Sellaite is a magnesium fluoride mineral with the formula MgF<sub>2</sub>. 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 n<sub>ÃÂ</sub> = 1.378 and n<sub>õ</sub> = 1.390.
Sellaite was first described in 1868 and named for Italian mining engineer and mineralogist Quintino Sella (1827âÂÂ1884). Its type locality is the 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 ; and in sodic alkali granite near .