Actinide Microdistributions Within Monazite

Abstract

AbstractThe results of an induced fission track mapping study of uranium in 12 pegmatitic monazite samples indicate that as the U concentration in monazite surpasses ca. 300–400 ppm, the microdistribution becomes much less homogeneous resulting in localized areas of very high uranium concentration (hot spots) in a phase undetectable by XRD. Microprobe analysis of three additional pegmatitic monazites shows the occurrence of exsolution of the two other endmembers of the monazite solid-solution series, huttonite and cheralite, as indicated by a normative mineralogy computed from the probe data. The presence of each mineral is confirmed by XRD. Also supporting this exsolution hypothesis is a strong similarity between ratios of the structural elements of monazite (LREE, P) within the Th-rich hot spots as compared to these ratios in the surrounding monazite. Gradational contacts between banded monazite and a Th-rich phase can be observed microscopically as well as through microprobe scans for Th, indicating the close genetic relationship of the two phases. To prevent their enhanced leachabilities, actinide redistribution must be prevented in any single- or multi-phase nuclear waste form.