Abstract
In sedimentary rocks, pyrite framboids are subspherical aggregates of microcrystals that form during decay of organic matter by sulfate reducing organisms (SRO). We report on pyrite found on a Permian silicified articulate brachiopod shell and address the implications for the pyritization and silicification processes. Four pyrite forms occur, in order of abundance: spherical to subspherical aggregates of microcrystals (framboids), rod-shaped aggregates of microcrystals (pyrite rods), euhedral crystals, and loose microcrystals. It is the pyrite rods that are of most interest here. Pyrite framboids and rods are found scattered in the silica that replaces the shell and in large groups in the surrounding silicified matrix. The microcrystals in pyrite rods have the same morphology and size distribution as those in framboids. The diameter of pyrite rods is slightly less than framboids, but the length is significantly greater. We interpret the pyrite rods as forming just like framboids by burst nucleation of pyrite microcrystals, but within a rod-shaped microbial cell, thus permineralizing that cell. Interestingly, the volume of rods and framboids are similar, suggesting that only cells of the right size and shape were recognizably permineralized. This is the first example of framboid-like pyrite permineralizing isolated microbial cells rather than multicellular microbial filaments. The abundance of pyrite framboids and rods embedded in silica strongly confirms the connection between microbial decay, here specifically including SRO, in a microbial ecosystem, and the silicification process.
