Investigation of the electronic and geometric structures of the (110) surfaces of arsenopyrite (FeAsS) and enargite (Cu₃AsS₄)

The (110) surfaces of arsenopyrite (FeAsS) and enargite (Cu ₃AsS₄) have been modelled using a Density Functional Theory (DFT) plane-wave pseudopotential method (CASTEP) in order to better understand aspects of the geometric and electronic structures of these minerals, which have important implications for the release of arsenic in acid mine drainage environments. In this study, bulk calculations of these minerals have been conducted to give consistent geometries for surface models and to establish reference states for changes at the surfaces in these models. Surface structure experimental data for enargite were collected using low-energy electron diffraction which confirmed an unreconstructed 1 × 1 (110) lattice, with a and b values of 6.38±0.3 Å and 9.92±0.5 Å, respectively. Surface calculations demonstrate geometric and electronic relaxation of both arsenopyrite and enargite (110) surfaces. Changes in atomic positions, interatomic distances, Mulliken charges and electronic configurations are reported. Enargite has a surface energy of ∼0.02 eV/Ų compared with arsenopyrite which has a surface energy of ∼0.11 eV/Ų, indicating that the enargite (110) surface is more energetically stable than that of arsenopyrite. The most stable surfaces are those which relax to restore the surface coordination and partial charge balance. For both minerals this is achieved by the formation of covalent bonds. Arsenic has the most positive Mulliken charge of all the surface atoms and is, therefore, predicted to be the most reactive atom at the arsenopyrite and enargite (110) surfaces. This implies that, according to these calculations, arsenic is most likely to react with oxidative species such as O₂ and H₂O in environments such as those associated with acid mine drainage, potentially releasing oxides and acids of arsenic into the environment.