The occurrence and mineral associations of 20 specimens of uraninite and thorianite are described. All the deposits occur in the Grenville Sub-province of the Canadian Shield.

The range in composition of these 20 specimens as determined by x-ray spectrography is U3O8 25-73%, ThO2 7-55% and PbO 7-14%. Lattice constants (a0) generally increase with increasing thorium content and range from 5.449 to 5.564 angstroms. After heating in air at 1000° C. for ten minutes the lattice constants are all reduced and more closely approach a lineal relation to the composition. The fact that none of the specimens oxidized to U3O8 on heating, indicates that they belong to the α group of Wasserstein.

The 20 specimens are readily subdivided into three distinct groups within each of which composition, lattice constants and type of deposits are distinctive.

An attempt to relate diffraction intensities of four different sets of planes to composition was inconclusive.

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