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a mineral of the nesosilicate subclass, with the chemical composition AlBe[SiO4](OH). Euclase contains admixtures of Ge, Mn, Cu, and, sometimes, Y, B, Sn, Zn, and Pb.
Euclase crystallizes in the monoclinic system. Its structure consists of zigzag-like chains of [BeO3(OH)] incrustated with SiO4 tetrahedrons. The latter are bound by aluminum atoms in octahedral coordination. Lamellar and short and long prismatic crystals are typical, as well as plumose and divergent aggregates, seldom larger than 1 cm across. Vertical striation is common on the facets of the prism. Euclase may be colorless, milky white, light yellow, pale blue, or greenish. It has a vitreous luster and is transparent to translucent. It typically has perfect cleavage. Its hardness on Mohs’ scale is 7.5, and its density is 3,020–3,100 kg/m3.
Euclase is usually found in hydrothermal deposits of the fluorite-phenakite-bertrandite formation, where it is associated with muscovite, quartz, fluorite, phenakite, and bertrandite or with tourmaline, fluorite, and chrysoberyl. It sometimes occurs in granitic pegmatites, where it develops together with bertrandite after beryl, and in alpine veins. Large transparent crystals of euclase are gems of order, or class, I. Euclase, together with phenakite, bertrandite, and sometimes chrysoberyl, is included in the composition of complex beryl ores.
REFERENCEMineralogiia gidrotermal’nykh mestorozhdenii berilliia. Moscow, 1976.
A. I. GINZBURG