Crystal chemical aspects of rare earth minerals

A rare earth mineral is defined as a mineral containing rare earth elements (REE: Sc, Y and lanthanoids) as essential constituents. So far, more than 280 species of RE minerals, as independent species, have been described after the official IMA-CNMNC approval. The chemical bonds between REE and anions possess largely ionic character and the coordination polyhedra of REE are not regular, but rather distorted, in almost all cases. The REE3+ ions exhibit 7 kinds of coordination number between 6 and 12, among which 8 is the most frequently observed. The coordination numbers of the larger Ce-group REE3+ are similar to those of Ca2+ and Th4+, and are generally higher than those of the smaller Y-group REE3+, which is similar to that of U4+. Isomorphous substitutions are commonly observed between cations having similar ionic radii and coordination numbers. The difference in the cation size between the Y- and Ce-group REE results in different crystal structures, when these structures consist of isolated anionic groups, such as CO32− and PO43−. The crystal structures having infinite frameworks, e.g., chains, sheets and 3-dimensional frameworks of silicate, niobates and others, sometimes accept both of the Y- and Ce-group REE in the spaces between/among the frameworks. The isomorphous substitutions between REE3+ and the other heterovalent cations found in the crystal structures of RE minerals are coupled substitutions with charge compensation mechanisms.