Monday, August 1, 2011

BEI COMPO Directed EDS

This is a secondary electron (SEI) image of the mineral syenite. Syenite is an igneous rock containing coarse grains of feldspar and other mineral phases. As it is insulating by nature, it was flash coated with ~ 10 nm of graphite to allow for imaging and EDS. While AuPd alloy coating is preferred for imaging, the graphite coating provides little attenuation of X-rays and is ideal for performing EDS on insulating materials. The SEI image of the mineral specimen shows some well defined grain structures in a matrix of material that cements these grains together. Based on the SEI image alone there is little to motivate subsequent EDS analysis other than the presence or absence of grains.

The second image is a back scattered image in compositional mode (BEI COMPO). This mode of imaging is sensitive to density or average atomic number, Z. It is immediately obvious that the presence of crystal habits or grains in the SEI image doesn't tell much of the story. There are high density grains in a matrix of low density material. Upon closer examination, many of the faceted surfaces in the low density region are likely due to the impression of high density grains that have been removed upon fracturing. Two EDS regions of interest are clearly identified: #1 being on a large high density grain; #2 in the low density cement.

The EDS spectrum in region #1 shows primarily Fe with a trace of the rare earth lanthanum. This syenite was of interest as a source of rare earths. The EDS detector window is dirty so the presence of oxygen would not be detected, and thus it is indeterminate whether this is metallic Fe or FeOx-- but presumed to be the later. Region #2 shows the presence of Fe-- except without the rare earth element La. Al, Si, Mg and Ca are also identified which is understood to be the tectosilicate (e.g. feldspar) portion of the syenite mineral sample.

Since one is looking at chemistry with EDS, BEI COMPO is a very useful tool to motivate one's analysis by identifying regions of interest that would be otherwise unidentified in SEI images.

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