Sellaite

Sellaite

Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 x 2.4 x 2 cm)
General
Category Halide mineral
Formula
(repeating unit)		 MgF2
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 P41/mnm
Unit cell a = 4.6213(2)
c = 3.0519(1) [Å]; Z = 2
Identification
Color Colorless to white
Crystal habit Prismatic crystals; fibrous, radial, spherulitic
Twinning On {011}
Cleavage Perfect on {010} and {110}
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 5–5.5
Luster Vitreous
Diaphaneity Transparent
Specific gravity 3.15
Optical properties Uniaxial (+)
Refractive index nω = 1.378 nε = 1.390
Birefringence δ = 0.012
References [1][2][3]

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

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. In the type locality sellaite occurred within a bitumen bearing dolomite-anhydrite rock 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. 1 2 3 Mineral Data Publishing: Handbook of Mineralogy
  2. 1 2 3 Mindat with location data
  3. ↑ Webmineral data
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