Geopuzzle #5

What mineral, or minerals, are we looking at in this Scanning Electron Microscope image?


Bonus points for people who can tell us anything about where you might find this stuff, and how it forms. If you’re struggling, click here for a picture of a possibly more familiar form of the same thing.
Update: Click through for the answer

Categories: geopuzzling

Comments (9)

  1. samk says:

    Pyrite. I don’t know where it’s found or how it forms.
    I did find out, though, that pyrite was used as a source of sulfur during World War II!

  2. Ron Schott says:

    I was going to say it’s a black and white image of fireworkite, but your hint gives a golden insight that only a fool could miss.
    The spherical(?) habit of these clusters is interesting, calling to mind framboidal pyrites that I’ve seen in much larger hand samples. The ones I’m thinking of (from the western Vermont slate belt) are musket ball sized and have spectacularly developed pressure shadows. These look pretty unmetamorphosed, with even a hint of an oolitic texture (albeit at a much smaller scale).

  3. Ellery says:

    I’ve seen similar pyrite structures using our electron microprobe. I’m guessing that this is a similar sample to ones I’ve seen and that the pyrite structures here are produced by bacteria in a coal deposit or slightly metamorphosed sediment — I can’t tell what the contrast level is in your backscattered-electron image, so I don’t know if the matrix is carbon, carbonate, silicate, etc… But that is my guess: it was produced by bacteria.

  4. Tegumai Bopsulai, FCD says:

    Clearly, it’s a picture of the virgin Mary.

  5. Maria Brumm says:

    I’ve seen detrital pyrites from your part of the world, but not such wee grains.
    I vote that it’s actually nonpareils.

  6. Ken Clark says:

    well, I am going to guess that it is a thin section of a chrondritic meteor high in metal content, they can have spheres on that scale, and can include pyrite.

  7. Miguel Vera says:

    The shape of the mineral(s) and the pyrite hint made me go for the easiest answer, framboidal pyrite.

  8. Andrew says:

    These are obviously pyritized nannobacteria.

  9. clay says:

    The only possible explanation I can think of is that it has an organic origin as Ellery suggested. Pyrite loves to form crystals and to get that many tiny, what appear to be spheroidal pyrites, clustered together in a larger spheroid without them all growing together into a solid mass I think would need some assistance.
    As for the surrounding matter, I cannot tell if it is silicate or organic (I’ll guess silicate as I think pyrite would be much more luminous vs carbonaceous material under a SEM). It is all appears very angular, poorly sorted and one would assume immature, we may be seeing a lot of clays. There are some non-pyrite fragments incorporated into the outer parts of the pyritic mass suggesting that the pyrite has grown insitu for at least the latter part of its development. There is a large (60um) linear fragment that has not been totally replaced in the center of the mass.
    ok, old fella on the block:
    My guess is a cluster of biogenic pyrite, either grown insitu or only transported a short distance hosted within a silty to muddy sediment (lacustrine) that most likely has some organic content (reducing conditions).