A small selection of soil samples was prepared for characterization using our group's ASPEX instrument. As a comparison method, I explored the use of X-ray maps acquired with more common electron beam instruments. Digital X-ray maps can be manipulated using software tools like IDL to evaluate the chemistry of individual grains. Crystal chemistry based criteria are used to sort the grains by mineral type, accomplishing a modal determination based on tens or hundreds of thousands of data points. If required, the modal data can be used to calculate a bulk composition. Importantly though, this method allows for the characterization of unique or minor chemical features, of even individual soil grains, that cannot be observed with bulk chemical assays.
While this method works well with sections of rock (closed or known system) where the mineralogy is determined prior to X-ray mapping, it requires extra effort to use with soil (open or unknown system) samples.