Here’s a type of fossil I’ve never seen: worm burrow casts made entirely of the mineral marcasite. These come from the George Chambers (’79) gift collection, so we know only that they were found in Bolivia. Despite the lack of information about them, they’re curious enough to be featured in our series.
As best as I can figure out, these started as tunnels burrowed into a muddy substrate by worms of some kind. Iron sulfide, in the form of the mineral marcasite, precipitated within the abandoned tunnels, eventually filling them completely. Later the mud matrix was eroded away, leaving these intertwined tubes of silvery marcasite.
The tunnel walls were probably coated with an organic slime from the original worms or later bacteria. Sulfate-reducing bacteria may have then colonized the organic material, precipitating the pyrite in a manner described by Schieber (2002) and Virtasalo et al. (2010). The marcasite would ordinarily have converted to the more common (and stable) form of iron sulfide, pyrite (see Murowchick, 1992), but for some reason this did not happen here.
A cool combination of mineralogy and paleontology!
Murowchick, J.B. 1992. Marcasite inversion and the petrographic determination of pyrite ancestry. Economic Geology 87: 1141–1152.
Schieber, J. 2002. The role of an organic slime matrix in the formation of pyritized burrow trails and pyrite concretions. Palaios 17: 104–109.
Virtasalo, J.J., Löwemark, L., Papunen, H., Kotilainen, A.T. and Whitehouse, M.J. 2010. Pyritic and baritic burrows and microbial filaments in postglacial lacustrine clays in the northern Baltic Sea. Journal of the Geological Society 167: 1185-1198.