The rare-earth elements are defined as a group of chemical elements composed of scandium, yttrium, and the lanthanides. The lanthanides are a group of 15 chemically similar elements with atomic numbers 57 through 71, inclusive. Although not a lanthanide, yttrium, atomic number 39, is included in the rare earths because it often occurs with them in nature, having similar chemical properties. Scandium, atomic number 21, is also included in the group, although it typically occurs in rare- earths ores only in minor amounts because of its smaller atomic and ionic size.
Rare-earths production is derived from the rare-earths ores bastnasite, monazite, xenontime, and ion-adsorption clay. Bastnasite is the world's principal source of rare earths and is produced in China and the United States. Significant quantities of rare earths are also recovered from the mineral monazite. Xenotime and ion-adsorption clays account for a much smaller part of the total production but are important sources of yttrium and other heavy-group rare earths.