THE ORIGIN OF TRUNCATED HEXAOCTAHEDRAL MAGNETITE IN MARTIAN METEORITE ALH84001

Martian meteorite ALH84001 contains mm-sized carbonate assemblages situated within cracks and fissures, which have been dated at 3.91 Ga. [1]. These are interpreted as secondary minerals formed at low temperature [2,3] in an aqueous medium [4,5]. Dispersed within these carbonates are nm-sized, single crystal magnetites. Using a combination of classical and tomographic transmission electron microscopy we have been able to demonstrate a fraction of these magnetites has chemical and physical properties nearly identical to intracellular magnetite crystals produced by magnetotactic bacteria strain MV-1 [6-8]. The MV-1 magnetite morphology has been described as truncated hexaoctahedral [9] and appears to be tightly regulated by the bacteria cell under genetic control. In the natural terrestrial environment, such truncated hexaoctahedral magnetite appears only to been produced through biogenic mediation [6 and refs. therein]. To date, no credible synthetic inorganic procedure has been able to reproduce magnetite with this morphology. Nor might it be expected, since the truncated hexaoctahedron possess hexagonal symmetry, where as the underlying crystal symmetry of the magnetite unit cell is itself cubic. A posteriori, we suggest those ALH84001 magnetites with truncated hexaoctahedral geometry, indistinguishable from MV-1 magnetite, were mediated by similar biogenic processes on early Mars.

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