Phenakite

This article is about a mineral. For the Michelin-starred restaurant in California, seePhenakite (restaurant).

Phenakite
Phenakite crystals
General
Category	Nesosilicate
Formula (repeating unit)	Be2SiO4
IMA symbol	Phk
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 scalehardness	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

Phenakiteorphenaciteis a fairly rarenesosilicatemineralconsisting of berylliumorthosilicate,Be₂SiO₄.Occasionally used as agemstone,phenakite occurs as isolatedcrystals,which arerhombohedralwith 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 nocleavage,and the fracture isconchoidal.TheMohs hardnessis high, being 7.5–8; thespecific gravityis 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 unlikequartz,for which indeed it has been mistaken.Its name comes fromAncient Greek:φέναξ,romanized:phénax,meaning "deceiver" due to its close visual similarity to quartz,named byNils Gustaf Nordenskiöldin 1833.

Largest phenakite

Largest phenakite found in Sri Lanka (616.9cts)

A large phenakite gemstone has been found inSri Lanka.Found on November 18, 2021, this gemstone weighs 616.9 carats which makes it the largest of its kind. It is owned by a gem dealer from Beruwala, Sri Lanka, and reported to be worth aroundSL Rs.1 billion(US $5 million).

Occurrence

Phenakite crystal from the Noumas IIPegmatite,South Africa (Size: 1.2 × 0.5 × 0.4 cm)

Crystal structure of phenakite viewed along the c axis

Phenakite is found in high-temperature pegmatite veins and inmica-schistsassociated with quartz,chrysoberyl,apatiteandtopaz.It has long been known from theemeraldand chrysoberyl mine on the Takovaya stream, nearYekaterinburgin theUralsofRussia,where large crystals occur in mica-schist. It is also found with topaz andamazonitein thegraniteof the Ilmen Mountains in the southern Urals and of thePikes Peakregion inColorado, US.Additionally in Colorado, phenakite is found in theMount Anteroarea withaquamarine,bertrandite,andfluorite.Small, gem grade individual crystals of phenakite showing a prismatic habit are noted inberyldissolution cavities at the Noumas II Pegmatite, part of the Orange River pegmatite belt in the Northern Cape ofSouth Africa.Large crystals of prismatic habit have been found in afeldsparquarry atKragerøinNorway.Framont near Schirmeck inAlsaceis 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 inMaine,but these arepseudomorphsof quartz after phenakite.

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 theBritish Museum.The indices of refraction are higher than those of quartz,berylor topaz; a faceted phenakite is consequently rather brilliant and may sometimes be mistaken fordiamond.