Thorite

THORITE, a rare mineral consisting of thorium silicate, crystallizing in the tetragonal system and isomorphous with zircon. The theoretical formula, ThSiCu, requires 81-5 % of thoria, but analyses show only 50-70%, there being also some uranium, cerium, &c. The mineral is almost always altered by hydration and is then optically isotropic and amorphous. Owing to differences in composition and to alteration, the specific gravity varies from 4-4 to 5-4. The colour is usually light brown, but in the variety known as " orangite " it is orange-yellow. The mineral occurs as isolated crystals and small masses in the augite-syenite near Brevik in South Norway; also at Arendal, and in the gem-gravels of Ceylon. If found in larger amount it would be an important source of thoria for incandescent gas mantles. (L. J. S.)