RBS study of galena thermal oxidation in air with a 6-MeV 16O 3+ ion beam

Abstract

Experiments performed on cleaved cm-sized single crystals of galena (PbS) show that heating in air during a few hours at temperatures varying from 400°C to 600°C induces an irridescence. This feature seems to be sought in North Africa to produce coloured eye make-up. Qualitative observation by X-ray diffraction shows the occurrence of anglesite (PbSO 4) and lanarkite (PbO.PbSO 4). The thickness of the oxidised layer is measured by means of the lead signal in the RBS spectrum, using 6-MeV 16O 3+ ions. This probing beam allows one to characterise layers of thickness extending from a few tens to a few hundreds nm. For larger values, a beam of 3-MeV 4He 2+ is used instead. This technique was used to investigate the kinetics of oxidation which involves a complex diffusion phenomenon associated with the formation of reactive phases at the interface between the altered layer and the pristine material. From the thickness of the altered layer and its refractive index we infer that the irridescence is due to an interference phenomenon.