APPLICATIONS OF AUGER-ELECTRON SPECTROSCOPY TO GEOCHEMISTRY

The composition of the uppermost 50 Angstrom of mineral surfaces is re adily determined by Auger electron spectroscopy (AES). Lateral resolut ion (analytical spot-size) less than 1 mu m can be achieved routinely, and the concentration of most elements can be quantitatively determin ed at levels as low as one-quarter to one weight percent. Auger instru ments typically include an ion sputter-gun, which provides the means t o obtain Angstrom-scale quantitative compositional depth-profiles of t he near-surface zones of minerals. Compositional depth-profiles throug h air-oxidized pyrrhotite surfaces are reported here, and demonstrate that Fe diffuses rapidly from the bulk to the surface of pyrrhotite gr ains during oxidation by air. Auger results of a goethite surface, com bined with electron-microprobe results, demonstrate that Al is distrib uted throughout the bulk of the mineral, and that Ni is concentrated a t the surface (uppermost 50 Angstrom). Sensitivity factors for Fe and O in goethile are derived from the data. The surface sensitivity of AE S, its high spatial resolution, and the ability to obtain compositiona l depth-profiles, yield an exceedingly powerful tool to study the near -surface regions of minerals. The technique is a boon to experimental petrologists and geochemists in that detection of Angstrom-thick leach ed veneers and authigenic overgrowths helps circumvent the ever-presen t problem of slow rates of reaction.