Reconstruction of fracture surfaces on bornite

Synchrotron-radiation X-ray Photoelectron Spectroscopy (SRXPS) and conventional X-ray Photoelectron Spectroscopy (XPS) have been used to study a pristine fracture-surface of bornite (Cu₅FeS₄). Comparisons of these high-resolution spectra reveal, for the first time, four distinct contributions to the S 2p spectra. The main symmetric peak of the S 2p spectra for bornite is located at about 163.51 eV and is derived from bulk S atoms. The broad nature of the bulk contribution, in comparison to other 3d transition-metal sulfides such as chalcopyrite, is consistent with the presence of eight crystallographically distinct S sites within the structure, these sites being energetically as well as structurally distinct. A second peak located at 160.1 eV is attributed to a surface monomeric species (S^2-) of lower coordination. The presence of a second broad surface-contribution at 162.1 eV likely represents surface polymeric species (S_N^2-). The presence of surface sulfur polymers indicates the stabilization of the bornite surface upon fracture through formation of S-S bonds. These data suggest that surface polymers form where polar surfaces are exposed during conchoidal fracture. A high binding-energy tail was observed at about 163 eV and extends to about 166 eV, the origin of which is uncertain. Conventional XPS Cu 2p and Fe 2p spectra collected from a pristine fracture-surface of bornite reveal a Cu¹⁺ peak centered at about 932.2 eV and a high-spin Fe³⁺ peak centered at 708 eV.