libutron: Septarian nodules of Marl with PyriteSeptarian nodules are a type of concretion; they are
libutron:Septarian nodules of Marl with PyriteSeptarian nodules are a type of concretion; they are masses of mineral matter formed when minerals in water are deposited about a nucleus (such as a leaf or shell or other particle) forming a rounded mass whose composition or cement is usually different from the surrounding rock. In contrast to most geodes, which are volcanic in nature, Septarians are a form of sedimentary geode. They formed on sea beds millions of years ago and, when they dried out, they cracked and formed hollow interiors. The interiors usually become either partly or completely full with other minerals.Locality: riverbank of the Volga, Ulyanovsk region, Russia.Photo credit: ©V. SlyotovReferences: [1] - [2]I gotta disagree on the “most geodes are volcanic in nature” part here. They can occur in both volcanic and sedimentary rocks, wherever you have open spaces, and I’m not sure there’s a good and true accounting of them. Most of the ones I’ve seen at rock sales are in sedimentary rocks, but that’s obviously not a good sampling either. -- source link
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