Phenakite

Phenakite or phenacite is a fairly rare nesosilicate mineral consisting of beryllium orthosilicate, Be₂SiO₄. Occasionally used as a gemstone, phenakite 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. There is no cleavage, and the fracture is conchoidal. The Mohs hardness is high, being 7.5 – 8; the specific gravity is 2.96. The crystals are sometimes perfectly colorless 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 has been mistaken.[4] Its name comes from Ancient Greek: φέναξ, romanized: phénax, meaning "deceiver" due to its close visual similarity to quartz,[5] named by Nils Gustaf Nordenskiöld in 1833.[4]

Phenakite
Phenakite crystals
General
Category	Nesosilicate
Formula (repeating unit)	Be2SiO4
Strunz classification	9.AA.05
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)
Space group	R3
Unit cell	a = 12.438 Å, c = 8.231 Å; Z = 18
Identification
Color	Colorless, yellow, pink, brown
Crystal habit	Tabular, prismatic to acicular crystals often as columnar aggregates, as spherulites and granular
Twinning	Penetration twins around {0001}
Cleavage	Distinct on {1120}, imperfect on {1011}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	7.5 – 8
Luster	Vitreous
Diaphaneity	Transparent to translucent
Specific gravity	2.93 – 3.00
Optical properties	Uniaxial (+)
Refractive index	nω = 1.650 - 1.656 nε = 1.667 - 1.670
Birefringence	δ = 0.017
Other characteristics	Bright blue cathodoluminescence
References	[1][2][3]

Occurrence

Phenakite crystal from the Noumas II Pegmatite, South Africa. (Size:1.2x0.5x0.4cm)

Crystal structure of Phenakite viewed along the c axis

Phenakite is found in high-temperature pegmatite veins and in mica-schists associated with quartz, chrysoberyl, apatite and topaz. It has long been known from the emerald and chrysoberyl mine on the Takovaya stream, near Yekaterinburg in the Urals of Russia, 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 Pikes Peak region in Colorado (US).[4] Small, gem grade individual crystals of phenakite showing a prismatic habit are noted in beryl dissolution cavities at the Noumas II Pegmatite, part of the Orange River pegmatite belt in the Northern Cape of South Africa.[6] Large crystals of prismatic habit have been found in a feldspar quarry at Kragerø in Norway. Framont near Schirmeck in Alsace is another well-known locality. Still larger crystals, measuring 12 inches (300 mm) in diameter and weighing 28 lb (13 kg). have been found at Greenwood in Maine, but these are pseudomorphs of quartz after phenakite.[4]

For gem purposes the stone is cut in the brilliant form, of which there are two fine examples, weighing 34 and 43 carats (6.8 and 8.6 g), in the British Museum. The indices of refraction are higher than those of quartz, beryl or topaz; a faceted phenakite is consequently rather brilliant and may sometimes be mistaken for diamond.[4]

References

1. Handbook of Mineralogy
2. Mindat.org
3. Webmineral data
4.  One or more of the preceding sentences incorporates text from a publication now in the public domain: Spencer, Leonard James (1911). "Phenacite". In Chisholm, Hugh (ed.). Encyclopædia Britannica. 21 (11th ed.). Cambridge University Press. p. 364.
5. Firefly Guide to Gems By Cally Oldershaw p.94
6. Cairncross, Bruce (2004). Field guide to rocks & minerals of Southern Africa (1st ed.). Cape Town: Struik. ISBN 1868729850.