Some Mineralogical Approaches to Study the Biocarbonate and the Carbonate-Siliceous Nodules

Nodules of different compositions from Paleozoic sedimentary rocks and those deposited by microbial communities in laboratory-scale experiments were studied by the use of electron paramagnetic resonance, X-ray diffraction, and scanning electron microscopy with energy-dispersive spectrometer. The study of the mineral composition of fossil nodules showed that they have monomineral composition of dolomite or chalcedony, mixed composition of dolomite-chalcedony or of opal-dolomite, and finally nodules can be composed of alternating opal and chalcedony layers cementing fine dispersed dolomite grains or clusters of irregular shape. Similarity in dolomite crystal lattice defects in both the nodules and the host rocks confirms their formation during synsedimentary early diagenesis. Bacterial activity during sedimentary nodules growth is evidenced by the presence of paramagnetic carbon-centered free radicals of fossilized protein substances and findings of fossil bacteria. Experimental laboratory-scale modeling of natural carbonate deposition by microbial communities confirms that bacteria can promote nodules formation.