Magnesium Fluoride, MgF₂

Magnesium Fluoride, MgF₂, unlike other magnesium halides, is very sparingly soluble in water: 7.6 mgm. of the amorphous variety dissolve in 1 litre of pure water. It is precipitated in a gelatinous form from magnesium sulphate solution by alkali fluorides. Cossa advises dissolving pure magnesia in hydrofluoric acid and expelling the excess of acid by heat. The white amorphous fluoride is converted by fusion, either with or without salt, into a crystalline variety which is identical with the mineral sellaite. Sellaite occurs as small, transparent tetragonal crystals, with a density of 2.95-3.10 and a hardness of 5.

Crystalline magnesium fluoride is very infusible, only slightly attacked by all acids except sulphuric, and can be made to fluoresce or phosphoresce. It forms readily fusible compounds with barium, strontium, and calcium sulphates, and interacts with aluminium sulphate to form aluminium fluoride.

A process has been patented for preparing magnesium fluoride by heating, in a dry or wet way, calcium fluoride with magnesium chloride, or with a double chloride of magnesium and potassium.

NaF.2MgF₂ is obtained by boiling magnesia with a solution of sodium fluoride, or crystalline by fusing magnesium chloride with excess of sodium fluoride and sodium chloride.

When a fusion of magnesia and potassium fluoride (or potassium hydrogen fluoride) is cooled, flattened quadratic prisms of MgF₂.3KF result. Crystalline, MgF₂.2KF, is simultaneously produced by increasing the proportion of magnesia in the melt. Both compounds are attacked by sulphuric and hydrochloric acids.

The following double fluorides have been reported: MgTiF₆.6H₂O, MgZrF₆.5H₂O, MgSnF₆.6H₂O