Supplementary material for Thomas-Keprta et al. (2001) Proc. Natl. Acad. Sci. USA 98 (5), 2164–2169. (10.1073/pnas.051500898)
Supplemental Figure 4Fig. 4.
Three idealized geometries of elongated bacterial magnetite (adapted from ref. 1): elongated cubo-octahedron, hexa-octahedron vibroid, and hexa-octahedron coccid. Each crystal habit is shown in three orientations: overview, side view (viewed perpendicular to the [1-10] zone axis), and viewed down the [111] zone axis. (a1) The cubo-octachedron is elongated along the [112] axis [parallel to a (111) face] and displays {111} green and {100} blue faces. (a2) Elongated cubo-octahedron viewed down the [1-10] zone axis is an irregular hexagon. The dihedral angle is »125°, and the angle between two {111} faces is »109°. (a3) Viewed down the [111] zone axis the elongated cubo-octahedron is a polygon with 12 edges. (b1) Hexa-octahedron vibroid, elongated along the [111] zone axis with eight {111} (green, octahedral) faces and six {110} (red, hexagonal) faces. Dihedral angle is »109°. All {111} faces are equivalent in length; {100} faces are absent. Previous studies have suggested this crystal habit for magnetite formed by some strains of magnetotactic bacteria including MV-1. (b2) Hexa-octahedron vibroid viewed down the [1-10] zone axis. (b3) Viewed down the [111] zone axis, hexa-octahedron vibroid has a hexagonal projection with {110} faces at »120°. (c1) The hexa-octahedron coccoid is elongated along the [111] zone axis and has {111} green capping faces bounded by {110} red and {100} blue faces. Note crystallographically equivalent {110} faces have nonequivalent surface areas. (c2) Hexa-octahedron coccoid viewed down the [1-10] zone axis. {111} and {100} faces intersect at »125°; {111} and {110} faces intersect at »145°. There are no intersecting {111} faces. (c3) Viewed down the [111] zone axis, hexa-octahedron coccoid has a hexagonal projection bounded by {110} faces at »120°. MV-1 and ALH84001 truncated hexa-octahedral magnetites have {111}, {100}, and {110} faces (Fig. 1). The elongated cubo-octahedron does not have {110} faces (Fig. 4 a1--a3). The hexa-octahedron vibroid (10) does not have {100} faces (Fig. 4 b1--b3) and the {111} faces are of equal surface area. The hexa-octahedron coccoid (10) does not have intersecting {111} faces (Fig. 4 c1--c3). Hence, we suggest that the crystal habit of MV-1 and ALH84001 magnetites described herein is similar to that of a hexa-octahedron vibroid (10), but is modified by the presence of {100} truncating faces to give a crystal habit we describe as a truncated hexa-octahedral (Fig.1).Reference:
1. Mann, S., Sparks, N. H. C. & Wade, V. J. (1991) in Iron Biominerals, eds. Frankel, R. B. & Blakemore, R. P. (Plenum, New York), pp. 21-49.