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a mineral of the class of nesosilicates, ThSiO4. The varieties of thorite are orangite (a transparent orange variety), uranothorite (up to 10 percent UO2), thorogummite (up to 15 percent H2O), ferrithorite (up to 13 percent Fe2O3), calciothorite, and auerlite (a phosphorus-containing thorite).
In addition, thorite contains aluminum, titanium, manganese, and rare earths as admixtures. Thorite crystallizes in a tetragonal system, forming prismatic or columnar crystals. It is usually encountered in the form of granular aggregates. Many thorites are metamict minerals. Thorite is black, red-brown, or orange. Unaltered uranothorite is transparent and olive green. The hardness of thorite on Mohs’ scale is 4.5–5, and its density is 4,100–6,700 kg/m3.
Thorite is highly radioactive. It is found as an accessory mineral in granites, some alkaline rocks, pegmatites, and hydrothermal veins. Large accumulations of thorite are usually not formed; the mineral is obtained from placers as a by-product with zircon andcassiterite.