Thorium occurs in several minerals including thorite (ThSiO), thorianite (ThO + UO) and monazite. He is most noted for first locating the mineral thorite. It is dimorphous with tetragonal thorite, and isostructual with monazite. Other minerals known to undergo metamictisation include allanite, gadolinite, ekanite, thorite and titanite. Coffinite is isostructural with the orthosilicates zircon (ZrSiO) and thorite (ThSiO). Unlike thorite, huttonite is not affected by metamictization. Because thorite is highly radioactive, specimens are often "metamict". Thorite is currently an important ore of uranium. Thorite is commonly metamict and hydrated, making it optically isotropic and amorphous.