Monazite

Phosphate mineral, cerium and lanthanum phosphate [(Ce, La)PO4], that is the major commercial source of cerium. Occurring as small, brown, resinous, rather heavy crystals in granitic and gneissic rocks and their detritus (called monazite sands), monazite frequently contains 10 - 12 percent thorium dioxide (ThO2) and thus represents a major commercial source of thorium as well.