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Combined X-ray diffraction and alpha particle X-ray spectrometer analysis of geologic materials

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Title: Combined X-ray diffraction and alpha particle X-ray spectrometer analysis of geologic materials
Author: Perrett, Glynis; Maxwell, John; Campbell, John
Department: Department of Physics
Abstract: The shallow interrogation depth of the lightest elements (Na, Mg, Al, Si) detected by the PIXE branch of the Curiosity Rover’s alpha particle X-ray spectrometer suggests that the X-rays of these elements very likely emerge from a single mineral grain. This reality violates the assumption of atomic homogeneity at the micron scale made in both existing spectrum reduction approaches for the APXS. Consequently, analytical results for these elements in igneous geochemical reference materials exhibit deviations from certified concentrations in a manner that can be related to the TAS diagram. A computer code is introduced here to provide quantitative prediction of these deviations using the mineral abundances determined from XRD. The latter are converted to area coverage fractions to represent the sample surface, and a fundamental parameters computation predicts the elemental X-ray yields from each mineral and sums these. In this process, the chemistry of each individual mineral has to be varied by an iterative Simplex approach; X-ray yields are computed and compared to the peak areas from the fit of the bulk sample. When the difference between mineral yields and peak areas for each element are minimized, the mineral formulae are set and elemental X-ray yields provided. The ratio between the summed mineral X-ray yields and the corresponding yields based on the homogeneity assumption may then be compared directly to the concentration deviations measured in our earlier work. For several rock types, good agreement is found, thereby consolidating our understanding of the effects of sample mineralogy on APXS results.
URI: http://hdl.handle.net/10214/10246
Date: 2017-02-17
Citation: Perrett, G. M., Maxwell, J. A., and Campbell, J. L. (2017) Combined X-ray diffraction and alpha particle X-ray spectrometer analysis of geologic materials. X-Ray Spectrom., doi: 10.1002/xrs.2743.


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