Uraninite

uraninite: see pitchblende.

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uraninite

Uraninite in pitchblende from Great Bear Lake, Northwest Territories, embedded (for display) in a …

(credit: Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard)

Uranium dioxide (UO₂), 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.

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uraninite [′yu̇r·ə·nə‚nīt]

(mineralogy)

UO₂A black, brownish-black, or dark-brown radioactive mineral that is isometric in crystallization; often contains impurities such as thorium, radium, cerium, and yttrium metals, and lead; the chief ore of uranium; hardness is 5.5-6 on Mohs scale, and specific gravity of pure UO₂is 10.9, but that of most natural material is 9.7-7.5. Also known as coracite; ulrichite.

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Uraninite 

a mineral, an anhydrous oxide of uranium (U⁴⁺) with the idealized formula UO₂, which occurs only in synthetic materials. All natural uraninites contain UO₃ along with UO₂. The ratio of UO₂ to UO₃ is expressed by the value of the oxygen coefficient, which ranges from UO_2.17 to UO_2.92.

A distinction is made between uraninite proper, which occurs in the form of distinct crystals, pitchblende, which occurs in the form of cryptocrystalline collomorphic aggregates, and uranium oxides, which occur as friable earthy aggregates. Uraninite proper forms an isomorphic series with both thorianite, ThO₂, and yttrocerianite, (Y,Ce)O₂. In addition, all uraninites contain products of the radioactive decay of uranium and thorium: potassium, actinium, polonium, helium, lead, calcium, and zinc. Taking into account the most common admixtures, the formula for uraninite is (U⁴⁺ + U⁶⁺, Th, TR, Pb, Ca)O_1.9–2.5.

Uraninite crystallizes in the isometric system. The structure of ideal uraninite is similar to that of fluorite. The symmetry of the crystal lattice in natural uraninites is diminished owing to the intrusion of uranyl groups UO²⁺ into the structure, resulting in a primitive isometric structure. The most commonly encountered crystal shapes are cubes, octahedrons, and combinations thereof.

A brittle mineral, uraninite is of black color and has a pitchlike luster. It has a hardness of 5–6 on Mohs’ scale and a density of 8,000–10,000 kg/m³ (6,000–9,200 kg/m³ for pitchblende).

Uraninite proper is a high-temperature mineral typical of granitic and syenite pegmatites in association with complex niobates, tantalates, and titanates of uranium (samarskite, columbite, pyrochlore) and with zircon and monazite. It also occurs in hydro-thermal, skarn, and sedimentary deposits. Pitchblende, which is formed primarily in low-temperature hydrothermal and sedimentary deposits, occurs in association with sulfides, arsenides, native bismuth, arsenic, silver, and carbonates. The other uranium oxides are especially typical of hydrothermal sulfide-uranium and sedimentary deposits.

Uraninite is easily altered in the oxidation zone and serves as the initial material for the formation of the hydroxides, silicates, phosphates, and other minerals of U⁶⁺. All the varieties of uraninite are ores of uranium. There are large uraninite deposits in Canada, the United States, Africa, Australia, and France.

REFERENCE

Mineraly: Spravochnik, vol. 2, fasc. 2. Moscow, 1965.

L. N. BELOVA