Uranoan thorite in lithophysal rhyolite - Topaz Mountain, Utah, U.S.A.

Uranoan thorite (SiO2 17.3, ThO2 56.8, UO2 25.4, = 99.5) occurs as a sparse constituent in lithophysae in alkali rhyolite flows (6.1-6.8 m.y.) on the flank of Topaz Mt in the Thomas Range; it is associated with sanidine, quartz, topaz, hematite, magnetite and calcite. The uranoan thorite forms euhedral, dark grass-green crystals; it is isotropic with n 1.86. This appears to be the first reported occurrence of thorite in rhyolitic volcanic rocks, but may account for part or all of the Th reported in alkali rhyolites.-R.A.H.