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(named after the British mineralogist T. Allan), also orthite, a mineral of the epidote group. Allanite is characterized by its pitch-black or greenish brown color and its vitreous luster. The mineral’s approximate formula is (Ca, Ce, Mn)2-(Fe2+, Fe3+, Mg) Al2[SiO4][Si2O7]O(OH). Allanite contains up to 6 percent Ce2O3 and frequent admixtures of Th and U; it sometimes contains up to 8 percent Y2O3. Hydrous varieties, which have become metamict, also exist.
Allanite generally crystallizes in the monoclinic system, forming slender prismatic crystals. It also may occur as irregular aggregates or grains. The mineral is brittle and has a hardness on Mohs’ scale of 6 and a density of 4,100 kg/m3 (reduced to 2,700 kg/m3 in converted varieties). Allanite is radioactive. The mineral occurs rarely in natural form but is usually found in association with other minerals in pegmatite veins. It also occurs in the form of individual crystals in granites. Allanite is valuable in the extraction of such rare-earth elements as Ce and Y.