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

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... 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.