Rare-earth minerals

Naturally occurring solids, formed by geological processes, that contain the rare-earth elements—the lanthanides (atomic numbers 57–71) and yttrium (atomic number 39)—as essential constituents. In a rare-earth mineral, at least one crystallographic site contains a total atomic ratio of lanthanides and yttrium that is greater than that of any other element. The mineral name generally has a suffix, called a Levinson modifier, indicating the dominant rare-earth element; for example, monazite-(La) [LaPO₄] contains predominantly lanthanum, and monazite-(Ce) [CePO₄] contains predominantly cerium. See also Mineral; Monazite; Periodic table; Rare-earth elements.

So far, about 170 distinct species of rare-earth minerals have been described. A large number of carbonates, phosphates, silicates, niobates, and fluorides are known as rare-earth minerals. It is necessary to obtain structural as well as chemical information about a mineral to judge the essentiality of its rare-earth elements (that is, whether the rare-earth element is part of the mineral's ideal formula or is an impurity). Sometimes, minerals with significant rare-earth content are treated as rare-earth minerals, even if the rare-earth element content appears unessential to the mineral. More than 60 mineral species, including the apatite group minerals, garnet group minerals, and fluorite, are in this category. See also Apatite; Fluorite; Garnet.

Rare-earth minerals can be observed as accessory minerals in igneous rocks, such as monazite-(Ce) in granite. Carbonatite is the typical host rock of rare-earth minerals such as bastnäsite-(Ce) [CeCO₃F] and monazite-(Ce). Rare-earth minerals also often occur in pegmatite. In both carbonatite and pegmatite, rare-earth elements are concentrated by primary crystallization from melt and by hydrothermal reactions. Carbonatite deposits containing rare-earth elements are found throughout the world. Chemically stable, rare-earth minerals are not weathered easily. As a result, they have been deposited as heavy minerals in beach sand. Such deposits are found in Southeast Asia and Western Australia. See also Carbonatite; Pegmatite.

Among the rare-earth minerals, bastnäsite-(Ce) is the most important source of rare-earth elements. Monazite-(Ce), synchysite-(Ce) [CaCe(CO₃)₂F], xenotime-(Y), britholite-(Ce) [(Ce₃Ca₂)(SiO₄)₃(OH)], and allanite-(Ce) [CaCeAl₂Fe(Si₂O₇) (SiO₄)O(OH)] are also sources. Rare-earth elements have been leached with acid from the surface of clay minerals. Rare-earth minerals containing radioactive nuclear species, such as thorium and uranium, are not used as source materials. See also Clay minerals.