nai

Project Progress

Mineral Biomarkers (dm)

This past year we made major progress in characterizing biogenic magnetite made by magnetotactic bacteria MV-1. This magnetite has an unusual non-equilibrium morphology: truncated hexa-octahedral. It also has a very pure composition consisting of only Fe and O at least down to the few hundred ppm level. The size distribution of this magnetite, when corrected for geometry effects of non-perpendicular projection shows a peculiar truncated non-normal distribution. The sizes are truncated before they exceed the single-domain size field. It then turned out that martian meteorite ALH84001 has essentially identical magnetite comprising a subpopulation that constitutes about 25% of all the magnetites in the carbonate globules.

Extensive literature search revealed no other magnetite, either natural or synthetic, that matched the unusual properties of this magnetite. Expermental work by a team at JSC has produced single domain magnetite by thermal decomposition of siderite, but this magnetite does not match more than one or two of the six characteristic properties of the MV-1 magnetite or the ALH84001 subpopulation magnetite. Experimental work by Chris Romanak and colleagues at the Savannah River Ecology Laboratory shows that both magnetite and siderite can be produced at relatively low temperatures from aqueous solutions under CO2 partial pressure equal to that on Mars.

Recent work used off-axis electron holography in the transmission electron microscope (TEM) to characterize the magnetic microstructure of magnetotactic bacteria MV-1 and MS-1. One result was the unexpected finding that superparamagnetic magnetite in chains, while to small to retain stable magnetic moments if isolated, are influenced by the single domain magnetite nearby so that they behave like single domain magnetite. This explains how the ends of magnetic chains in magnetotactic bacteria, while often too small to have stable moments, can be influenced by the rest of the chain and can therefore contribute to the overall magnetic moment of the chain. Studies of submicrometer greigite (iron sulfide) show that its crystallization can be biologically induced or biologically mediated, and it is possible to tell the difference from the size distribution. Greigite may provide a suitable biomarker if it can be well characterized.

Studies of Mn precipitates in caves by show that Mn minerals are almost certainly formed by the action of Mn-oxidizing microbes. Diagenesis of amorphous and nanocrystalline Mn oxides and hydroxides as well as biofilms form the mineral todorokite. The possibility that Mn minerals may make excellent stable mineral biomarkers can be applied to potential Mars locations and samples.