a rare mineral, mainly thoria, ThO2, but also containing uranium, cerium, and other rare-earth metals, occurring in small, black, cubic crystals: notable for its radioactivity and used as a minor source of thorium.
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The richest known source is thorianite, which consists mainly of thorium oxide, and contains 9.5 cc.
Much the best practical source of helium is thorianite, a mineral imported from Ceylon for the manufacture of thoria.