Abstract

The effects of ion bombardment on homogeneous samples of quartz, albite, calcite, and anhydrite have been studied with X-ray photoelectron spectroscopy (xps) in order to better understand the usefulness and limitations of sputter depth profiles of minerals. Under the experimental conditions used (45° incident 1-kV Ar ions for a total of 20 min per sample; total ion dose = 1017 ions/cm2), between 10 and 20 nm have been sputtered from each sample, xps measurements have shown that compositional and structural modifications occur in the upper few nanometers of the mineral surface within the first minute of sputtering and after less than 1 nm of surface removal. The extent of the ion-induced modifications in the near-surface region reaches an apparent near-steady state after 3 to 4 min of sputtering. Therefore, after this time, significant changes in atomic concentrations as measured by xps or Auger electron spectroscopy (aes) should indicate a true compositional gradient inherent to an unknown sample and not to the sputtering process itself. In addition, a sputter profile from a standard homogeneous sample similar to an unknown can provide a basis for interpreting the entire unknown profile despite preferential sputtering. Unfortunately, preferential sputtering and structural modifications preclude useful interpretations of oxidation state and atomic structure information from the photoelectron spectra during depth profiling. In addition, although the use of implanted Ar as a charge-reference standard is theoretically sound, we have found that the structural damage to mineral near-surfaces during even light implantations is significant; thus, the usefulness of this method of charge referencing for minerals is greatly diminished.

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