Molecular and elemental characterisation of mineral particles by means of parallel micro-Raman spectrometry and Scanning Electron Microscopy/Energy Dispersive X-ray Analysis

The “fingerprinting” of a molecular structure obtained by micro-Raman spectroscopy (MRS) can be successfully complemented by means of X-ray spot analysis through the application of scanning electron microscopy equipped with an X-ray detector (SEM/EDX). The elemental composition revealed by SEM/EDX is essential for a correct interpretation of the collected Raman spectra. The results presented here illustrate how the two techniques can be combined to characterize geological samples, especially in the case of individual particles. The samples involved in the experiments were Zr- and Ti-bearing sand from South Africa (with major minerals such as zircon and rutile) and U mine tailings from Hungary (rich with feldspars, quartz and sulphate minerals). Mineral phases detected by MRS were identified according to their respective main Raman shifts, with a spatial resolution up to 1 μm, depending on the parameters set. Some unusual and sometimes inexplicable Raman activity was observed, which was ascribed to and rationalized by the presence of accompanying elements as detected with EDX. The relocation of a particle by means of the two instruments was facilitated with TEM grids. Although the limitations of the sequential use of SEM/EDX and MRS, such as different beam sizes, probing depth and surface topography, should be considered in their application to the analysis of individual geological particles, the two methods appeared to be complementary. Not only do they provide correlated chemical information about the sample, but also enable chemical characterization that would be otherwise incomplete when analyzed on a stand-alone basis.