sedimentary
Shale with Plant Fossils / Carbonaceous Shale
Laminated Mudrock (Siliciclastic)
Hardness: 3 on Mohs scale (fairly soft/fissile); Color: Grey, black, or dark brown due to organic content; Luster: Dull/Earthy; Crystal structure: Non-crystalline (composed of clay and silt-sized particles); Fracture/Cleavage: Fissile, meaning it breaks easily into thin layers.
- Hardness
- 3 on Mohs scale (fairly soft/fissile)
- Color
- Grey, black, or dark brown due to organic content
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3 on Mohs scale (fairly soft/fissile); Color: Grey, black, or dark brown due to organic content; Luster: Dull/Earthy; Crystal structure: Non-crystalline (composed of clay and silt-sized particles); Fracture/Cleavage: Fissile, meaning it breaks easily into thin layers.
Formation & geological history
Formed from the slow deposition of silt and clay-sized mineral particles in low-energy environments like swamps, lake bottoms, or deep-ocean basins. The dark coloration and presence of plant-matter imprints suggest a swampy or flood-plain environment during the Carboniferous period (approx. 300 million years ago).
Uses & applications
Major source of natural gas and oil when processed; historically used in brickmaking and cement production; often collected by hobbyists for the plant fossils contained within the layers.
Geological facts
Shale is the most common sedimentary rock on Earth. This specific specimen appears to contain compressions of prehistoric flora, which helped form the great coal beds of the world during the Paleozoic era.
Field identification & locations
Identify in the field by its layered appearance and the fact that it splits into flat sheets when struck; it will not fizz with acid (distinguishing it from limestone). Commonly found in sedimentary basins worldwide, particularly near coal mines or river valleys.
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