rare earths

rare earths in chemistry, oxides of the rare-earth metals . They were once thought to be elements themselves. They are widely distributed in the earth's crust and are fairly abundant, although they were once thought to be very scarce. Generally, the name of an earth is formed from the name of its element by replacing -um with -a ; e.g., the earth of cerium is ceria. Mixed rare earths are used in glassmaking, ceramic glazes, glass-polishing abrasives, carbon arc-light electrode cores, and catalysts for petroleum refining. Individual purified rare earths have many uses, e.g., in lasers and as color-television picture tube phosphors. Important rare-earth minerals include bastnasite, cerite, euxenite, gadolinite, monazite, and samarskite.

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 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.