phenakite

{{infobox mineral | name = Phenakite | boxwidth = | boxbgcolor = | image = Fenacitas.jpg | imagesize = 260px | alt = | caption = Phenakite crystals | category = Nesosilicate | formula = Be2SiO4 | IMAsymbol = Phk | molweight = | strunz = 9.AA.05 | dana = | system = Trigonal | class = Rhombohedral () H-M symbol: () | symmetry = R | unit cell = a = 12.438 Å, c = 8.231 Å; Z = 18 | color = Colorless, yellow, pink, brown | colour = | habit = Tabular, prismatic to acicular crystals often as columnar aggregates, as spherulites and granular | twinning = Penetration twins around {0001} | cleavage = Distinct on {110}, imperfect on {101} | fracture = Conchoidal | tenacity = Brittle | mohs = 7.5–8 | luster = Vitreous | streak = | diaphaneity = Transparent to translucent | gravity = 2.93–3.00 | density = | polish = | opticalprop = Uniaxial (+) | refractive = nω = 1.650 – 1.656 nε = 1.667 – 1.670 | birefringence = δ = 0.017 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = Bright blue cathodoluminescence | prop1 = | prop1text = | references = }}

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 , meaning "deceiver" due to its close visual similarity to quartz, named by Nils Gustaf Nordenskiöld in 1833.