Septarian nodules are round concretions of claystone or rhyolite (a volcanic deposit) with an interior structure that is star shaped or formed by haph

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Septarian nodules are round concretions of claystone or rhyolite (a volcanic deposit) with an interior structure that is star shaped or formed by haphazardly radiating cracks subsequently filled with another mineral, often crystalline calcite. The cracked structure comes about by the drying up and shrinkage of the concretion and the cracks and voids are later filled up with some other mineral. Cut in half and polished, these nodules display fascinating configurations, like seen in this example from the Drome region in southeastern France, the infill of the cracks being the mineral barite. Actual diameter is 44 cm.

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Photo credit: © DIRK WIERSMA/SCIENCE PHOTO LIBRARY / Alamy / Afripics
License: Licensed
Model Released: No

Keywords: barite, calcite, claystone, cracks, drome, nodules, rhyolite, septarian, volcanic