Uranoan Thorite in Lithophysal Rhyolite—Topaz Mountain, Utah, USA

Eugene E. Foord, Robert R. Cobban and Isabelle K. Brownfield
US Geological Survey, Box 25046, DFC, MS 905, Denver, Colorado 80225
Department of Geology, Denver Museum of Natural History, City Park, Denver, Colorado 80205
US Geological Survey, Box 25046, DFC, MS 916, Denver, Colorado 80225

Abstract: Uranoan thorite crystals have been found occurring as a sparse constituent in lithophysae in 6.1 to 6.8 Ma alkali rhyolite flows at Thomas Mountain, Utah, USA. The crystals are associated with sandidine, quartz, topaz, hematite, magnetite, and calcite; they are leek to dark grass green, transparent, well-formed, euhedral prisms, showing development of forms {100}, {101}, and {111}. The mineral is both optically isotropic, with a refractive index of 1.86(1), and X-ray amorphous, indicating its metamict state. Electron microprobe analysis yields: SiO2 17.3, ThO2 56.8, UO2 25.4, total 99.5%, and a structural formula of (Th0.72U0.31)Σ1.03Si0.97O4.

This appears to be the first reported occurrence of thorite in volcanic rocks of rhyolitic composition, and may account for part or all of the Th reported in alkali rhyolites and so-called ‘topaz-rhyolites’ of the western USA.

Keywords: thorite • uranium • rhyolite • Topaz Mountain • Utah • USA

Mineralogical Magazine; December 1985 v. 49; no. 354; p. 729-731; DOI: 10.1180/minmag.1985.049.354.13
© 1985, The Mineralogical Society
Mineralogical Society (www.minersoc.org)