Sellaite

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.

Sellaite
Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 x 2.4 x 2 cm)
General
CategoryHalide mineral
Formula
(repeating unit)		MgF2
Strunz classification3.AB.15
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space groupP42/mnm
Unit cella = 4.6213(2)
c = 3.0519(1) [Å]; Z = 2
Identification
ColorColorless to white
Crystal habitPrismatic crystals; fibrous, radial, spherulitic
TwinningOn {011}
CleavagePerfect on {010} and {110}
FractureConchoidal
TenacityBrittle
Mohs scale hardness5–5.5
LusterVitreous
DiaphaneityTransparent
Specific gravity3.15
Optical propertiesUniaxial (+)
Refractive indexnω = 1.378 nε = 1.390
Birefringenceδ = 0.012
References[1][2][3]

Discovery and occurrence

It was first described in 1868 and named for Italian mining engineer and mineralogist Quintino Sella (1827–1884). The type locality is Gébroulaz glacier, Val Thorens, Savoie, Rhône-Alpes, France. At its type locality, sellaite occurred within a bitumen bearing dolomite-anhydrite clasts within a moraine deposit.[1][2]

It has been reported from Bleicherode, Thuringia, Germany, where it occurs in an evaporite deposit. in Vesuvius, Italy, it occurs within volcanic ejecta and fumaroles. In Serra das Éguas, Brazil, sellaite occurs in a magnesite deposit that has been metamorphosed. Near Lake Gjerdingen, Nordmarka, Oppland, Norway, it occurs in an sodic alkali granite.[1][2]

References
  1. Mineral Data Publishing: Handbook of Mineralogy
  2. Mindat with location data
  3. 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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