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Reflectance spectroscopy and imaging spectroscopy of rare earth element-bearing mineral and rock samples Turner, David James

Abstract

A significant knowledge gap between the fields of reflectance spectroscopy and rare earth element (REE) mineralogy prompted this research effort. It narrows the knowledge gap through detailed study of thirty three samples representing three important mineral classes: REE fluorocarbonates (bastnaesite, synchysite, and parisite), REE phosphates (monazite, xenotime, and britholite), and REE-bearing silicates (cerite, mosandrite, kainosite, zircon and eudialyte). Reflectance spectroscopy was carried out in the visible to short wave infrared regions (500 nm to 2500 nm) and each sample was characterized using scanning electron microscopy and electron microprobe analysis. Spectral features of these minerals are primarily related to numerous 4f-4f intraconfigurational electronic transitions of trivalent lanthanides (Ln³+), as well as 5f-5f electronic transitions of uranium and vibrational overtones and combinations of CO₃²-, H₂O, PO₄³- and OH- where applicable. In general, the respective spectra of these REE minerals are sufficiently distinct for spectral classifications, and compilation diagrams with representative spectra are given. Broadly speaking, the light REE-enriched minerals are dominated by sharp absorptions related to Nd³+, Sm³+, and Pr³+ with lesser input from Eu³+, whereas heavy REE-enriched minerals are dominated by sharp spectral features related to Er³+, Dy³+ and Yb³+ with lesser input from Nd³+, Tb³+, Ho³+ and Tm³+ depending on their specific concentrations. For those minerals that do not show strong preference for light or heavy REE, a mixed set of absorption patterns is seen. Spectral variability of specific 4f-4f absorptions were substantial between different minerals and these variations are interpreted to be driven by the specific anion coordination at the Ln³+site across various crystal structures. Shifts in wavelength position and relative strengths of related absorptions can be significant enough to be highly relevant for hyperspectral remote sensing. This is especially applicable for REE mineral identification in field-based settings and high spatial resolution imaging spectroscopy. Three themes of ongoing and potential research are discussed: additional REE mineral spectroscopy, exploitation of diagnostic features for REE mineral detection and identification, and REE ore grade estimation. Overall, the research presented in this dissertation sets the foundation for future interpretation and exploitation of reflectance spectra for the REE minerals.

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Attribution-NonCommercial-NoDerivs 2.5 Canada

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