Radioactive Mineral

a mineral containing natural radioactive elements (long-lived isotopes of the radioactive series of ²³⁸U, ²³⁵U, and ²³²Th) in amounts considerably exceeding their average content in the earth’s crust. There are about 250 known radioactive minerals that contain uranium, thorium, or both elements; radium minerals have not been reliably established. The variety of radioactive minerals, which belong to different classes and groups, results from the existence of uranium in tetravalent and hexavalent forms and the isomorphism of tet-ravalent uranium with thorium, rare earths (TR), zirconium, and calcium, as well as the isomorphism of thorium with rare earths of the cerium subgroup.

A distinction is made between radioactive minerals in which uranium or thorium is present as the main component (uranium and thorium minerals, respectively) and radioactive minerals in which the radioactive elements are present as an isomorphic admixture (uranium-containing and/or thorium-containing minerals). Minerals containing mechanical admixtures of radioactive minerals (mineral mixtures) or radioactive elements in adsorbed form are not considered radioactive minerals.

Uranium minerals are divided into two groups. One group encompasses the U⁴⁺ minerals, which always contain a certain amount of U⁶⁺, and is represented by the oxide uraninite, UO₂, and the silicate coffinite, U(SiO₄)_1–x(OH)_4x. Pitchblende (a type of uraninite) and coffinite are the main industrial minerals of hydrothermal and exogenous uranium deposits. Uraninite is also encountered in pegmatites and albitites. Powdered oxides (uranium blacks) and uranium hydroxides form significant accumulations in oxidation zones of various uranium deposits. Uranium titanates (for example, brannerite, UTi₂O₆) are known in pegmatites and in certain hydrothermal deposits.

The second group includes minerals containing U⁶⁺, among them the hydroxides becquerelite, 3UO₃·3H₂O?, and curite, 2PbO·5UO₃·5H₂O; the silicates uranophane, Ca(H₂O)₂·U₂O₄(SiO₄)·3H₂O, and kasolite, Pb[UO₂][SiO₄]H₂O; the phosphates autunite, Ca[UO₂]₂[PO₄]₂·8H₂O, and torbernite, Cu[UO₂]₂[PO₄]₂·12H₂O); the arsenate zeunerite, Cu[UO₂]₂·[AsO₄]₂·12H₂O; the vanadate carnotite, K₂[UO₂]₂[VO₄]₂·3H₂O; the molybdate iriginite; the sulfate uranopilke; and the carbonate uranothallite. All these minerals are found in the oxidation zones of uranium deposits.

Thorium minerals—the oxide thorianite (ThO₂) and the silicate thorite (ThSiO₄)—are less common in nature. They are encountered as accessory minerals in granites, syenites, and pegmatites. They sometimes form significant concentrations in various alluvial deposits.

Uranium-containing and/or thorium-containing minerals are titanates (davidite), tantaloniobates (columbite, pyrochlore), titanotantaloniobates (samarskite), phosphates (mona-zite), and silicates (zircon). These minerals are found mainly in igneous and sedimentary rocks, causing their natural radioactivity. Only a few minerals (davidite and monazite) form significant concentrations and serve as sources for uranium and thorium. In radium-containing barite, an isomorphic substitution of radium for barium has been proposed.

The metamict state is characteristic of many radioactive minerals. Inclusions of radioactive minerals in grains of other minerals are accompanied by radiation damage halos, such as pleochroic halos. The ability to form autoradiographs is another specific feature of radioactive minerals. The accumulation of stable isotopes at a steady rate in radioactive minerals makes possible determination of the absolute age of geological formations.