DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Cut and polished septarian nodule. Septarian nodules are rounded concretions of claystone or rhyolite with an interior structure that is formed by haphazardly radiating cracks filled with another mineral, often crystalline calcite (calcium carbonate). The cracks occur by the drying and shrinking of the host rock. They are then filled up with calcite. Specimen measures 150 mm in diameter.
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