Thorium, which makes up about 0.0015% of the earth's crust, occurs in a large number of minerals in association with the rare earths and uranium. The major source of thorium is monazite (a phosphate of the light-weight rare earths) usually associated to silica. The main deposits of monazite are located in India, Brazil, Australia, Malagasy, Ceylon, South Africa, Canada and U.S.A. From all these the most important is that of India, with huge deposits of ore with a concentration that averages about 9% of thorium oxide.