Cabella, R.

Lucchetti, G.

Marescotti, P.

Vanko, D. A.

English

Marescotti P.

Cabella R.

Lucchetti G.

Vanko D. A.

University of Szeged

Acta Minera Iogica-Petrograph ica, Szeged, XL1, Supplementum B, 2000 GREIGITE FROM ROCKS: BIOGENIC OR NON-BIOGENIC? MÁRTON, E., (Paleomagnetic Laboratory, Eötvös Loránd Geophysical Institute, Budapest, Hungary), MÁRTON, P. (Department of Geophysics, Eötvös University, Budapest, Hungary), CZINER, K. & PÓSFAI, M. (Department of Earth and Environmental Sciences, University of Veszprém, Veszprém, Hungary) Iron sulfides that commonly occur in rocks include pyrrhotite (Fe, XS) of magmatic or metamorphic origin, pyrite and marcasite (both FeS2), and greigite (Fe3S4). Pyrite and marcasite are paramagnetic, i.e., they contribute only to the magnetic susceptibility. Monoclinic pyrrhotite and greigite have ferrimagnetic properties; thus, they may also contribute to the remanence. When greigite was discovered and identified in rocks, it was first considered to be rare. However, greigite and pyrrhotite as carriers of the natu-ral remanent magnetization in sediments have been increasingly reported over the last decade, and magnetic methods to screen rocks for the presence of ferrimagnetic iron minerals and for their identification have been developed. Iron sulfides in sediments can be of either inorganic or biogenic origin. Biologically-induced mineralization (BIM) is known to produce greigite as an intermediate in a reac-tion sequence that starts with precipitated amorphous iron sulfide and terminates with pyrite (MORSE et al., 1987). Greigite is also produced by biologically-controlled min-eralization (BCM) in magnetotactic bacteria (HEYWOOD et al., 1990). Except for a study that described presumably BCM greigite from soil (STANJEK et al., 1994), we have no information about the potential contribution of bacterial greigite to the magnet-ic signal of sediments or rocks. In the Pannonian basin and its surroundings there are several occurrences of dark grey clay and marl that have been studied in the last three years primarily for obtaining paleo-magnetic directions of tectonic value or for magnetostratigraphy; these rocks display magnetic properties characteristic of magnetic iron sulfides. We selected half a dozen samples from the above outcrops in which greigite was the likely carrier of the rema-nence, and studied them using analytical transmission electron microscopy (ATEM). Electron diffraction and energy-dispersive X-ray spectra confirmed the presence of greigite and pyrite in two samples, a marl that deposited in a brackish environment (Laki, Poland), and a salty marl that formed in a hypersaline basin (Mihalovce, Slovakia). In the Laki specimen greigite crystals occurred in clusters, attached to the surfaces of clay min-erals. The crystals typically showed non-uniform, blotchy contrast in the TEM (Fig. lb). Similar spotty contrast was observed in BCM greigite from magnetotactic bacteria (Fig. la), and interpreted as resulting from a partially C9mpleted solid-state transformation of a precursor sulfide (mackinawite) into greigite (PÓSFAI et al., 1998). Greigite grains in the Mihalovce sample looked different from those in the Laki rock; they occurred in large clusters together with pyrite, and showed uniform black contrast (Fig. lc). A knowledge of the crystal size distribution (CSD) of a population of crystals is use-ful for assessing possible growth mechanisms (EBERL et al., 1998). We compared the CSDs of three populations of iron sulfide crystals, including bacterial (BCM) greigite and greigite from two rock specimens (Fig. 2). The CSD of bacterial greigite shows an almost perfect bell-shaped or normal distribution (Fig. 2a), in contrast to CSDs of BCM magnetite crystals that are asymmetric (DEVOUARD et al., 1998). The CSD of greigite from the Laki sample (Fig. 2b) is similar to that of bacterial greigite; however, the max-imum of the curve is at 120 nm, indicating that greigite crystals in this rock are about twice as large as those from the MMP. The CSD for the Mihalovce sample differs sig-nificantly from both the bacterial and the Laki greigite CSDs: it has an asymmetric "tail" extending to large crystal sizes, and a maximum of the frequency at 400 nm (Fig. 2c). 50 Acta Minera Iogica-Petrograph ica, Szeged, XL1, Supplementum B, 2000 The CSD for the Mihalovce sample can result from surface-controlled growth in an open system (EBERL et al., 1998), and both the CSD and the peak value are consistent with a framboidal (BIM) origin (WILKIN et al., 1998). On the other hand, the CSD and typ-ical contrast features of greigite crystals in the Laki sample make it very likely that these grains are of BCM origin, even though they are larger than greigite crystals produced by contemporary magnetotactic bacteria. Figure 1. Bright-field electron micrographs of clusters of greigite crystals from (a) a multicellular magnetotactic prokaryote (MMP), and from rock specimens from (b) Laki, and (c) Mihalovce. Note that in (a) and (b) greigite crystals show uneven, spotty contrast in the TEM, whereas in (c) the rounded and relatively thick crystals produce uniform and dark contrast. ÎhldMnnrfGcigMePMdrf Run Si« Lfctitu»«iofr*«l* l̂ rtiti hfini ttr Ikotukm cjf (Irigte and PuKr ftrtdet Ctfcoftt»M.P »IfcckSpK*T»rM™iljk HomakKfcSfKcnKiRunNMow* . J i l . l i l l i i it ,1,1. ,..lj,., , ... Figure 2. Size distributions of iron sulfide crystals from specimens as indicated on top of the figures References DEVOUARD, B., PÖSFAI, M., HUA, X., BAZYLINSKI, D.A., FRANKEL, R.B. & BUSECK, P.R. (1998). Am. Mineral., 83: 1387-1399. EBERL, D.D., DRITS, V.A. & SRODON, J. (1998). Am. J. Sei., 298: 499-533. HEYWOOD, B.R, BAZYLINSKI, D.A., GARRATT-REED, A., MANN, S. & FRANKEL, R.B. (1990). Naturwissenschaft., 77: 536-538. MORSE, W., MILLERO, F.J., CORNWELL, J.C & RICKARD, D. (1987). Earth Sei. Rev., 24: 1-42. PÖSFAI, M., BUSECK, PR., BAZYLINSKI, D.A. & FRANKEL, R.B. (1998). Science, 280: 880-883. STANJEK, H., FASSBINDER, J.W.E., VALI, H., WÄGELE, H. & GRAF, W. (1994). Eur. J. Soil Sei., 45: 97-103. WILKIN, R.T., BARNES, H.L. & BRANTLEY, S.L. (1996). Geochim. Cosmochim. Acta, 60: 3897-3912. This study was financially supported by Hungarian Scientific Research Fund (OTKA) project No. T029805 and a grant from the Hungarian Academy of Sciences (AKP #98-94 2.5). 51 

Microbial interactions in the alteration processes of natural basaltic glass : textual evidences in pillow basalts from the eastern flank of the Juan de Fuca Ridge

MARESCOTTI, PIETRO

CABELLA, ROBERTO

LUCCHETTI, GABRIELLA

VANKO D. A.

Archivio istituzionale della ricerca - Università di Genova

Microbial interactions in the alteration processes of natural basaltic glass: textural evidences in pillow basalts from the eastern flank of the Juan de Fuca Ridge.

http://acta.bibl.u-szeged.hu/25059/1/mineralogica_041_suppl_b_050-051.pdf

Microbial interactions in the alteration processes of natural basaltic glass: textural evidences in pilow basalts from the eastern flank of the Juan de Fuca Ridge

Microbial interactions in the alteration processes of natural basaltic glass: textural evidences in pilow basalts from the eastern flank of the Juan de Fuca Ridge

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University of Szeged

Archivio istituzionale della ricerca - Università di Genova