sedimentary
Shale
Shale (composition varies: primarily clay minerals and quartz)
Hardness: 3 on Mohs scale; Color: Gray to black; Luster: Dull/Earthy; Crystal structure: Non-crystalline/fine-grained; Cleavage: Fissile (splits into thin sheets); Specific gravity: 2.4 - 2.8
- Hardness
- 3 on Mohs scale
- Color
- Gray to black
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3 on Mohs scale; Color: Gray to black; Luster: Dull/Earthy; Crystal structure: Non-crystalline/fine-grained; Cleavage: Fissile (splits into thin sheets); Specific gravity: 2.4 - 2.8
Formation & geological history
Formed through the lithification of mud and silt in low-energy environments like lake beds or deep ocean basins. It is common across all geological eras, particularly the Paleozoic and Mesozoic.
Uses & applications
Used in the manufacture of cement, brick, and tile. It is also an important source rock for natural gas and oil through hydraulic fracturing.
Geological facts
Shale is the most common sedimentary rock on Earth. It often contains delicate fossils that are preserved between its thin, flat layers.
Field identification & locations
Identify by its fine-grained texture and its ability to break into thin, flat plates (fissility). It will not fizz with acid unless it contains calcite. Commonly found in river deltas and ancient sea beds.
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