Sellaite
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.
| 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 | P42/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] |
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.[1][2]
References
- Palache, C., H. Berman, and C. Frondel (1951) Dana’s system of mineralogy, (7th edition), v. II, pp. 37–39