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a. 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.
Syn: septarian nodule; septarian concretion;
beetle stone;
turtle stone.
AGI
b. A crystal-lined crack or fissure in a septarium. Pl: septaria.
AGI
Source:
Dictionary of Mining, Mineral, and Related Terms
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