Micro-structures associated with uraninite alteration

Abstract Primary uraninite from the Canadian Proterozoic unconformity-type uranium deposits have exceptionally low, but variable, δ 18 O values (−32 to −15‰). Although these uranium deposits have been studied extensively, the oxygen isotope systematics in uraninite from these deposits are poorly understood. X-ray powder-diffraction patterns of uraninite with both low and high δ 18 O values show that these uraninite samples are consistent with a composition of UO 2 and are texturally similar. Micro-diffraction and lattice images obtained using high-resolution transmission electron microscopy (HRTEM) of uraninite with the lowest δ 18 O values show that this uraninite is well crystallized and essentially defect-free. Energy dispersive X-ray spectroscopy of well-crystallized uraninite indicates relatively high Pb contents but low Si and Ca contents. In contrast, micro-diffraction and lattice images of uraninite with δ 18 O values near −18‰ show that this uraninite is polycrystalline, with micro-diffraction patterns that often show both streaking and concentric patterns. High-resolution images reveal sub-grain formation and rotation, formation of edge dislocations, low-angle grain-boundaries, and bent lattice-fringes. The uraninite is also characterized by relatively high Si and Ca contents and variable Pb contents. This indicates that incipient alteration occurs on a micro-scale as revealed by HRTEM. The sub-grains in this uraninite were preserved either because the annealing process in uraninite was retarded by the Si and Ca impurities, or they were formed during a late alteration event and the uraninite has had little time to anneal.