Sellaite is a magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 fluoride

Fluoride

Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...

 mineral with formula: MgF₂. It crystallizes in the tetragonal crystal system

Crystal system

In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...

 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

Specific gravity

Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

 of 2.97 to 3.15. Refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 values are nω = 1.378 and nε = 1.390.

It was first described in 1868 and named for Italian mining engineer and mineralogist Quintino Sella (1827-1884). The type locality

Type locality (geology)

Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

 is Gébroulaz glacier, Val Thorens, Savoie, Rhône-Alpes

Rhône-Alpes

Rhône-Alpes is one of the 27 regions of France, located on the eastern border of the country, towards the south. The region was named after the Rhône River and the Alps mountain range. Its capital, Lyon, is the second-largest metropolitan area in France after Paris...

, France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

.