Magnetic Domain State and Coercivity Predictions for Biogenic Greigite (Fe3S4): 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 abo~t the. magnetic challl~ristics of greigite. For this reason, and the present inability to grow these microorg~~s ~ pure ~re, It u ~ot known .whether ~r not the magnetosomes in the newly discovered greigiteprea~tatmg bactena ~re of smgle-d~ (SD) sJZe, as are the magnetosomes from magnetite-precipitating bactena. The hypotheSls of natural selection for magnetotactic behavior predicts that the greigite-bearing magnetos~es should also be s~~e magnetic domains. Using previously reported magnetic properties and crystalloglllphic featul7s for gre1g1te, we have calculated the size and shape boundaries expected for SD and superpa~gnetlc (SP~ behavior~ this m!neral. For further characterization ofthe greigite crystals, we analyzed the ~am state at vanous length/Width ratios assuming crystal shapes of parallelepipeds and prolate spheroids. Mag~etlte was used as control for the current theories supporting these calculations. We also present a simple alg~~thm to cal~te the upper size ~t of single-domain glllins. Our resuhs show that the crystals of bacterial greigite characte~ so far are l~ted m the region close to the single-domain superparamagnetic boundary and should have relatlv_ely lo~ .coerCJ.Vlty. If these crystals contribute to the magnetization ci sediments, remanence ~rodu~ by bacterial gre1g1te could be mistaken for large, multidomain magnetite in alternating field demagnetization studies.