Elongated prismatic magnetite crystals in ALH84001 carbonate globules: Potential Martian magnetofossils

Using transmission electron microscopy (TEM), we have analyzed magnetite (F e3O4) crystals acid-extracted from carbonate globules in Martian meteorite ALH84001. We studied 594 magnetites from ALH84001 and grouped them into thr ee populations on the basis of morphology: 389 were irregularly shaped, 164 were elongated prisms, and 41 were whisker-like. As a possible terrestrial analog for the ALH84001 elongated prisms, we comp ared these magnetites with those produced by the terrestrial magnetotactic bacteria strain MV-1. By TEM again, we examined 206 magnetites recovered fr om strain MV-1 cells. Natural (Darwinian) selection in terrestrial magnetot actic bacteria appears to have resulted in the formation of intracellular m agnetite crystals having the physical and chemical properties that optimize their magnetic moment. In this study, we describe six properties of magnet ite produced by biologically controlled mechanisms (e.g., magnetotactic bac teria), properties that, collectively, are not observed in any known popula tion of inorganic magnetites. These criteria can be used to distinguish one of the modes of origin for ma gnetites from samples with complex or unknown histories. Of the ALH84001 ma gnetites that we have examined, the elongated prismatic magnetite particles (similar to 27% of the total) are indistinguishable from the MV-1 magnetit es in five of these six characteristics observed for biogenically controlle d mineralization of magnetite crystals. Copyright (C) 2000 Elsevier Science Ltd.