Magnesium fluoride

Magnesium fluoride (MgF2) is a white crystalline salt composed of one magnesium ion and two fluoride ions, and is used in the electrolysis of aluminium ore. It is a tetragonal, birefringent crystal.

Refractive index at 500 nm: no = 1.37397, ne = 1.3916.

Magnesium fluoride is transparent over an extremely wide range of wavelengths. Windows, lenses, and prisms made of this material can be used over the entire range of wavelengths from 0.120 μm vacuum (ultraviolet) to 8.0 μm (infrared). Good structure synthetic VUV grade MgF2 is quite expensive, in the region of $3000/kg (2007) but the real cost of optics in this material is due to relatively low volume manufacture. However, with lithium fluoride it is one of the two materials that will transmit in the vacuum ultraviolet range at 121nm (Lyman alpha )and this is where it finds its application. Poor grade (cheaper) MgF2 is sometimes used in the infrared but here it is inferior to calcium fluoride. As an infrared optical material, it was sometimes known by the Eastman Kodak trademark Irtran-1 but this term is quite obsolete. MgF2 is tough and works and polishes well, but it is slightly birefringent and should be cut with the optic axis perpendicular to the plane of the window or lens.

Thin layers of MgF2 are frequently applied to the surfaces of optical elements as part of optical coatings such as anti-reflective coatings.

As a mineral, it is known as Sellaite.