Abstract: Dwornikite, (Ni_1−xFe_x)SO₄ · H₂O is a member of the kieserite group, monoclinic with space group C2/c. Specimens from Minasragra, Peru with x ∼ 0.1 have a unit cell with a = 6.839(2), b = 7.582(2), c = 7.474(2) Å, and β = 117.85(2)°. The six strongest lines of the powder pattern are: 3.342 (1¯12, 100), 4.732 (110, 70), 3.024 (200, 70), 4.754 (1¯11, 50), 3.293 (021, 35), 2.491 (022, 35). The mineral occurs as fine grained white aggregates associated with vanadium sulphide ores containing patronite and bravoite, mixed with other oxidation products. New unit cell data for the synthetic end-member compounds NiSO₄ · H₂O and FeSO₄ · H₂O, and new X-ray powder data for retgersite (NiSO₄ · 6H₂O) are provided.