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Septarian concretions (or septarian nodules) are carbonate-rich concretions containing angular cavities or cracks. The cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate. Although it has commonly been assumed that concretions grew incrementally from the inside outwards, the fact that radially oriented cracks taper towards the margins of septarian concretions is taken as evidence that in these cases the periphery was stiffer while the inside was softer, presumably due to a gradient in the amount of CaCO3 precipitate progressively cementing the mud porosity from outside to inside.
The combination of natural processes giving rise to the formation of the carbonate-rich septaria remains unclear, but likely involves microbial activity and oxidation of the organic matter in the clay sediment as an internal source of carbonates. The calcium ions present in seawater, or in the pore water of the surrounding clay sediments, slowly diffuse towards the center of the initially soft concretion and progressively precipitate in contact with the carbonate anions present along their path and produced by the decaying organic matter. Indeed, when the pore water of the clay sediment becomes locally saturated with respect to calcium carbonate, this latter precipitates and progressively starts to cement the porosity of the initial medium (decaying marine organisms?). The cementation process seems to proceed from outside (harder) to inside (softer) along with the Ca2+ ions diffusion transport.
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