Schwertmannite is a new oxyhydroxysulphate of iron from the Pyhäisalmi sulphide mine, Province of Oulu, Finland. It occurs there, and elsewhere, as an ochreous precipitate from acid, sulphate-rich waters. Associated minerals at other localities may include jarosite, natrojarosite, goethite and ferrihydrite. Schwertmannite is a poorly crystalline, yellowish brown mineral with a fibrous morphology under the electron microscope. A high specific surface area in the range of 100 to 200 m²/g, rapid dissolution in cold, 5 M HCl or in ammonium oxalate at pH 3, and pronounced X-ray diffraction line broadening are consistent with its poorly crystalline character.

Colour parameters for the type specimen as related to CIE illuminant C are L* = 53.85, a* = + 15.93, and b* = +47.96. Chemical analysis gives Fe₂O₃, 62.6; SO₃, 12.7; CO₂, 1.5; H₂O⁻, 10.2; H₂O⁺, 12.9; total 99.9 wt.%. These data yield an empirical unit cell formula of Fe₁₆O₁₆(OH)_9.6(SO₄)_3.2·10H₂O after exclusion of CO₂ and H₂O⁻. The most general simplified formula is Fe₁₆O₁₆(OH)_y(SO₄)_z·nH₂O, where 16 − y = 2z and 2.0 ⩽ z ⩽ 3.5. Schwertmannite has a structure akin to that of akaganéite (nominally β-FeOOH) with a doubled c dimension. Its X-ray powder diffraction pattern consists of eight broad peaks [d_obs in (I_obs) (hkl)] 4.86(37)(200,111); 3.39(46)(310); 2.55(100)(212); 2.28(23)(302); 1.95(12)(412); 1.66(21)(522); 1.51(24)(004); and 1.46(18)(204,542), giving a = 10.66(4), c = 6.04(1) Å, and V = 686(6) ų for a primitive, tetragonal unit cell. The probable space group is P4/m. Upon heating, schwertmannite transforms to hematite with Fe₂(SO₄)₃ occurring as an intermediate phase. Bidentate bridging complexes between Fe and SO₄ are apparent in infrared spectra. Mössbauer data show the Fe in schwertmannite to be exclusively trivalent and in octahedral coordination; it has a Néel temperature of 75 ± 5 K and a saturation magnetic hyperfine field of about 45.6 T. Pronounced asymmetry of the Mössbauer spectra indicates different locations for Fe atoms relative to SO₄ groups in the structure. The name is for Udo Schwertmann, professor of soil science at the Technical University of Munich.