Spatially-resolved analysis of natural minerals as carriers of high-tech metals and Rare Earth Elements: comparison of EPMA, PIXE and Sy-µXRF

The assessment of supply risk of high technology elements requires quality assurance of their natural carriers such as ores. One path to achieve this goal is through the use of reference materials (RMs). Providing reliable data on elemental concentrations at the µg/g level and on their spatial distribution is achieved by employing spatially-resolved micro-analytical methods.
In order to produce synthetic RMs with optimized chemical composition, three natural mineral analogues (sanidine, pyrite, columbite) have been tested for chemical homogeneity with three methods based on X-ray detection: EPMA (Electron Probe Micro Analyzer), Sy-µXRF (Synchrotron radiation-induced X-ray Fluorescence) and PIXE. EPMA analyses were carried out at the TU Bergakademie Freiberg using an accelerating voltage of 20 keV and a beam size of 2 µm. Sy-µXRF measurements were performed at the hard X-ray beamline “BAMline” at the synchrotron facility BESSY in Berlin. Samples were exposed in atmosphere to monochromatic X-rays of 20 keV focused with a compound refractive lens to 3x3 µm² or 5x7.4 µm². PIXE data were obtained using a 3 MeV proton beam of about 5x5 µm² from a 3 MV tandem accelerator at the HZDR, Dresden.
Quantitative (EPMA, PIXE) and qualitative (Sy-µXRF) elemental distribution maps have been obtained for major, minor and trace elements for each scan. At least five trace elements were detected in each of the matrices, showing irregular distribution patterns. Compared to PIXE, Sy-µXRF spectra were characterized by lower peak to background ratios, thus, better detection limits could be achieved and more trace elements identified, such as Ge and Pb in sanidine, Mn and Ti in columbite, and Zn and Se in pyrite.
Compared to EPMA and PIXE, only Sy-µXRF seems to be thorough and robust enough for swift homogeneity testing for trace elements distribution in light, medium-heavy matrices, thus, proving to be an invaluable method in quality assurance of synthetic candidate RMs.