Chemistry Reference
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symmetry related faces of the {100} form and of the eight symmetry related faces of the
{111} form. The habits of the non-equidimensional crystals in some strains can be
described as combinations of {100}, {111} and {110} forms (Devouard et al. 1998). In
these cases, the six, eight and twelve symmetry related faces of the respective forms that
constitute the habits do not develop equally (Mann and Frankel 1989; Devouard et al.
1998). Examples of these crystal types are illustrated in Figure 4.
Whereas the cubooctahedral form of magnetite occurs in inorganically-formed
magnetite crystals (Palache et al. 1944), the elongated, pseudo-prismatic structures,
corresponding to the unequal development of some symmetry related faces, might occur
either because of anisotropy in the growth environment (for example, concentration
and/or temperature gradients) or the growth sites (Mann and Frankel 1989). Anisotropy
could derive from an anisotropic flux of ions through the magnetosome membrane
surrounding the crystal, or from anisotropic interactions of the magnetosome membrane
with the growing crystal (Mann and Frankel 1989). In these cases, the growth process
could break the symmetry of the faces of each form. These elongated crystals are so
unusual that their presence in recent and ancient sediments and in the Martian meteorite
ALH84001 has led to them being referred to as magnetofossils (Chang and Kirschvink
1989) and being used as evidence for the past presence of magnetotactic bacteria in
aquatic habitats and sediments (Chang and Kirschvink 1989; Stolz et al. 1986, 1990) and
life on ancient Mars (McKay et al. 1996; Thomas-Keprta et al. 2000, 2001, 2002) (this is
discussed at length later in this chapter).
The most anisotropic crystal habits are those of the tooth-, bullet- or arrowhead-
shaped magnetite crystals (Fig. 3b). The synthesis of these crystal habits appears to be
Figure 4.
Idealized magnetosome crystal habits based on combinations of {100} (cube), {110}
(dodecahedron), and {111} (octahedron) forms. All the habits except the equidimensional cubo-
octahedron (lower left) have elongated projected shapes which could result from anisotropy in the
crystal growth site. (Figure adapted from Devouard et al. 1998).
