Examination of the proposition that Cu(II) can be required for charge neutrality in a sulfide lattice — Cu in tetrahedrites and sphalerite

Abstract

Synchrotron XPS and Cu L2,3-edge NEXAFS spectroscopic data for a natural tetrahedrite surface prepared by fracture under UHV were in accord with the composition of the mineral and its expected semiconductivity. The 2p binding energy for the 6-coordinate S atoms was found to be not detectably greater than that for the 4-coordinate S atoms, and surface species were not clearly discernible in either surface-optimized S 2p or Cu 2p spectra. The Cu 2p and Cu L2,3-edge spectra indicated that all Cu in the mineral was indisputably Cu(I). The Cu L2,3-edge spectra of relatively pure natural sphalerite treated with mildly acidic aqueous cupric solution revealed the presence of Cu(II) in the outermost layer of the fracture surfaces, but it was concluded that most of the Cu near the surface of the mineral was in formal oxidation state Cu(I), albeit with higher than normal d9 character. The Cu(I) absorption peak was at an energy much lower than for the tetrahedrite absorption edge, but still consistent with Cu(I) in 4-fold coordination by S. The Cu(II) was consistent with Cu bonded both to S atoms in the outermost layer of the sphalerite and to O atoms in chemisorbed water. S 2p spectra determined at different photon energies revealed high binding energy components arising from oligosulfide-like environments in the outermost layers, but not necessarily in a completely restructured lattice and not in a Cu oligosulfide only. The data indicated some loss of Zn in addition to the Zn that had been replaced by Cu in the outermost layers of the sulfide lattice. The presence of these oligosulfide-like environments precluded the detection of S with formal oxidation state greater than (-II) that might have arisen only from Cu(I) in the S lattice. No evidence was obtained for the presence of Cu(II) in a sulfide lattice, but it was not possible to exclude the possibility of a very low concentration because of the presence of the Cu(II) bonded to both S and O at the surface of the treated sphalerite.Key words: tetrahedrite, sphalerite, copper uptake, XPS, NEXAFS.