Sellaite

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Sellaite
Sellaite-39033.jpg
Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 x 2.4 x 2 cm)
General
Category Halide mineral
Formula
(repeating unit)
MgF2
IMA symbol Sel [1]
Strunz classification 3.AB.15
Crystal system Tetragonal
Crystal class Ditetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space group P42/mnm
Unit cell a = 4.6213(2)
c = 3.0519(1) [Å]; Z = 2
Identification
ColorColorless to white
Crystal habit Prismatic crystals; fibrous, radial, spherulitic
Twinning On {011}
Cleavage Perfect on {010} and {110}
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness5–5.5
Luster Vitreous
Diaphaneity Transparent
Specific gravity 3.15
Optical propertiesUniaxial (+)
Refractive index nω = 1.378
nε = 1.390
Birefringence δ = 0.012
References [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]

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