Hello,
I've found this enigmatic iron oxide (only Fe and O in EDS spectra) under SEM in lacustrine sediments under volcanic influence (acidity) ... Locally, the crystal shows erroded and subeuhedral face (on the right lower corner), but it is not clear if the small voids observed on the rest of the crystals correspond to dissolution features or to microbial-mediated iron oxide precipitation. Has anybody already observed such a peculiar feature?
Idem for the second photograph that show "bubble structure" systematically associated to microsphere of iron oxides (probably pigmentary hematite)... is anybody has an idea of how these iron microspheres precipitated?
Thanks!
cheers
eric
Water does not show up well in these analyses, so you could be dealing with FeOOH or good old goethite. It is hard to tell from the photo but it looks like goethite I have found that replaced or was formed from dissolved pyrite. Such material is often mixed with bio materials of all kinds. An IR analysis would tell you more of the water content. But if the colour is anywhere between yellow and black, and it is relatively soft my bet would be on "limonite".
I have seen limonite as well as powdery hematite that is red. Where was it found? Associations? Environment?
I found these particles in a red clay layer from lacustrine sediments of the Deccan magmatic province. Kfd values and FORC diagram indicate that this red clay layer is rich in superparamagnetic particles. Below this red clay level we have a charcoal level, probably corresponding to allogenic coal produced by widespread wildfire. So my guess is that the red clay capping the coal represent a moment where the lake was dried and without vegetal cover, the rock surface suffered severe oxidation processes (explaining the presence of Fe3+ and red color).
Aridity and acidity (due to acid aerosols) may play a major role in the formation of the iron microspheres... but the striking well organisation of the "bubble structure" strongly suggest a microbial origin. My guess is the following: in such acidic and volcanic settings, opportunist algae may develop rapidly. The algae were not able to synthesize magnetosomes and iron was further oxidized in Fe3+ to form hematite/limonite pigment, giving the red color to the sediment....
what do you think?
I think your assessment is extremely sound. Has anyone else done any work on this locality?
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