sedimentary
Shale
Shale (Fine-grained clastic sedimentary rock)
Hardness: 3 on Mohs scale; Color: Gray to dark gray; Luster: Dull or earthy; Crystal Structure: Non-crystalline (fine-grained matrix); Cleavage: Perfect fissility (splits into layers); Specific Gravity: 2.4-2.8
- Hardness
- 3 on Mohs scale
- Color
- Gray to dark gray
- Luster
- Dull or earthy
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Physical properties
Hardness: 3 on Mohs scale; Color: Gray to dark gray; Luster: Dull or earthy; Crystal Structure: Non-crystalline (fine-grained matrix); Cleavage: Perfect fissility (splits into layers); Specific Gravity: 2.4-2.8
Formation & geological history
Formed through the compaction of silt and clay-sized mineral particles, commonly referred to as mud. This usually occurs in low-energy environments like lake bottoms, river deltas, and floodplains over millions of years.
Uses & applications
Mainly used in the manufacture of cement, brick, and tile. It is also a significant source of natural gas and petroleum when it contains high organic content (oil shale).
Geological facts
Shale is the most common sedimentary rock on Earth, making up approximately 70% of the sedimentary rock in the Earth's crust. It often contains fossils of marine life or plants.
Field identification & locations
Identify by its fine-grained texture and its tendency to break along thin laminations or parallel layers (fissility). It is found globally in sedimentary basins. For collectors, look for layers that may contain fossil imprints.
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