sellaite

{{infobox mineral | name = Sellaite | boxwidth = | boxbgcolor = | image = Sellaite-39033.jpg | imagesize = 150px | alt = | caption = Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 × 2.4 × 2 cm) | category = Halide mineral | formula = MgF2 | IMAsymbol = Sel | molweight = | strunz = 3.AB.15 | dana = | system = Tetragonal | class = Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m) | symmetry = P42/mnm | unit cell = a = 4.6213(2) Å c = 3.0519(1) Å Z = 2 | color = Colorless to white | colour = | habit = Prismatic crystals; fibrous, radial, spherulitic | twinning = On {011} | cleavage = Perfect on {010} and {110} | fracture = Conchoidal | tenacity = Brittle | mohs = 5–5.5 | luster = Vitreous | streak = | diaphaneity = Transparent | gravity = 3.15 | density = | polish = | opticalprop = Uniaxial (+) | refractive = nω = 1.378nε = 1.390 | birefringence = δ = 0.012 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | references = }} Sellaite is a magnesium fluoride mineral with the formula MgF2. It crystallizes in the tetragonal crystal system, typically as clear to white vitreous prisms. It may be fibrous and occur as radiating aggregates. It has a Mohs hardness of 5 to 6 and a specific gravity of 2.97 to 3.15. Refractive index values are nω = 1.378 and nε = 1.390.

==Discovery and occurrence== Sellaite was first described in 1868 and named for Italian mining engineer and mineralogist Quintino Sella (1827–1884). Its type locality is the in France, where it occurred inside bitumen-bearing dolomite-anhydrite clasts within a moraine deposit. It has been reported in an evaporite deposit at Bleicherode; within volcanic ejecta and fumaroles at Vesuvius; in a metamorphic magnesite deposit at ; and in sodic alkali granite near .