Oxidation of synthetic and natural samples of enargite and tennantite: 2. X-ray photoelectron spectroscopic study

The surface oxidation of synthetic and natural samples of enargite and tenn antite has been monitored by X-ray photoelectron spectroscopy, XPS. The min erals were conditioned at pH 11.0 in an aqueous solution purged with nitrog en gas for 20 min or with oxygen gas for 60 min. The XPS results show that the oxidation layer on the mineral surface is thin. The surface oxidation p roducts comprise copper and arsenic oxide/hydroxide, sulfite, and a sulfur- rich layer made of metal-deficient sulfide and/or polysulfide. The proporti on of all of these oxidation products at the mineral surface is more import ant when the minerals are treated in more oxidizing conditions (i.e., with oxygen gas and for a longer time) for tennantite than for enargite and for the natural samples than for the synthetic samples. Different arsenic sulfi de species have been found at the surfaces of enargite and tennantite: As4S 4 Or As2S3 constitutes the major arsenic sulfide species at the surface of enargite, but these are the minor arsenic sulfide species at the surface of tennantite and in the bulk of both minerals. This difference is not relate d to a surface impurity in the natural enargite sample as it is also observ ed in the synthetic enargite sample.