EVIDENCE OF RADIATION DAMAGE IN MICROLITE FROM THE HARDING PEGMATITE, TAOS COUNTY, NEW MEXICO: AN EDS AND EBSD STUDY

The characterization of the spatial distribution of radiation damage in minerals is relevant to the problem of nuclear waste stabilization that challenges governments worldwide. Naturally occurring radioactive minerals serve as analogs for the long-term behavior of synthetic materials that might be used to encapsulate radioactive waste. This project characterizes radiation damage and chemical alteration in natural microlite (Na,Ca,U)₂(Ta,Nb)₂O₆(O,OH,F) from the Harding Pegmatite, New Mexico. These minerals contain approximately 0.1 to 5 wt% UO₂ and crystallized about 1.4 billion years ago. Backscatter electron images show that grains with high UO2 concentration (between 3 and 5 wt% UO₂) exhibit abundant fractures and a distinctive mottled texture. We used electron backscatter diffraction to characterize the relative amount of radiation damage within three crystals of microlite, based upon the image quality of the diffraction patterns. X-ray diffraction was used to map the spatial distribution of UO₂ within each crystal. Using these methods we established correlations between regions of damage and UO₂ concentration across entire crystals (0.1-1.0 mm). One grain with minor UO₂ zoning (rim, 0.86%, core 0.08%) also showed significant variations in image quality. Results show that higher UO₂ concentrations correlate with low image quality, as expected. X-ray maps were collected for Pb, Ca, and Na. Highest Pb concentrations occur within fractures. Zones of chemical alteration typically extend 0.01-0.05 mm away from fractures. These areas are depleted in Na and enriched in Ca. The spatial distribution of Pb, Ca, and Na are evidence of fluid induced chemical alteration in regions of heavy radiation damage.