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

Abstract

The shallow interrogation depth of the lightest elements (Na, Mg, Al, and Si) detected by the particle-induced X-ray emission 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 alpha particle X-ray spectrometer. 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 total alkali-silica diagram. A computer code is introduced here to provide quantitative prediction of these deviations using the mineral abundances determined from X-ray diffraction. 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 with 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 with 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 alpha particle X-ray spectrometer results. Copyright © 2017 John Wiley & Sons, Ltd.