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. Its name comes from Ancient Greek: φέναξ, romanized: phénax, meaning "deceiver" due to its close visual similarity to quartz, named by Nils Gustaf Nordenskiöld in 1833.
| 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 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 | |