Sellaite

Sellaite
Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 × 2.4 × 2 cm)
General
Category	Halide mineral
Formula	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) 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	[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 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  [ 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]}

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

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