenian pyrite and marcasite are predicted to occur at low (X-As < 0.05) and high (X-As > 0.05) As bulk compositions, respectively.