Iron sulfides from magnetotactic bacteria: Structure, composition, and phase transitions

Using transmission electron microscopy, we studied the structures and compo sitions of Fe sulfides within cells of magnetotactic bacteria that were col lected from natural habitats. Ferrimagnetic greigite (Fe3S4) occurred in al l types of sulfide-producing magnetotactic bacteria examined. Mackinawite ( tetragonal FeS) and, tentatively, sphalerite-type cubic FeS were also ident ified. In contrast to earlier reports, we did not find pyrite (FeS,) or pyr rhotite (Fe1-xS). Mackinawite converted to greigite over time within the ba cteria that were deposited on electron microscope grids and stored in air. Orientation relationships between the two minerals indicate that the cubic- close-packed S substructure remains unchanged during the transformation; on ly the Fe atoms rearrange. Neither mackinawite nor cubic FeS are magnetic, and yet they are aligned in chains such that when converted to magnetic gre igite, the probable easy axis of magnetization, [100], is parallel to the c hain direction. The resulting chains of greigite are ultimately responsible for the magnetic dipole moment of the cell. Both greigite and mackinawite magnetosomes can contain Cu, depending on the sampling locality. Because ba cterial mackinawite and cubic FeS are unstable over time, only greigite cry stals are potentially useful as geological biomarkers.