sedimentary
Shale
Fissile Clay-dominant Mudrock (SiO2, Al2O3, K2O, Na2O, CaO, MgO)
Hardness: 3 on Mohs scale (soft); Color: typically gray or black, but can be red or brown; Luster: dull/earthy; Crystal structure: lacks visible crystals; Cleavage: exhibits fissility (splits into thin layers); Specific gravity: 2.4 - 2.8.
- Hardness
- 3 on Mohs scale (soft)
- Color
- typically gray or black, but can be red or brown
- Luster
- dull/earthy
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Physical properties
Hardness: 3 on Mohs scale (soft); Color: typically gray or black, but can be red or brown; Luster: dull/earthy; Crystal structure: lacks visible crystals; Cleavage: exhibits fissility (splits into thin layers); Specific gravity: 2.4 - 2.8.
Formation & geological history
Formed by the compaction of silt and clay-sized mineral particles over millions of years, typically in slow-moving environments such as river deltas, lake bottoms, or deep ocean floors. It is the most common sedimentary rock found in the Earth's crust.
Uses & applications
Used in the production of bricks, tile, and ceramics. It is a critical source of cement. In the energy industry, organic-rich shales (oil shale) are a major source of natural gas and petroleum through hydraulic fracturing.
Geological facts
Black shale gets its color from high amounts of organic carbon (carbonaceous material). It is often a host rock for well-preserved fossils because the fine-grained sediment protects delicate remains from oxygen and scavengers.
Field identification & locations
Identify in the field by its distinct layering and tendency to flake into thin, flat sheets when struck. It feels smooth/clay-like to the touch compared to coarser sandstone. Look for it in road cuts or creek beds where stratified sedimentary layers are exposed.
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