sedimentary
Concretion in Shale
Siltstone/Shale with Siderite or Calcareous concretions
Hardness: 3-4 (matrix 2-3). Color: Gray, tan, or brown. Luster: Dull/Earthy. Structure: Massive/Fine-grained with nodular inclusions. Specific gravity: ~2.5-3.0.
- Hardness
- 3-4 (matrix 2-3)
- Color
- Gray, tan, or brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (matrix 2-3). Color: Gray, tan, or brown. Luster: Dull/Earthy. Structure: Massive/Fine-grained with nodular inclusions. Specific gravity: ~2.5-3.0.
Formation & geological history
Formed through the precipitation of mineral cement within the spaces between sediment grains (shale or silt), often during early diagenesis in marine or lake environments. Many are from the Paleozoic or Mesozoic eras.
Uses & applications
Primarily used for geological study and fossil collecting; some concretions contain well-preserved fossils inside. Large deposits may be used for aggregate or fill.
Geological facts
Concretions are often mistaken for fossils, eggs, or meteorites. Some of the most famous concretions are the Moeraki Boulders in New Zealand.
Field identification & locations
Identify by a rounded or disc-shaped hardened mass sitting within softer layers of sedimentary rock. Common in shale cliffs, riverbeds, and coastal regions.
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