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 |
| 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) 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 | |
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