Sellaite

Sellaite
Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 × 2.4 × 2 cm)
General
Category	Halide mineral
Formula (repeating unit)	MgF2
IMA symbol	Sel
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) Å 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

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_ω = 1.378 and n_ε = 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 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; and in sodic alkali granite near Gjerdingen.

References

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