SCANNING-TUNNELING-MICROSCOPY STUDIES OF GALENA - THE MECHANISM OF OXIDATION IN AIR

Scanning tunnelling microscopy (STM) was used to study the surface oxidation of galena in air. The main aim of the study was to compare the surface oxidation behaviour of synthetic and natural galena samples and to contrast the different mechanisms for the air oxidation process. Topographical imaging of the galena surfaces showed the growth of nanometre-scale oxidation products with time. The position of these oxidation products and the rate of their formation depended strongly on galena type. Oxidation products formed on natural galena after a few minutes exposure to air, whereas synthetic galena required an induction period of over an hour before oxidation products could be detected. X-ray photoelectron spectroscopy confirmed the different rates of oxidation and showed lead hydroxide and sulphate to be the main oxidation products. Oxidation product growth on the synthetic sample occurred preferentially on edges and dislocations rather than on the faces of the galena surface. Oxidation product growth on the natural galena showed no such preference, with indiscriminate coverage of edges and faces. It is proposed that impurities present at the surface of the natural galena, which are absent in the synthetic galena, are responsible for the significantly different oxidation behaviour.