The Crystal Structure of Olivine and the Effect of Crystal Orientation on X-ray Absorption Spectroscopy Thesis uri icon

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abstract

  • Thesis (Ph.D., Geology) -- University of Idaho, 2017 | X-ray absorption near edge structure (XANES) spectra have been shown to be dependent on crystal orientation. This study characterizes the variation in spectra along the a, b, and c crystallographic axes for olivine group minerals of varying compositions with the formula MgxFe1-xSiO4. Single crystal X-ray diffraction was used to collect the unit cell parameters, atomic coordinates, and bond lengths. The same single crystals were then oriented optically using a spindle stage for X-ray absorption spectroscopy, and for some crystals, the composition was determined by electron probe microanalysis (EPMA) afterward. The compositions from refinement were compared to EPMA results from analysis of the bulk mineral samples, or the same single crystal when possible, and are nearly identical. The partitioning of Mg and Fe into the M1 and M2 crystallographic sites was found to be correlated to the total %Mg in each sample. A small site preference was observed at high levels of total iron, where there is a higher percentage of the total magnesium in the M2 site than there is in the M1 site. The pattern is not as strong for iron in the M1 site at high levels of total magnesium. Neither site seems to be influenced by the concentration of Fe3+. The three crystallographic axes are distinct in the XANES spectra, however samples with lower iron are more easily distinguishable, possibly due to self-absorption. There is a correlation at the pre- edge between peak intensity and total iron, particularly along the b axis.

publication date

  • June 1, 2017

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