Evidence of biogenic greigite (ferrimagnetic Fe₃S₄) in soil

Abstract

The occurrence of greigite (Fe3S4) in soils is reported for the first time. It forms irregularly-shaped aggregations within plant cells in the Gr2 horizon of a gley soil developed from colluvial material. Greigite was identified by X-ray diffraction and magnetic measurements and was investigated by optical and transmission electron microscopy. Biogenic formation is proposed, based on the elongated shape of single greigite crystals, and sulphur isotope analyses, which showed a depletion in 34S relative to the soil-water sulphate. The cell-edge length of 0.98639 f0.00003 nm is significantly smaller than values reported for sedimentary greigite. The mean coherence length of 27nm agrees with TEM observations and indicates that the single greigite crystals lie in the superparamagnetic region. However, the fine aggregates show magnetically single-domain behaviour. Greigite is the only carrier of a stable magnetic remanence in the soil profile studied.

Comparison of Pedogenic and Sedimentary Greigite by X-ray Diffraction and Mössbauer Spectroscopy

“…The decrease of MCLuhh > MCLhho > MCL,oo indicates for both greigites that the single crystallites are elongated along 111. This is supported by TEM observations, which show strong elongations for this soil greigite (Stanjek et al, 1994). In all cases strain is negligibly small.…”

“…The most interesting feature, however, is the fact that the soil and sedimentary greigite show magnetic order at room temperature, although their particle sizes lie within the region that is characteristic for SP behavior (Figure 3; Ricci and Kirschvink, 1992). This discrepancy has also been described for the macroscopic single-domain behavior (Stanjek et aL, 1994). It has been explained by the dense aggregation ofgreigite particles, which have different crystallographic orientation towards each other--hence giving small crystalline sizes in X-ray diffraction--but common magnetic orientations.…”

“…In a recently described occurrence, 6 S 34 isotope analyses and TEM observations indicated a greigite formed in a gley soil to be of biogenic origin (Stanjek et at., 1994). Preliminary X-ray diffraction and TEM work indicate the mean particle size of this soil greigite to lie in the superparamagnetic region as defined by Ricci and Kirschrink (1992), but its macroscopic behavior corresponds to that of single-domain particles.…”

Effects of parent material and climate on the magnetic susceptibility of Saskatchewan soils

“…Magnetic horizonation results from formation and destruction of magnetic minerals, their transport in the solum, and their dilution or concentration due to build-up or removal of non-magnetic soil constituents (Fine et al 1989). Magnetite and maghemite are the main contributors to χ, although in some soils iron sulfides may also make a significant contribution (Mullins 1977;Stanjek et al 1994). Collectively these minerals are referred to as ferrimagnetics.…”

“…The small size of the pedogenic ferrimagnetics leads to increased χ and increased dependency of χ on the frequency of the measuring field. Maghemite is not stable under reducing conditions and low values of χ are found in gleyed soils (Dearing et al 1985;Mullins 1977) unless certain iron sulfides are present (Stanjek et al 1994).…”