Thorium dioxide occurs as the rare mineral thorianite. Thorium occurs in several minerals including thorite (ThSiO), thorianite (ThO + UO) and monazite. Thorianite and uraninite form a complete solid solution series in synthetic and natural material. Thorianite is the name of the mineralogical form of thorium dioxide. Thorianite is a rare mineral and may contain up to about 12% thorium oxide.