Crystal-size distributions and possible biogenic origin of Fe sulfides

Sedimentary greigite (Fe3S4) can form either by "biologically controlled" o r by "biologically induced mineralization" (BCM and BIM, respectively). In order to identify the origin of magnetic Fe sulfides, we studied and compar ed the sizes and morphologies of greigite crystals produced by a magnetotac tic microorganism (previously described and referred to as the "many-celled magnetotactic prokaryote", MMP) and Fe sulfides from two specimens of Mioc ene sedimentary rocks (from Laka: in the foredeep of the Western Carpathian s and from Michalovce, in the Transcarpathian Depression). Greigite grains from the MMP and the Laka rock show nearly Gaussian crystal-size distributi ons (CSDs), whereas the CSD is lognormal for Fe sulfides from the Michalovc e rock. We simulated various crystal-growth mechanisms and matched the calc ulated and observed CSDs; crystals from the MMP and the Laka rock have CSDs that are consistent with random grow-th of crystal nuclei in an open syste m, whereas the CSD of the Michalovce Fe sulfides is consistent with surface -controlled growth followed by supply-controlled growth in an open system. On the basis of CSDs and characteristic contrast features in the transmissi on electron microscope, greigite in the Laka rock is likely of BCM origin, whereas the Fe sulfide crystals in the other rock sample were produced by B IM processes. Our results indicate that the methods we applied in this stud y may contribute to the identification of the origin of magnetic Fe sulfide minerals in sedimentary rocks.