Ferrous aluminium sulphate, FeSO₄.Al₂(SO₄)₃

Ferrous aluminium sulphate, FeSO₄.Al₂(SO₄)₃.24H₂O, occurs in nature as halotrichite, and may be prepared in the laboratory by concentration of a solution containing ferrous and aluminium sulphates in equimolecular proportions. At first ferrous sulphate crystallises out, but this is followed by the double salt.

The salt is washed with alcohol and ether. It is white in colour, but exhibits a green fluorescence. It melts on warming, and decomposes to oxides at high temperatures.

The salt has also been found as an efflorescence on bricks which have been continuously exposed to sulphur dioxide in bleaching chambers. When the efflorescent mass is broken up it exhibits a silken or fibrous mass of white crystals not unlike asbestos in texture, whence the popular names of hair salt and feather alum.