Comparison of Pedogenic a N D Sedimentary Greigite by X - R a Y Diffraction a N D Mossbauer Spectroscopy

-Greigites from a gtey and a Tertiary sediment were investigated by X-ray diffraction and M6ssbauer spectroscopy. The cell-edge length a of 9.8639/~ + 0.0003 A, for the soil greigite was significantly smaller than that of the sedimentary greigite (9.8737 A, + 0.0004 A,), but both cell-edge lengths were smaller than the value given on JCPDS card #16-713 (9.876 ~,). Both greigites had 440 as strongest peak rather than 311 (as indicated on JCPDS card #16-713), but the other relative intensities did not deviate from the values given on this card within experimental error. Mean X-ray diffraction coherence lengths of 23 + 2 nm for the soil greigite and of 60 + 5 nm for the sedimentary greigite suggest superparamagnetic behavior. M6ssbauer spectra nevertheless comprised two sextets with hyperfine fields of about 31.2 T (tetrahedral sites) and 30.7 T (octahedral sites), which resemble published values. It is postulated that aggregation may play an important role in determining the magnetic properties of the described samples. Key Words--Greigite, M6ssbauer, Sediment, Soil, XRD. I N T R O D U C T I O N The ferrimagnetic mineral greigite (Fe 2+ Fe23+ $4) has been described from marine, brackish and freshwater lacustrine sediments (Jedwab, 1967; Bemer, 1969; Dell, 1972; Snowball, 1988; Mann et aL, 1990). 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 Kirschr i nk (1992), but its macroscopic behavior corresponds to that of single-domain particles. In an attempt to elucidate this contradictory behavior, room-temperature M6ssbauer spectra were taken of this soil greigite and of a sedimentary greigite from the Sokolov Basin in Western Bohemia (previously described by Hoffmann, 1992). MATERIALS AND METHODS The soil greigite was formed in a gley developed from colluvial material eroded from the Hesselberg, Bavaria. The zone of greigite formation begins at a depth of about 70 cm (Gr2) and ends at about 100 cm in the Gr3 horizon. A detailed description of this occurrence has been given by Auerswald et al. (1992). The sedimentary greigite was extracted magnetically from Miocene claystones (for further details see Krs et al., 1990). X-ray diffraction analyses were carried out with a Huber Guinier diffractometer using Co Ka~ radiation. * Present address: Bayerisches Geologisches Landesamt, Concordiastrasse 28, D-96049 Bamberg, Germany. Copyright 9 1994, The Clay Minerals Society Step scans were run from 100-100~ with an increment of 0.02" and a counting time of 30 sec. Silicon powder with a particle size <20 ttm (Merck, #12497) with a unit-cell dimension a -5.4307 ~ (Klug and Alexander, 1974) served as an internal standard. The scans were fitted with a program based on the IR-spectra fitting program by Janik and Ranpach (1977) and ported to C by Stanjek. In order to better approximate the peak shapes by the fitted Pseudo-Voigt functions, each greigite peak was fitted with three individual peaks. The program CDruck was used to sum up these single peaks to a combined peak from which the integral intensity and centroid were calculated. Peak intensities were corrected for the Lorentzand polarization factor (De Wolf and Visser, 1988) but not for sample absorption. Instrumental broadening was determined using annealed Fe2Oa and quartz. To calculate the effective particle size and strain, the measured integral widths were corrected for instrumental broadening by applying correction functions derived from folding procedures (Stanjek, 1991). Based on the postulation that particlesize broadening yields Lorentzian profiles and strain broadening Gaussian profiles, both effects can be separated by the formula given by Klug and Alexander (1974, p. 665): (620) 2 K~[~ 620 o] tan200 ~ -L-an 00 sin 0 + 16e 2 (1) where 6 20 is the integral breadth corrected for instrumental broadening, 00 is the angle of reflection, K is the Scherrer constant, ~, is the wavelength, L is the mean coherence length of the crystallites (MCL), and e is the strain. K was taken from Langford and Wilson (1978) assuming the crystallites to be octahedral in shape. A