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Published November 10, 1992 | Published
Journal Article Open

Magnetic domain state and coercivity predictions for biogenic greigite (Fe_3S_4): A comparison of theory with magnetosome observations


The discovery of bacteria that precipitate greigite within intracellular organelles (magnetosomes) offers new evidence about the origin of greigite in natural environments. Unlike magnetite, only scarce information is available about the magnetic characteristics of greigite. For this reason, and the present inability to grow these microorganisms in pure culture, it is not known whether or not the magnetosomes in the newly discovered greigite-precipitating bacteria are of single-domain (SD) size, as are the magnetosomes from magnetite-precipitating bacteria. The hypothesis of natural selection for magnetotactic behavior predicts that the greigite-bearing magnetosomes should also be single magnetic domains. Using previously reported magnetic properties and crystallographic features for greigite, we have calculated the size and shape boundaries expected for SD and superparamagnetic (SPM) behavior in this mineral. For further characterization of the greigite crystals, we analyzed the domain state at various length/width ratios assuming crystal shapes of parallelepipeds and prolate spheroids. Magnetite was used as control for the current theories supporting these calculations. We also present a simple algorithm to calculate the upper size limit of single-domain grains. Our results show that the crystals of bacterial greigite characterized so far are located in the region close to the single-domain superparamagnetic boundary and should have relatively low coercivity. If these crystals contribute to the magnetization of sediments, remanence produced by bacterial greigite could be mistaken for large, multidomain magnetite in alternating field demagnetization studies.

Additional Information

© 1992 by the American Geophysical Union. Received August 15, 1991; revised March 20, 1992; accepted April 10, 1992. This work was supported partially by NIH grant GM41635 and is contribution 5043 from the Division of Geological and Planetary Sciences of the California Institute of Technology.

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