Ferroso-ferric sulphate, FeSO₄.Fe₂(SO₄)₃

Ferroso-ferric sulphates occur in nature. Raemerite is a mineral with yellow trielinie crystals and a composition represented by FeSO₄.Fe₂(SO₄)₃.12H₂O. Its crystallographic elements are: -

a: b: c = 0.8791: 1: 0.8475, α = 89° 44', β = 102° 17', γ = 85° 18'.

The salt may be prepared artificially by allowing a mixture of powdered ferrous sulphate and acid ferric sulphate to remain in contact with moist air for several months. It is then obtained as a reddish brown, crystalline powder. It decomposes in moist air, yielding ferrous and ferric sulphates, the latter giving α-copiapite and the acid sulphate.

In voltaite the iron is partly replaced by other metals yielding isomorphous salts. By adding ferrous sulphate to fused ammonium sulphate a compound of ferroso-ferric sulphate with ammonium sulphate has been obtained - namely, ferroso-ferric tetrammonium sulphate, FeSO₄.Fe₂(SO₄)₃.4(NH₄)₂SO₄.3H₂O - as colourless needles, of density 2.02. It dissolves slowly in cold water, yielding a basic salt on warming. When heated with ammonium sulphate it yields ferric tri-ammonium sulphate, referred to above.

The following acid salts have also been obtained: -

FeSO₄.Fe₂(SO₄)₃.2H₂SO₄,
NiSO₄.Fe₂(SO₄)₃.2H₂SO₄, and
2MnSO₄.Fe₂(SO₄)₃.3H₂SO₄.