XPS and ab initio calculation of surface states of sulfide minerals: Pyrite, chalcopyrite and molybdenite

Sulfide minerals contain sulphur in a large variety of coordination environments. Consequently, the S 2p XPS of various mineral surface states undergo different shifts in binding energy (BE) relative to the bulk, depending on the charge distribution on the surface. This in turn depends on the number, type and position of the atoms on the fracture surface, which is determined by the fracture mechanism. We have investigated three sulfide minerals: pyrite (tetrahedrally-coordinated S), chalcopyrite (tetrahedrally-coordinated S) and molybdenite (layered structure with trigonally-coordinated S). Comparison of conventional with surface sensitive synchrotron XPS shows that the S 2p spectrum displays two additional doublets at lower BE than the bulk signal for pyrite, and one doublet each at lower and at higher BE for chalcopyrite. Each of these signals is derived from surface states. Molybdenite shows no additional states. A BE shift to lower (higher) BE suggests a charge increase (decrease) on the S atoms relative to those in the bulk because of higher (lower) charge screening. We have used ab initio density functional calculations to validate this interpretation of the experimental evidence, obtaining Mulliken population analyses for the possible fracture surfaces and comparing their charge distribution with the corresponding bulk charge distribution. Our calculations support the assignments of S 2p surface contributions as follows: the lower BE peak of chalcopyrite (160.84eV) as under-coordinated surface S states, the higher BE peak of chalcopyrite (161.88eV) as surface S polymers, the lowest BE peak of pyrite (161.3eV) as surface S monomers, and the next lowest BE peak of pyrite (162.0eV) as under-coordinated surface S dimers. The absence of any surface states in molybdenite is also confirmed by the models.