Septarian

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.

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