The tiny granules of magnetite found in magnetized sediments come in various crystalline forms. Inorganic magnetite precipitated from molten rock is octahedral, while the particles manufactured by bacteria are cubes, hexagonal prisms, or noncrystalline teardrops. The magnetite found in marine sediments appears to be organically formed -- at least the shapes of the particles are characteristic of bacterial manufacture. Apparently these industrious bacteria have been busy producing magnetite ever since "lowly" life forms appeared in the Precambrian.
These facts pose at least four questions:
How much of the earth's iron ore has been concentrated biologically and is there a connection with the Gaia Hypothesis?
Is it possible that magnetic field reversals, now believed to be of purely geophysical origin, might be biological artifacts (that is, due to population and/ or species changes of magnetic bacteria)?
If magnetic field reversals are of geophysical origin, how do the magnetic bacteria find their food sources during the long periods of near-zero field?
Lab experiments prove that magnetic bacteria require free oxygen to secrete magnetite, but the Precambrian atmosphere and oceans were supposedly devoid of oxygen until 2.3 billion years ago. How did the magnetic bacteria prosper before then?
(Simon, C.; "Tiniest Fossils May Record Magnetic Field," Science News, 124:308, 1983.)