Sellaite
Sellaite is an exceedingly rare gem. Crystals from Brazil are the only current facetable material. Crystals are very small with a very little transparent area for faceting. Sellaite is a Halide mineral along with Boleite, Fluorite, and Villiaumite.
Sellaite is an interesting mineral in that it forms in a wide variety of geologic environments. It forms in a bituminous dolomite-anhydrite rock (Gebroulaz glacier, France); in evaporites (Bleicherode, Germany); in volcanic ejecta and fumarolic deposits (Vesuvius, Italy); in marble (Carrara, Italy); in a metamorphic magnesite deposit (Brumado mine, Brazil); and in an alkalic granite (Lake Gjerdingen, Norway).
| Category: | Halide mineral |
| Formula: | MgF2 |
| magnesium fluoride | |
| Crystallography: | Tetragonal – Ditetragonal Dipyramidal |
| Crystal Habit: | Crystals are stout prismatic to acicular. Also fibrous aggregates. |
| Twinning: | On [011] |
| Cleavage: | [100] Distinct, [110] Distinct |
| Fracture: | Conchoidal |
| Tenacity: | Brittle |
| Moh’s Hardness: | 5.0 – 5.5 |
| Density: | 3.14 – 3.15 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Other: | Very slightly soluble in water. |
| Color: | Colorless, White |
| Transparency: | Transparent |
| Luster: | Vitreous |
| Refractive Index: | 1.378 – 1.390 Uniaxial ( + ) |
| Birefringence: | 0.0120 |
| Dispersion: | 0.014 (low) |
| Pleochroism: | None |
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