High-resolution TEM image of a single-domain magnetite (Fe₃O₄) crystal from the human cerebellum^[1]. This image shows the pattern of intersecting {111} and {022} crystal lattice fringes, with particle elongation in the [111] lattice direction. The morphology and structure of these crystals resemble strongly those produced by the magnetotactic bacteria and salmon. Although biogenic magnetite is present in trace levels (1-100 ppb) in most human tissue samples, we do not yet know what biological function it has, if any.

[1] J. L. Kirschvink, A. Kobayashi-Kirschvink, and B. J. Woodford, "Magnetite biomineralization in the human brain.," Proceedings of the National Academy of Sciences, vol. 89, pp. 7683-7687, 1992.