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When UO 2 is oxidised to U 3 O 7 , the positions in the crystal lattice of all the uranium atoms and of about 70% of the oxygen atoms are hardly affected. The remaining oxygen atoms occupy new sites which are located 310 pm along 〈1 1 0〉 vectors from the holes in the fluorite framework of UO 2 . These results, which are based on the analysis of neutron diffraction powder data, are consistent with the concept that excess oxygen in U 3 O 7 is accommodated in cuboctahedral anionic clusters.

Samples of uraninite and pitchblende annealed at 1200°C in H 2 , and untreated pitchblende were sequentially oxidized in air at 180–190°C, 230°C, and 300°C. Uraninite and untreated pitchblende oxidized to the U 4 O 9 -type oxide, and their X-ray symmetry remained isometric up to 300°C. Reduced pitchblende after oxidation to UO 2 + x and U 4 O 9 -type oxides transformed into α-U 3 O 8 at 300°C. Two major mechanisms control uraninite and untreated pitchblende stability during oxidation: (1) Th and/or REE maintain charge balance and block oxygen interstitials near impurity cations; (2) the uraninite structure saturates with respect to excess oxygen and radiation-induced oxygen interstitials. Untreated pitchblende during oxidation behaved similarly to irradiated UO 2 in spent nuclear fuel; whereas, reduced pitchblende resembled nonirradiated UO 2 . An analysis of the data in the literature, as well as our own efforts to identify U 3 O 7 in samples from Cigar Lake, Canada, failed to provide conclusive evidence of the natural occurrence of tetragonal α-U 3 O 7 . Most probably, reported occurrences of U 3 O 7 are mixtures of isometric uraninites of slightly different compositions.

A revised structural formula for uraninite, nominally UO 2 , is presented:(U 1−x−y−z 4+ U x 6+ REE y 3+ M z 2+ )O 2+x−(0.5y)−z , which emphasizes the role of cationic substitutions. This formula clarifies the relationship between structural properties (e.g., unit cell parameters), oxidation state (U 6+ /U 4+ ratio), and phase transformations with increasing concentrations of excess oxygen. The formula demonstrates that the amount of excess oxygen required to compensate for increased positive charge due to U 6+ is less than suggested by the amount of U 6+ alone. This may increase the stability field of uraninite under oxidizing conditions relative to synthetic UO 2+ x .

Optical absorption spectra of single crystal UO2 + x have been measured between 0.5 and 2.2 eV, for various values of x between 0.0 and 0.06, and previous spectra up to 6 eV have been reassessed. A steady increase in absorbance at all wavelengths was observed and was linear with x increase. Thus from absorption spectra the value of x in a single crystal of UO2 + x may be determined. The sensitivity of optical absorption spectroscopy permits changes in x as small as 0.0002 to be reliably determined. As excess oxygen entered the lattice a small peak appeared at 0.78 eV, and is identified with the formation of U5+. The oxidation processes in single crystal UO2 + x may therefore be investigated dynamically in-situ and in considerable detail by optical absorption spectroscopy.