DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Septarian nodule. Section through a septarian nodule, showing its inner structure. Septarin nodules are round mineral deposits of claystone (brown) filled with cracks formed when the claystone dried up. Claystone is a clastic sedimentary rock composed primarily of clay-sized particles. The cracks fill with other minerals, often calcite (calcium carbonate) forming a variety of patterns that are often prized for their beauty. This specimen is from Newcastle, UK.
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