Energy Dispersive Analysis
Scanning electron microscopy is useful not only for examining sediment textures, but, when equipped with an energy dispersive analyser, it can be used effectively for mineral identification and semi-quantitative chemical analysis. The analyses are rapid (seconds) and require relatively little sample preparation. In most cases, small chips of the sample can be mounted on a small plug with no polishing or cutting required. The sample is then coated with a gold-palladium alloy (or other conductive metal) and is inserted into the SEM.
Here, a potassium feldspar from the Cretaceous Frontier Formation of Wyoming is shown with an accompanying analytical spectrum. Although the grain might be identifiable as a feldspar on the basis of its crystal shape, cleavage, and other features alone, the energy dispersive analysis provides additional chemical data which allows positive identification.
The analytical trace (lower photo) shows major peaks for Si and K (the main K peak has the long, pale blue line over it) with only very minor peaks for other elements. This indicates a rather pure K-feldspar composition.
Although energy dispersive analysis on the SEM provides an excellent tool for mineral identification, it is not ideally suited for quantitative analytical work. Detailed determination of mineral composition or analysis of trace element contents of small crystals is best done using polished samples on an electron microprobe.