Uraninite is a highly radioactive cubic mineral of chemical formula UO2. It is ubiquitous in granite but in small quantities. It is the main source of uranium in ore deposits and it contains also minor amounts of radium, thorium, and helium. Pitchblende is the name given to ore that contains a mixture of uraninite and some other minerals. The presence of detrital uraninite in ca. 2.7–2.9 Witwatersrand conglomerates in South Africa is taken as evidence of the reduced state of the Archean atmosphere. The appearance of large uranium ore deposits in the early Proterozoic is related to the Great Oxygenation Event.