EVIDENCE OF BIOGENIC GREIGITE (FERRIMAGNETIC FE3S4) IN SOIL

The occurrence of greigite (Fe3S4) in soils is reported for the first time. It forms irregularly-shaped aggregations within plant cells in t he Gr2 horizon of a gley soil developed from colluvial material. Greig ite was identified by X-ray diffraction and magnetic measurements and was investigated by optical and transmission electron microscopy. Biog enic formation is proposed, based on the elongated shape of single gre igite crystals, and sulphur isotope analyses, which showed a depletion in S-34 relative to the soil-water sulphate. The cell-edge length of 0.98639 +/- 0.00003 nm is significantly smaller than values reported f or sedimentary greigite. The mean coherence length of 27 nm agrees wit h TEM observations and indicates that the single greigite crystals lie in the superparamagnetic region. However, the fine aggregates show ma gnetically single-domain behaviour. Greigite is the only carrier of a stable magnetic remanence in the soil profile studied.