Magnetic Domain and Coercivity predictions for Biogenic Greigite (Fe3S4): A Comparison of Theories with Magnetosome observations

DÍAZ RICCI, JUAN C.; KIRSCHWINK, JOSEPH L.

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 1992 vol. 97 p. 17309 - 17315

0148-0227

Thediscovery of bacteria that precipitate greigite within intracellular organelles(magnetosomes) offers new evidence about the origin of greigite in naturalenvironments. Unlike magnetite, only scarce information is available about the. magnetic characteristics of greigite. For thisreason, and the present inability to grow these microorganisms inpure culture, it is not known whether or not the magnetosomes in the newlydiscovered greigite-precipitating bacteria are of singled-domain (SD) size, as are the magnetosomesfrom magnetite-precipitating bacteria. The hypothesis of natural selection formagnetotactic behavior predicts that the greigite-bearing magnetosomes should also be single magnetic domains. Usingpreviously reported magnetic properties and crystallographic features for greigite,we have calculated the size and shape boundaries expected for SD and superparamagnetic (SPM) behaviour in this mineral. For furthercharacterization of the greigite crystals, we analyzed the domain state at various length/width ratios assuming crystal shapesof parallelepipeds and prolate spheroids. Magnetitewas used ascontrol for the current theories supporting these calculations. We also presenta simple algorithm to calculatethe uppersize limit of single-domain grains. Our resultsshow that the crystals of bacterial greigite characterizedso far are located in the region close to the single-domain superparamagneticboundary and should have relatively low coercivity. If thesecrystals contribute to the magnetization of sediments, remanence produced by bacterial greigite could be mistaken for large,multidomain magnetite in alternating field demagnetization studies.