Oxidation of uraninite: Does tetragonal U 3O 7 occur in nature?

Abstract

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 4O 9-type oxide, and their X-ray symmetry remained isometric up to 300°C. Reduced pitchblende after oxidation to UO 2 + x and U 4O 9-type oxides transformed into α-U 3O 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 3O 7 in samples from Cigar Lake, Canada, failed to provide conclusive evidence of the natural occurrence of tetragonal α-U 3O 7. Most probably, reported occurrences of U 3O 7 are mixtures of isometric uraninites of slightly different compositions.