Monazite, mineral, an anhydrous phosphate of the rare earth elements of the cerium group, usually containing some thorium; its general formula is (Ce,La,Y,Th)PO₄. The mineral crystallizes in the monoclinic system (see Crystal) and is yellowish, red, or brown in color, with a resinous luster; the hardness ranges from 5 to 5.5 and the specific gravity from 4.9 to 5.3. Monazite is found in association with the older crystalline rocks, mainly in gravel deposits of Brazil, India, and Malaysia; in the United States it is found in Idaho, Florida, North Carolina, and South Carolina. Monazite is the main source of the radioactive metallic element thorium.