Postprint version. Published in Science, Volume 280, Issue 5365, May 8, 1998, pages 880-883. Copyright © 1998 American Association for the Advancement of Science. The definitive version is available at http://dx.doi.org/10.1126/science.280.5365.880.
Some bacteria form intracellular nanometer-scale crystals of greigite (Fe3S4) that cause the bacteria to be oriented in magnetic fields. Transmission electron microscope observations showed that ferrimagnetic greigite in these bacteria forms from nonmagnetic mackinawite (tetragonal FeS) and possibly from cubic FeS. These precursors apparently transform into greigite by rearrangement of iron atoms over a period of days to weeks. Neither pyrrhotite nor pyrite was found. These results have implications for the interpretation of the presence of pyrrhotite and greigite in the martian meteorite ALH84001.