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PHENACITE

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Originally appearing in Volume V21, Page 364 of the 1911 Encyclopedia Britannica.
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PHENACITE , a See also:

mineral consisting of See also:beryllium orthosilicate, Be2SiO4, occasionally used as a See also:gem-See also:stone. It occurs as isolated crystals, which are See also:rhombohedral with parallel-faced hemihedrism, and are either lenticular or prismatic in See also:habit: the lenticular habit is determined by the development of faces of several obtuse rhombohedra and the See also:absence of See also:prism faces (the accompanying figure is a See also:plan of such a crystal viewed along the triad, or See also:principal, See also:axis). There is no cleavage, and the fracture is conchoidal. The hardness is high, being 71—8; the specific gravity is 2.98. The crystals are sometimes perfectly colourless and transparent, but more often they are greyish or yellowish and only translucent; occasion-ally they are See also:pale See also:rose-red. In See also:general See also:appearance the mineral is not unlike See also:quartz, for which indeed it had been mistaken; on this See also:account it was named, by N. See also:Nordenskiold in 1833, from Gr. OivaE (a deceiver). Phenacite has See also:long been known from the See also:emerald and See also:chrysoberyl mine on the Takovaya stream, near See also:Ekaterinburg in the Urals, where large crystals occur in See also:mica-schist. It is also found with See also:topaz and See also:amazon-stone in the See also:granite of the Ilmen mountains in the See also:southern Urals and of the See also:Pike's See also:Peak region in See also:Colorado. Large crystals of prismatic habit have more recently been found in a See also:felspar See also:quarry at Kragero in See also:Norway. Framont near Schirmeck in See also:Alsace is another well-known locality.

Still larger crystals, measuring 12 in. in See also:

diameter and weighing 28 lb, have been found at See also:Greenwood in See also:Maine, but these are pseudomorphs of quartz after phenacite. For gem purposes the stone is cut in the brilliant See also:form, of which there are two See also:fine examples, weighing 43 and 34 carats, in the See also:British Museum. The indices of See also:refraction (w=1.6540, e=1.6527) are higher than those of quartz, See also:beryl or topaz; a faceted phenacite is consequently rather brilliant and may sometimes be mistaken for See also:diamond. (L. J.

End of Article: PHENACITE

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