SCHWERTMANNITE, A NEW IRON OXYHYDROXYSULPHATE FROM PYHASALMI, FINLAND, AND OTHER LOCALITIES

Schwertmannite is a new oxyhydroxysulphate of iron from the Pyhasalmi 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 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).10H(2)O after exclusion of CO2 and H2O-. The most general simplified formula is Fe16O16(OH)(g)amma(SO4)(z).nH(2)O, where 16 - gamma = 2z and 2.0 less than or equal to z less than or equal to 3.5. Schwertmannite has a structure akin to that of akaganeite (nominally beta-FeOOH) with a doubled c dimension. Its X-ray powder diffraction pattern consists of eight broad peaks [d(obs) in Angstrom(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) Angstrom, and V = 686(6) Angstrom(3) for a primitive, tetragonal unit cell. The probable space group is P4/m. Upon heating, schwertmannite transforms to hematite with Fe-2(SO4)3 occurring as an intermediate phase. Bidentate bridging complexes between Fe and SO4 are apparent in infrared spectra. Mossbauer data show the Fe in schwertmannite to be exclusively trivalent and in octahedral coordination; it has a Neel temperature of 75 +/- 5 K and a saturation magnetic hyperfine field of about 45.6 T. Pronounced asymmetry of the Mossbauer 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.