Evaluating Synchrotron-Based Scanning Laue Microdiffraction for Mineralogy Mapping in Heterogeneous Samples

Abstract

Synchrotron Laue microdiffraction imaging (MDI) is a well-used technique in material science research and environmental research to determine the strain/stress and orientation of quartz and calcite crystallites. However, Laue MDI has unrealized potential to provide spatially resolved mineralogical information for geochemical and contaminated site samples. In this manuscript, three constructed mineral mixtures and two environmental samples were analyzed with Laue MDI to illustrate the strengths, limitations/challenges, and applicability of the technique for environmental research. Mixture 1 (quartz, calcite, and magnetite), mixture 2 (quartz, calcite, magnetite, and clinochlore), and mixture 3 (calcite, hematite, sphalerite, and arupite) were constructed to highlight the potential limitations of the technique. The mixtures illustrate the potential of Laue MDI and X-ray fluorescence imaging for clearly identifying environmentally relevant contaminant and sorbent minerals while demonstrating that minerals smaller than the incident beam are challenging to index. Two environmental samples were then used for comparison to the synthetic mixtures. Laue MDI analysis was successful in identification of a Zn-substituted magnetite/non-stoichiometric franklinite rather than a pure franklinite phase in a Zn smelter-contaminated soil. Laue MDI was also crucial in the evaluation of the particle size of carbonates to distinguish between parent material (calcite) and secondary carbonate minerals (dolomite) in a calcareous soil undergoing phosphorus nutrient applications to stimulate hydrocarbon remediation. Despite challenges and limitations, Laue MDI, when correctly used, is a valuable technique for earth and environmental science researchers.