The Llandovery Folly Sandstone near Presteigne, Powys, contains disseminated thorium mineralization. The thorium occurs as inclusions of thorite and thorianite within sub-millimetre-scale bitumen nodules throughout the sandstone. The nodules grew replacively in situ. The two square kilometres outcrop/subcrop of the sandstone may contain over 500 tonnes thorium.

An electron microprobe study of Y-REE-Th phosphate, silicate and Nb-Ta-Y-REE accessory-mineral assemblages revealed the compositional variations and evolution in post-orogenic, hypersolvus Permian A-type metagranite from Turčok, in the Gemeric Unit, of the Western Carpathians, eastern Slovakia. Prismatic zircon I and allanite-(Ce) are primary magmatic phases. However, the late-magmatic to early-subsolidus processes led to the formation of a more complex younger assemblage: bipyramidal zircon II, xenotime-(Y), thorite, gadolinite-hingganite-(Y), Nb-Ta-Y-REE oxide phases [fergusonite-(beta)/ samarskite-(Y), aeschynite/polycrase-(Y), and Nb-rich rutile?] and possibly monazite-(Ce). However, monazite-(Ce) and the partial alteration of allanite-(Ce), xenotime-(Y) and the Nb-Ta-Y-REE minerals are probably connected with a younger Alpine metamorphic overprint of the granite. Thorite appears as a solid solution in the thorite-xenotime-zircon series; it is also enriched in Al. Fergusonite-(beta)/ samarskite-(Y) and especially aeschynite/polycrase-(Y) show increased P, Si and Al contents.

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.

A new compound Th0.9Ce0.1SiO4, iso-typic to zircon, was prepared by heating predried ThO2, CeO2, and SiO2 (in the mole ratio 0.9:0.10:1.0) by a two-step heating protocol. The polycrystalline sample obtained was characterized by Rietveld refinement of the observed neutron diffraction data with the starting model of tetragonal ThSiO4. It has a body centered tetragonal structure with space group I41∕amd and four formula units per unit cell. The unit-cell parameters are a=7.1238(4) Å and c=6.3186(95) Å. The RP, Rwp, and Re factors are 7.77%, 10.9%, and 4.85%, respectively. The incorporation of about 10 mol % cerium was used to stabilize the tetragonal modification of ThSiO4.