What is "sellaite"

n. (context mineral English) A tetragonal-ditetragonal dipyramidal mineral containing fluorine and magnesium.

Sellaite is a magnesium fluoride mineral with the formula MgF. 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.