Sellaite

(repeating unit)MgF2IMA symbolSel[1]Strunz classification3.AB.15Crystal systemTetragonalCrystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)Space groupP42/mnmUnit cella = 4.6213(2)
c = 3.0519(1) [Å]; Z = 2IdentificationColorColorless to whiteCrystal habitPrismatic crystals; fibrous, radial, spheruliticTwinningOn {011}CleavagePerfect on {010} and {110}FractureConchoidalTenacityBrittleMohs scale hardness5–5.5LusterVitreousDiaphaneityTransparentSpecific gravity3.15Optical propertiesUniaxial (+)Refractive indexnω = 1.378
nε = 1.390Birefringenceδ = 0.012References[2][3][4]

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 glacier de Gébroulaz [fr] 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 Serra das Éguas [pt]; and in sodic alkali granite near Gjerdingen [no].[2][3]

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Mineral Data Publishing: Handbook of Mineralogy
  3. ^ a b Mindat with location data
  4. ^ Webmineral data
  • Palache, C., H. Berman, and C. Frondel (1951) Dana’s system of mineralogy, (7th edition), v. II, pp. 37–39


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