Figure 1.

Idealized truncated hexa-octahedral crystal habit of magnetite from the magnetotactic bacterium strain MV-1. Previous reports describing the MV-1 crystal habit misidentified this type of crystal, defining it as a hexa-octahedron (2, 10). [Note: In the microbiological community these crystals have been referred to as “parallelepiped”(10).] Magnetite with hexa-octahedral crystal habit is elongated along one of the [111] axes, with eight {111} octahedral (green) faces and six {110} hexagonal (red) faces (see supplemental data, Fig. 4 b1–b3). In contrast, magnetite with truncated hexa-octahedral crystal habit has eight {111} octahedral faces, six {110} hexagonal faces, and six {100} cubic (blue) faces. The two {111} faces perpendicular to the axis of elongation are equivalent to each other but not to the remaining six {111} faces that are not parallel to the elongation axis. Other biogenic magnetite geometries reported in the literature include elongated cubo-octahedrons (in wild-type bacteria), hexa-octahedrons (in vibroid bacteria), and hexa-octahedrons (in cocci) (for a complete discussion see ref. 10 and supplemental data, Fig. 4). (a1) Orthographic projection of a truncated hexa-octahedron. (a2) Truncated hexa-octahedron viewed down the [1–10] zone axis. (a3) Truncated hexa-octahedron viewed down the [111] zone axis. (b) The transition from hexa-octahedral to truncated hexa-octahedral crystal habit is achieved by adding cubic {100} faces (blue surfaces). The degree of truncation appears variable in MV-1 and ALH84001 magnetites, with hexa-octahedral crystal habit being one end member (far left) of the progression shown here.