A digital image-processing technique was modified to provide a more accurate modal analysis of sediments than the standard point-counting procedure. A digital backscattered-electron image covering an area that includes at least 300 grains is processed to form a binary image template for beam control. The automated process re-examines each feature or grain and collects an X-ray spectrum with an energy-dispersive spectrometer. The advantage of this procedure is that instead of analyzing a single spot on a grain, our system rasters the electron beam over the entire grain surface. The eds spectra therefore average chemical variations over the entire grain. The stored analyses are background corrected, and the element intensities are utilized to calculate oxide weight percentages for mineral identification. All analyses were by a Tracor Northern image-processing system (ipa-5700) adapted to a jeol 733 Superprobe.

To demonstrate the utility of the analytical technique, six grain mounts of sand- and silt-sized particles from the Cubits Gap crevasse-splay, Mississippi Delta, were analyzed. There were no size-dependent differences in the mineralogy of the samples. Quartz makes up 50-60 vol% of the samples analyzed. Feldspars and lithic fragments occur in approximately equal quantities, and each may constitute 20-30 vol% of the material. The carbonates and accessory minerals account for less than 5% of the rock volume. Other mineral and rock fragment categories may be selected by modifying the elements to be analyzed and adjusting the classification procedure accordingly.

The technique—referred to as chemod—provides more reliable estimates of feldspar abundances than optical methods because it is based on chemical composition. This chemical approach provides estimates of compositional diversity within groups of minerals or rock types. The image-analysis technique also measures grain size and shape parameters. The outcome is an accurate and reproducible chemical, mineral, and textural analysis from a single polished specimen.

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