OXIDATION OF URANINITE - DOES TETRAGONAL U3O7 OCCUR IN NATURE

Samples of uraninite and pitchblende annealed at 1200 degrees C in H-2 , and untreated pitchblende were sequentially oxidized in air at 180-1 90 degrees C, 230 degrees C, and 300 degrees C. Uraninite and untreate d pitchblende oxidized to the U4O9-type oxide, and their X-ray symmetr y remained isometric up to 300 degrees C. Reduced pitchblende after ox idation to U4O(2+x) and U4O9-type oxides transformed into alpha-U3O8 a t 300 degrees C. Two major mechanisms control uraninite and untreated pitchblende stability during oxidation: (1) Th and/or REE maintain cha rge balance and block oxygen interstitials near impurity cations; (2) the uraninite structure saturates with respect to excess oxygen and ra diation-induced oxygen interstitials. Untreated pitchblende during oxi dation behaved similarly to irradiated UO2 in spent nuclear fuel; wher eas, reduced pitchblende resembled nonirradiated UO2. An analysis of t he data in the literature, as well as our own efforts to identify U3O7 in samples from Cigar Lake, Canada, failed to provide conclusive evid ence of the natural occurrence of tetragonal alpha-U3O7. Most probably , reported occurrences of U3O7 are mixtures of isometric uraninites of slightly different compositions.