Rocks are typically not simple materials but are composed of different chemical compounds—minerals. The formation mechanism and environment of a rock can determine the composition, shape, and distribution of minerals in the rock. Understanding the spatial distribution and chemical compositions of minerals in these rocks is vital to determining the mineral and textural development and history.
I’ve outlined below an experiment we conducted using Energy Dispersive Spectroscopy (EDS) acquisitions to rapidly locate and identify sub-millimeter scale mineral phases in the sample.
A thin section of contact metamorphosed Leadville Limestone from Colorado, USA was sectioned, polished, mounted to a glass slide, and finally polished to transparency. It was examined in a Field Emission Scanning Electron Microscope (FESEM) using an X-ray microanalysis system with a fully integrated EDS detector.
Images were collected in the SEM using secondary electrons. EDS Spectral Imaging (SI) mapping acquisitions were performed to collect the distribution of all of the elements in the map analysis region. Spectral Imaging is a technique that collects every X-ray from each pixel location in a scanned region of a map and stores them into a spectrum for each pixel. In this manner, a 3D histogram of X-ray intensities is stored for the dimensions (x, y, Energy). From this 3D histogram, spectra can be extracted for selected regions in the x-y image plane and elemental X-ray maps can be extracted for any elemental lines along the energy axis.
A primary benefit to Spectral Imaging data acquisitions is the ability to perform Multivariate Statistical Analyses (MSA) on the whole 3D histogram. MSA uses only the 3D data set as an input and analyzes all of the contained spectra in a self-consistent manner, comparing the spectra at each pixel to the spectra at every other pixel, looking for similarities and differences. When two pixels have a statistically similar shape (elemental ratios), they are grouped into a common map. When two spectra have statistically different shapes (elemental ratios), the pixels are stored in separate maps. The final result is a series of spectrum-map pairs of the unique chemical materials in the analysis area. These analyses do not rely on the experience level of the analyst to select any data inputs, like elements of interest, but uses only the X-ray intensities within the 3D SI histogram to find common spectral shapes. Because each mineral has a unique spectral shape, each mineral will be found and located at its acquisition location in the scanned region. From these map and spectral results, complete mineralogical phase distributions can be determined.
The secondary electron image of the sample area revealed a number of different phases with interesting phase boundary contrasts. However, it should be noted that intensity and contrast in the electron image was determined by the secondary electron emission of the sample, but is only indirectly related to the composition of the region.
Two areas with similar secondary electron intensities can possess very different compositions. This means that not all of the minerals present in the electron image may be distinguished from other neighboring minerals. Only EDS chemical analyses can provide unique mineral distributions.
The cumulative spectrum from all of the scanned pixels in the SI acquisition is the primary starting data result for most analyses. The peak identification of this spectrum indicates that there are a number of elements to be investigated beginning with the expected O, Mg, Al, Si, K, and Ca, and also the slightly less likely C (probably from beam contamination), S, Fe, and Ba. Note that the intensity of peaks in this spectrum depends on the composition of the minerals analyzed, but also depends on the area fraction of the mineral phases. Large area regions, like the mid-gray that dominates the electron image, provides an over abundance of X-ray intensity, and a small mineral particle which contributes to the spectrum from only a few pixels may not contribute enough elemental intensity in a peak to be observed.
You can see a chart of analytical conditions, instruments used, electron images, and spectra in the application note: EDS Phase Mapping of a Contact Metamorphosed Calc-Silicate Rock.
Our next article will discuss the various mapping conducted, including traditional elemental mapping, quantitative elemental mapping, manual phase mapping, and automated phase mapping.