THORITE  , a rare

mineral consisting of thorium silicate, crystallizing in the tetragonal system and isomorphous with zircon . The theoretical formula, ThSiO4, 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 .