riebeckite

{{infobox mineral | name = Riebeckite | category = Silicate mineral | boxwidth = | boxbgcolor = | image = Crystals of black riebeckite in alkaline pegmatite, near Évisa (Corsica, France).jpg | imagesize = 260px | alt = | caption = Black riebeckite in an alkaline pegmatite from Lindinosa near Évisa, Corsica, France | formula = Na2(Fe2+3Fe3+2)Si8O22(OH)2 | IMAsymbol = Rbk | molweight = | strunz = | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = C2/m | unit cell = a = 9.76 Å, b = 18.04 Å c = 5.33 Å; β = 103.59°; Z = 2 | color = Black, dark blue; dark blue to yellow-green in thin section | colour = | habit = As prismatic crystals, commonly fibrous, asbestiform; earthy, massive | twinning = Simple or multiple twinning parallel to {100} | cleavage = Perfect on {110}, intersecting at 56° and 124°; partings on {100} and {010} | fracture = Conchoidal to uneven | tenacity = Brittle | mohs = 6 | luster = Vitreous to silky | streak = Pale to bluish gray | diaphaneity = Semitransparent | gravity = 3.28–3.44 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.680–1.698 nβ = 1.683–1.700 nγ = 1.685–1.706 | birefringence = δ = 0.005–0.008 | pleochroism = X = blue, indigo; Y = yellowish green, yellow brown; Z = dark blue | 2V = Measured: 68° to 85°, Calculated: 62° to 78° | dispersion = Strong | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }} Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na2(Fe2+3Fe3+2)Si8O22(OH)2. It forms a solid solution series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. Cleavage is perfect, two directions in the shape of a rhombus; fracture is uneven, splintery. It is often translucent to nearly opaque.

==Name and discovery== Riebeckite was first described in 1888 for an occurrence on Socotra Island, Aden Governorate, Yemen, and named for German explorer Emil Riebeck (1853–1885).