Sunday, October 12, 2008
The first answer was already the correct one! Lockwood correctly identified the shown foto as a septarian nodule. Dispite the breccia like appearance the cement is orange calcite (just tested it with a bit of acid). Also the components are carbonate rich mud. A bit more accurate septarian nodules are concretions in carbonate rich sedimentary rock like clays and marls that develop cracks due to desiccation. They are said to form due to local enrichment of carbonate because of decomposition of organic matter. Usually the cavities are filled by calcite but cases of quartz and other minerals that can be precipated at low temperatures are also known. The minerals form from circulating solutions. Congrats to Lockwood!