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(or phenakite), a mineral of the nesosilicate class. Phenacite, whose composition is Be2[SiO4], contains 42–45 percent beryllium oxide. It crystallizes in the trigonal system. Its structure is based on trigonal prisms consisting of three chains connected at the vertices. In each of the chains, two BeO4 tetrahedrons alternate with an SiO4 tetrahedron; the latter is set spirally on the prisms. The crystals are usually small and prismatic; sometimes they are rhombohedral. Radiating concretions and spherulites are also common. Phenacite may be colorless, yellowish, pinkish, gray, or white. It has a vitreous luster and conchoi-dal fracture. It is transparent to translucent. Phenacite has a hardness of 7.5–8.0 on Mohs’ scale and a density of 2,940–3,000 kg/m3.
Phenacite is found with fluorite, sulfides, and micas in hydro-thermal metasomatic formations that occur after carbonate rocks (in which case phenacite is associated with chrysoberyl, euclase, or bertrandite), basic rocks (with bavenite), ultrabasic rocks (with emerald and alexandrite), or acidic aluminosilicate rocks (with beryl). Deposits are genetically or paragenetically linked to the leucocratic or subalkalic and alkaline granites. Phenacite sometimes occurs in feldspathic metasomatites in association with genthelvite and is occasionally observed in granitic pegmatites. Beryllium ore is found in hydrothermal deposits of the ber-trandite-phenacite-fluorite formation. Large transparent crystals of phenacite are gems of order (or class) II.
REFERENCESMineraly: Spravochnik, vol. 3, fasc. 1. Moscow, 1972.
Berillievye mineraly perspektivnykh tipov mestorozhdenii. Moscow, 1973.
A. I. GINZBURG