rare-earth element (REE, lanthanide) One of those elements with atomic numbers between 57 and 71, that have closely similar chemical properties. The ionic radius decreases with increasing atomic number, a phenomenon referred to as the lanthanide contraction. Rare-earth elements occur in minerals only in trace amounts, sometimes replacing Ca²⁺ in apatite and hornblende. They tend to become concentrated in the residual fluid of magmas, and in some pegmatites the REE cerium replaces the calcium in epidote to form the mineral allanite. Lunar rocks, apart from anorthosite, show considerable enrichment in most of the rare-earth elements relative to the REE cosmic abundance. See EUROPIUM ANOMALY.

rare earth (also rare earth el·e·ment or rare earth met·al) • n. Chem. any of a group of chemically similar metallic elements comprising the lanthanide series and (usually) scandium and yttrium. They are not esp. rare, but they tend to occur together in nature and are difficult to separate from one another.

rare earth See lanthanide series