Electronic environments in carrollite, CuCo 2S 4, determined by soft X-ray photoelectron and absorption spectroscopy

Abstract

Fracture surfaces of a natural carrollite specimen have been characterised by synchrotron and conventional X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy. For the synchrotron X-ray measurements, the mineral surfaces were prepared under clean ultra high vacuum and were unoxidised. The characterisation was undertaken primarily to establish unequivocally the oxidation state of the Cu in the mineral, but also to obtain information on the electronic environments of the Co and S, and on the surface species. Experimental and simulated Cu L 2,3-edge absorption spectra confirmed an oxidation state of Cu I, while Co 2 p photoelectron and Co L 2,3 absorption spectra were largely consistent with the Co III established previously by nuclear magnetic resonance spectroscopy. S 2 p photoelectron spectra provided no evidence for S to be present in the bulk in more than one state, and were consistent with an oxidation state slightly less negative than S –II. Therefore it was concluded that carrollite can be best represented by Cu ICo III 2(S 4) –VII. The Cu I oxidation state is in agreement with that expected for Cu tetrahedrally coordinated by S, but is in disagreement with the Cu II deduced previously from some magnetic, magnetic resonance and Cu L-edge X-ray absorption spectroscopic measurements. A significant concentration of S species with core electron binding energies both lower and higher than the bulk value were formed at fracture surfaces, and these entities were assigned to monomeric and oligomeric surface S species. The density of Cu d states calculated for carrollite differed from that previously reported but was consistent with the observed Cu L 3 X-ray absorption spectrum. The initial oxidation of carrollite in air under ambient conditions was confirmed to be congruent, unlike the incongruent reaction undergone by a number of non-thiospinel sulfide minerals.