Greigite is isostructural with magnetite and is also ferrimagnetically ordered at room temperature. The greigite particles are characterized by narrow particle size distributions and species-specific crystal forms [ Heywood et al., 1991]. Electron microscopy of the greigite particles in several organisms has revealed at least two idealized particle morphologies: (1) cubo-octahedral and (2) elongated cubic with the axis of elongation along the <100> direction [ Heywood et al., 1991; Bazylinski et al., 1994].
The intrinsic magnetic properties of greigite are poorly
known, but the biogenic particles provide some useful information.
First, greigite magnetosomes, which should theoretically fall
within the SD range for magnetotaxis, have dimensions between
67-100 nm. These particle dimensions observed for greigite
magnetosomes are consistent with some simple calculations for the
SD size limit in Fe
S
[ Diaz Ricci and Kirschvink,
1993]. Second, the greigite magnetosomes are oriented with their
<100> axes aligned along the chain axis [ Heywood et al.,
1990; 1991], instead of <111> directions as observed in their
magnetite cousins, implying that the <100> direction is the
magnetic easy axis in greigite. Although the magnetocrystalline
anisotropy constant (K
) has never been measured for greigite,
the chain arrangement of the biogenic particles indicates that the
sign of K
must be positive at room temperature. Micromagnetic
calculations for greigite should therefore be consistent with a
positive anisotropy constant.
Although particular species of MTB are usually characterized by a unique magnetosome morphology and mineral composition, the magnetotactic bacteria found in sulfidic habitats are interesting exceptions. The MMP contains single or double chains of greigite and pyrite particles, each exhibiting several different crystal morphologies but all within a narrow size range of 50-90 nm [ Bazylinski and Frankel, 1992; Bazylinski et al., 1994; Heywood et al., 1990]. The pyrite particles seem to be the dominant phase but the arrangement of the greigite/pyrite particles within the chains or the function of the pyrite particles are unknown [ Heywood et al., 1991]. In addition, copper has been identified in association with some of the greigite-pyrite particles in the MMP and is the first evidence that a transition metal other than iron could be biomineralized by MTB [ Bazylinski et al., 1993b]. Lastly, the MTB described by Bazylinski et al. [1993a] produce both magnetite and greigite magnetosomes. Both ferrimagnetic mineral phases are co-organized in the same magnetosome chain, but each phase has a distinct crystal morphology and crystallographic orientation. The magnetite magnetosomes are arrowhead-shaped, whereas the greigite ones are rectangular. Furthermore, the magnetite and greigite crystals are aligned with their respective <111> and <100> easy axes along the chain direction.