X-RAY examination of monazite has been carried out by Hadding1, Gliszczynski2 and Parrish3, and the space group and crystal class uniquely determined. According to Goldschmidt4, the thorium which is usually present is ‘‘captured during crystallization of the phosphate from its parent magma because of its favourable ionic size. To compensate for the increased cationic charge of the thorium ion, the resulting lattice must possess a defect structure in which some of the sets of equivalent cationic positions are only partially occupied. The quantity of thorium which can be tolerated in such a structure is not known, but monazites the thoria content of which varies from nil to at least 18 per cent (approximately 0·2 atoms per atom of rare earth element) have been reported3,5.