Sector Zoning in Uraninite

A uraninite crystal from the Roode pegmatite in southern Norway displays clear sector zoning mostly defined by the main U-Th substitution, with sharp changes in chemical compositions between the cubic and the octahedral sectors; a weaker concentric zoning defined by the same chemical variations is also present. The {100} sectors incorporate slightly more U whereas the {111} sectors incorporate slightly more Th, Y, and REEs. The cubic sectors contain on average ca. 77.0 wt.% UO₂, ca. 6.6 wt.% ThO₂, ca. 0.6 wt.% ΣREE₂O₃, and ca. 0.45 wt.% Y₂O₃; the octahedral sectors contain on average ca. 73.8 wt.% UO₂, ca. 9.1 wt.% ThO₂, ca. 0.8 wt.% ΣREE₂O₃, and ca. 0.55 wt.% Y₂O₃. The development of sector zoning in uraninite is possibly due to the interplay between the difference in effective ionic radius between U and Th and the difference in protosite density between the {100} and {111} sectors.