sedimentary
Siderite Concretion
Ironstone (Siderite) Concretion, FeCO3
Hardness: 3.5-4.5. Color: Dark brown to black core with tan or ochre weathering rind. Luster: Dull or earthy. Structure: Nodule/Concretionary. Specific gravity: Approx 3.8.
- Hardness
- 3
- Color
- Dark brown to black core with tan or ochre weathering rind
- Luster
- Dull or earthy
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Physical properties
Hardness: 3.5-4.5. Color: Dark brown to black core with tan or ochre weathering rind. Luster: Dull or earthy. Structure: Nodule/Concretionary. Specific gravity: Approx 3.8.
Formation & geological history
Formed by the chemical precipitation of mineral cement in the spaces between particles in sedimentary layers, often around a nucleus like a shell or leaf. Found in shale or sandstone beds.
Uses & applications
Primarily used as a minor ore of iron historically; today, valuable for education and collection. Some concretions contain fossils.
Geological facts
These are often mistaken for meteorites or fossils due to their unusual shapes. They can range from the size of a marble to several meters across.
Field identification & locations
Look for rounded or disc-shaped stones with a distinct core and outer 'shell' layer. Commonly found in riverbeds or eroded sedimentary cliffs.
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