Rare-earth element

Rare-earth ore (shown with a 19 mm diameter US 1 cent coin for size comparison)

Refined rare-earth oxides are heavy, gritty powders usually brown or black, but can be lighter colors as shown here.

Legend:
gadolinium · praseodymium · cerium
samarium · lanthanum
neodymium

The rare-earth elements (REE), also called the rare-earth metals or rare earths or, in context, rare-earth oxides, and sometimes the lanthanides (although yttrium and scandium, which do not belong to this series, are usually included as rare earths),^[1] are a set of 17 nearly indistinguishable lustrous silvery-white soft heavy metals. Compounds containing rare earths have diverse applications in electrical and electronic components, lasers, glass, magnetic materials, and industrial processes.

Scandium and yttrium are considered rare-earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties, but have different electrical and magnetic properties.^[2][3] The term 'rare-earth' is a misnomer because they are not actually scarce, although historically it took a long time to isolate these elements.^[4][5]

These metals tarnish slowly in air at room temperature and react slowly with cold water to form hydroxides, liberating hydrogen. They react with steam to form oxides and ignite spontaneously at a temperature of 400 °C (752 °F). These elements and their compounds have no biological function other than in several specialized enzymes, such as in lanthanide-dependent methanol dehydrogenases in bacteria.^[6] The water-soluble compounds are mildly to moderately toxic, but the insoluble ones are not.^[7] All isotopes of promethium are radioactive, and it does not occur naturally in the earth's crust, except for a trace amount generated by spontaneous fission of uranium-238. They are often found in minerals with thorium, and less commonly uranium.

Though rare-earth elements are technically relatively plentiful in the entire Earth's crust (cerium being the 25th-most-abundant element at 68 parts per million, more abundant than copper), in practice this is spread thin across trace impurities, so to obtain rare earths at usable purity requires processing enormous amounts of raw ore at great expense, thus the name "rare" earths.

Because of their geochemical properties, rare-earth elements are typically dispersed and not often found concentrated in rare-earth minerals. Consequently, economically exploitable ore deposits are sparse.^[8] The first rare-earth mineral discovered (1787) was gadolinite, a black mineral composed of cerium, yttrium, iron, silicon, and other elements. This mineral was extracted from a mine in the village of Ytterby in Sweden; four of the rare-earth elements bear names derived from this single location.