septarian nodule

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septarian nodule A concretion, roughly spheroidal in shape, usually of clay ironstone, and characterized by an internal structure of angular blocks separated by radiating mineral-filled blocks. The mineral filling the cracks is usually calcite. The structure results from the formation of a hard exterior to the nodule due to the development of an aluminous gel on the exterior, followed by dehydration of the colloidal mass (see COLLOID) in the interior, leading to cracking and subsequent mineral infilling of the radiating pattern of cracks.