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Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 x 2.4 x 2 cm)
CategoryHalide mineral
(repeating unit)
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
ColorColorless to white
Crystal habitPrismatic crystals; fibrous, radial, spherulitic
TwinningOn {011}
CleavagePerfect on {010} and {110}
Mohs scale hardness5-5.5
Specific gravity3.15
Optical propertiesUniaxial (+)
Refractive indexn? = 1.378 n? = 1.390
Birefringence? = 0.012

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 n? = 1.378 and n? = 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. 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]


  • Palache, C., H. Berman, and C. Frondel (1951) Dana's system of mineralogy, (7th edition), v. II, pp. 37-39

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