Schwertmannite, a new iron oxyhydroxysulphate from Pyhäsalmi, Finland, and other localities

Abstract

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 m 2/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 Fe 2 O 3, 62.6; SO 3, 12.7; CO 2, 1.5; H 2 O−, 10.2; H 2 O+, 12.9; total 99.9 wt.%. These data yield an empirical unit cell formula of Fe 16 O 16(OH)9.6(SO 4)3.2·10H 2 O after exclusion of CO 2 and H 2 O−. The most general simplified formula is Fe 16 O 16(OH)y(SO 4)z·n H 2 O, where 16 − y = 2 z 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) Å 3 for a primitive, tetragonal unit cell. The probable space group is P4/m. Upon heating, schwertmannite transforms to hematite with Fe 2(SO 4)3 occurring as an intermediate phase. Bidentate bridging complexes between Fe and SO 4 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 SO 4 groups in the structure. The name is for Udo Schwertmann, professor of soil science at the Technical University of Munich.