A High Resolution Electron Microscopic Investigation of Bacterial Magnetite. Implications for Crystal Growth

Magnetite particles isolated from magnetococcoid bacterial cells have been studied by high resolution transmission electron microscopy (h.r.t.e.m.). The particles show lattice images consistent with the magnetite structure and have a hexagonal prism habit truncated by {011} and {100} planes. The particles have a high degree of crystal perfection and are single domain structures. Some crystals showed a superimposed `graininess' and ill-defined edges. In some crystals regular growth points on the crystal edge could be observed while in other crystals the material at the crystal edge appeared to be amorphous. These results suggest that the crystals are often surrounded by an amorphous phase. These observations are consistent with a model of crystal formation which involves nucleation of the magnetite phase at one primary site followed by slow crystal growth. It is proposed that crystal growth proceeds by dissolution of kinetically favoured precursor iron oxide phases aggregated or precipitated at the magnetite surface of the primary crystal followed by incorporation of soluble ferric-ferrous hydroxo-complexes into the magnetite lattice. The possible roles of the surrounding membrane in magnetite synthesis are discussed.