THE PHOTOELECTROCHEMISTRY OF GALENA

The photoresponse of galena to chopped illumination has been studied to elucidate the surface oxidation of this mineral relevant to flotation. The results substantiated previous findings that sulfur remains in the mineral lattice during the initial stages of oxidation. For cleaved surfaces, the potential dependence of the characteristics of photocurrent transients and the photocurrent observed on potential scans have been interpreted in terms of light-induced acceleration of anodic oxidation to remove lead ions and leave lead vacancies in the lattice and of the corresponding cathodic reduction process resulting in sulfur vacancies. Rotating ring-disk electrode studies confirmed that anodic and cathodic processes leading to dissolved Pb2+ or H2S occur at all potentials. Investigations with galena of 99.99% purity showed that the photoelectrochemical phenomena were not a result of impurity atom migration to the surface. XPS studies on the pure galena showed incongruent initial oxidation as reported previously for natural galena containing minor elements. Natural galena samples from the same cluster of crystals range from highly n-type to slightly p-type. However, most of the natural samples were highly n-type and exhibited only anodic photocurrrents. Abrading the surfaces under water in equilibrium with air greatly reduced the magnitude of the photoresponse and changed the shape of the photocurrent-time and photocurrent-potential curves. The results indicated that abrasion produces a thick p-type layer at the galena surface.