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Definition of septarium

i. A large, roughly spheroidal concretion, 8 to 90 cm in diameter, usually of an impure argillaceous carbonate, such as clay ironstone. It is characterized internally by irregular polyhedral blocks formed by a series of radiating cracks that widen toward the center and that intersect a series of cracks concentric with the margins; these cracks are invariably filled or partly filled by crystalline minerals (most commonly calcite) that cement the blocks together. Its origin involves the formation of an aluminous gel, case hardening of the exterior, shrinkage cracking due to dehydration of the colloidal mass in the interior, and vein filling. The veins sometimes weather in relief, thus producing a septate pattern.

Synonym of: septarian nodule, septarian concretion, beetle stone, turtle stone

Ref: AGI

ii. A crystal-lined crack or fissure in a septarium. Pl: septaria.

Ref: AGI

Further refs.:
W. Alfred Richardson (1919): On the origin of septarian structure. Mineralogical Magazine 18, 327-338.
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