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Ochreous precipitates containing 5.5-69.8 g/kg As were isolated from mine drainage in Finland and were composed of schwertmannite, ferrihydrite, and goethite. Schwertmannite formation was favored at pH 3-4, but its structure was degraded at high As levels. A series of coprecipitates were therefore prepared from mixed iron arsenate/sulfate solutions to define the limits of schwertmannite stability. Schwertmannite was replaced as the dominant phase by a poorly crystalline ironIII hydroxy arsenate (FeOHAs) when As/Fe mole ratios exceeded 0.15. The FeOHAs gave an X-ray diffraction pattern similar to that obtained from an "amorphous" ironIII arsenate (As/Fe = 1.0) with broad peaks at 0.30 and 0.16 nm. The FeOHAs possessed a magnetic hyperfinefield of 41.9T at 4.2 K that was intermediate to those of schwertmannite (46.1 T) and the ironIII arsenate (24.8 T). These data indicate a strong disruptive effect of arsenate on magnetic ordering and structure development in schwertmannite. Equilibration of 0.01 M arsenate solutions with freshly prepared schwertmannite and 2-line ferrihydrite at pH 3.0 for up to 60 d gave sorbed As contents of 175 and 210 g/kg, respectively. Arsenate sorption degraded the host schwertmannite and ferrihydrite, perhaps due to the formation of an FeOHAs surface phase.

Schwertmannlte is a new oxyhydroxysulphate of iron from the Pyh~isalmi 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 HCI 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 FezO3, 62.6; SO3, 12.7; CO2, 1.5; HzO-, 10.2; H20 +, 12.9; total 99.9 wt.%. These data yield an empirical unit cell formula of FelsOIs(OH)9.6(SO4)3.2.10H20 after exclusion of CO2 and HzO-. The most general simplified formula is Fe16O16(OH)y(SO4)z.nH20, where 16 -y = 2z and 2.0 ~< z ~< 3.5. Schwertmannite has a structure akin to that of akaganrite (nominally 13-FeOOH) with. a doubled c dimension. Its X-ray powder diffraction pattern consists of eight broad peaks [dobs in A(Iobs) (hk/)] 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) M6ssbauer data show the Fe in schwertmannite to be exclusively trivalent and in octahedral coordination; it has a Nrel temperature of 75 __+ 5 K and a saturation magnetic hyperfine field of about 45.6 T. Pronounced asymmetry of the M6ssbauer 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.