1911 Encyclopædia Britannica/Phenacite

by Leonard James Spencer

PHENACITE, a mineral consisting of beryllium orthosilicate, Be₂SiO₄, occasionally used as a gem-stone. It occurs as isolated crystals, which are rhombohedral with parallel-faced hemihedrism, and are either lenticular or prismatic in habit: the lenticular habit is determined by the development of faces of several obtuse rhombohedra and the absence of prism faces (the accompanying figure is a plan of such a crystal viewed along the triad, or principal, axis). There is no cleavage, and the fracture is conchoidal. The hardness is high, being 71/2 – 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; occasionally they are pale rose-red. In general appearance the mineral is not unlike quartz, for which indeed it had been mistaken; on this account it was named, by N. Nordenskiöld in 1833, from Gr. φέναξ (a deceiver).

Phenacite has long been known from the emerald and chrysoberyl mine on the Takovaya stream, near Ekaterinburg in the Urals, where large crystals occur in mica-schist. It is also found with topaz and amazon-stone in the granite of the Ilmen mountains in the southern Urals and of the Pike’s Peak region in Colorado. Large crystals of prismatic habit have more recently been found in a felspar quarry at Kragerö in Norway. Framont near Schirmeck in Alsace is another well-known locality. Still larger crystals, measuring 12 in. in diameter and weighing 28 ℔, have been found at Greenwood in Maine, but these are pseudomorphs of quartz after phenacite.

For gem purposes the stone is cut in the brilliant form, of which there are two fine examples, weighing 43 and 34 carats, in the British Museum. The indices of refraction (ω=1.6540, ε=1.6527) are higher than those of quartz, beryl or topaz; a faceted phenacite is consequently rather brilliant and may sometimes be mistaken for diamond.  (L. J. S.)