sedimentary
Shale
Shale (comprised mostly of clay minerals and quartz)
Hardness: 3 on Mohs scale; Color: Gray, black, or reddish-brown; Luster: Dull or earthy; Crystal structure: Non-crystalline/amorphous; Cleavage: Fissile (splits into thin, flat layers); Specific gravity: 2.4 - 2.8
- Hardness
- 3 on Mohs scale
- Color
- Gray, black, or reddish-brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 3 on Mohs scale; Color: Gray, black, or reddish-brown; Luster: Dull or earthy; Crystal structure: Non-crystalline/amorphous; Cleavage: Fissile (splits into thin, flat layers); Specific gravity: 2.4 - 2.8
Formation & geological history
Formed through the compaction and lithification of silt and clay-sized mineral particles (mud). It forms in low-energy environments like lake bottoms, river deltas, and floodplains. Most shales date back to the Paleozoic or Mesozoic eras.
Uses & applications
Used in the manufacture of cement, brick, and tile. It is also an important source rock for oil and natural gas (shale gas) via hydraulic fracturing.
Geological facts
Shale is the most common sedimentary rock on Earth, accounting for about 70 percent of the rocks found in the earth's crust. 'Black shale' is rich in unoxidized organic matter.
Field identification & locations
Identify by its ability to split into thin sheets along parallel planes (fissility). It looks similar to slate but is softer and lacks the slight sheen and hardness of metamorphic slate. Found globally in sedimentary basins.
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