Uranium dioxide (UO), a major oxide mineral of uranium. Uraninite is radioactive and usually forms black, gray, or brown crystals that are moderately hard and generally opaque. The elements uranium and radium were first extracted from uraninite ore from what is now the Czech Republic. It has also been mined in Germany and Canada, and in the Colorado Plateau (U.S.). See also pitchblende.
Uraninite in pitchblende from Great Bear Lake, Northwest Territories, embedded (for display) in a
—Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard
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