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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