Fe₂(SO₄)₃·H₂SO₄·28H₂O, a low-temperature water-rich iron(III) sulfate

Analysis of the title compound, diiron(III) tris­ulfate-sulfuric acid-water (1/1/28), at 100 and 200 K reveals the structural features of an alum, (H₅O₂)Fe(SO₄)₂·12H₂O. The Fe(H₂O)₆ unit is located on a centre of inversion, while the H₅O₂⁺ cation is located about an inversion centre. The compound thus represents the first oxonium alum, although the unit cell is ortho­rhom­bic.