thoria

Thoria is the common name for thorium oxide, a chemical compound with the formula ThO2. It is a white to pale yellow, highly refractory oxide of thorium. In nature, thorium occurs in several minerals, and ThO2 is the principal oxide associated with thorium-rich phases; rare minerals such as thorianite and thorite are related thorium-bearing minerals.

Thoria crystallizes in a fluorite-type cubic structure. It is highly heat resistant, with a melting point around 3300 °C and a density of about 11.7 g/cm3. The material is chemically stable, largely insoluble in water, and resistant to many common reagents, though it can be attacked by strong acids under appropriate conditions to form thorium salts.

Naturally occurring thorium is radioactive, dominated by the isotope Th-232, which has a half-life of about 1.4 × 10^10 years and decays via alpha emission through a long decay series to stable lead. This radiological nature requires proper safety measures in handling and processing thoria-containing materials.

Applications have varied over time. Thoria gained prominence in gas lighting as a component of Welsbach mantles due to its bright emission when heated. It has also been used as a dopant in tungsten electrical filaments (thoriated tungsten) to improve electron emission, and as a ceramic oxide or pigment in certain high-temperature ceramics and glass. In nuclear technology, thorium oxide is studied and used as a ceramic fuel in thorium-based reactor cycles, because Th-232 can be bred to fissile U-233 under reactor conditions.

Environmental and safety considerations reflect its radioactivity; regulatory controls govern its mining, processing, and use.