Quantitative X-ray photoelectron spectroscopy study of enargite (Cu3AsS4) surface

dThe surface chemical state and composition of natural and synthetic enargi te samples were characterized with X-ray photoelectron spectroscopy (XPS). Freshly cleaved, powdered and 'as received' samples were analysed at liquid nitrogen temperature. For the natural 'as received' samples as well as for the natural and synthetic powdered samples the binding energies of Cu 2p(3 /2), As 3d(5/2) and S 2p are consistently found at 932.5 +/- 0.2, 43.9 +/- 0.2 and 162.2 +/- 0.2 eV, respectively. A second component of S 2p at 163.9 +/- 0.2 eV is present and may be assigned to elemental sulphur. Freshly cl eaved enargite samples showed binding energies of 43.2 and 161.3 eV, respec tively, for As 3d(5/2) and S 2p(3/2) At the surface of crystals 'as receive d', an additional signal at similar to 169 eV, referable to sulphate sulphu r, is also present, as well as additional signals for Cu 2p(3/2) (similar t o 934 eV), and As 3d(5/2) (similar to 45.5 eV). The thickness and the compo sition of this outer oxidized layer together with the composition of the in terface beneath the film are calculated on the basis of a model developed f or this purpose, taking into account the attenuation of the photoelectrons due to the contamination and the oxidized layer. An oxidized film of simila r to0.5 nm is present at the surface of the 'as received' crystals whose co mposition was found to be similar to 45% copper, 24% arsenic and 31% sulphu r; hence, the oxidized layer appears to be enriched in arsenic compared wit h the bulk composition. The composition of the material beneath the oxidize d layer was found to be enriched in copper and depleted in sulphur with res pect to the bulk analysis. Any further study of the interaction of natural enargite samples with the environment has to take into account the existenc e of this thin outer layer. Copyright (C) 2001 John Wiley & Sons, Ltd.