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 symmetry	Tetragonal, ditetragonal dipyramidal H-M symbol: (4/m 2/m 2/m) Space group: P 41/mnm
Unit cell	a = 4.6213(2) Å, c = 3.0519(1) Å; Z=2
Identification
Color	Colorless to white
Crystal habit	Prismatic crystals; fibrous, radial, spherulitic
Crystal system	Tetragonal
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

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