Thorianite (with Pen for scale, Collected 1893 by A. E. Foote) - Locality: Galle, Ceylon - Mineral collection of Brigham Young University Department of Geology, Provo, Utah. BYU index 4-8034a — Photo: Andrew Silver · Public domain
Thorianite
Thorium dioxide — a rare, dense, radioactive black mineral resembling uraninite.
Intro adapted from Wikipedia (CC BY-SA).
Thorianite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula ThO₂.
It crystallizes in the cubic crystal system (space group Fm3m). It is a member of the Fluorite structural group.
Key properties: Mohs hardness 6.5–7; specific gravity 9.7–9.8; Gray to greenish gray streak; Submetallic when fresh, altering to resinous or hornlike luster.
IMA symbol Tho, grandfathered status — known before the IMA’s founding, named in 1904.
An accessory mineral in granite pegmatites, more rarely in carbonatites or serpentinites; commonly detrital in heavy mineral concentrates.
It is found associated with Thorite, zircon, ilmenite, cassiterite, allanite (Sri Lanka); phlogopite, diopside, spinel (Andolobe, Madagascar).
The name refers to THORium in the composition.