sedimentary
Shale (sub-type: Carbonaceous Shale)
Lutaceous Sedimentary Rock
Hardness: 3 on Mohs scale; Color: Black, dark gray; Luster: Dull/Earthy; Crystal Structure: Clastic (clay-sized particles); Cleavage: Fissile (splits into thin layers); Specific Gravity: 2.4 - 2.8
- Hardness
- 3 on Mohs scale
- Color
- Black, dark gray
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3 on Mohs scale; Color: Black, dark gray; Luster: Dull/Earthy; Crystal Structure: Clastic (clay-sized particles); Cleavage: Fissile (splits into thin layers); Specific Gravity: 2.4 - 2.8
Formation & geological history
Formed through the compaction of silt and clay-sized mineral particles over millions of years. This specific dark specimen suggests a high organic content, likely formed in low-energy, oxygen-poor environments like swamps or deep lake basins.
Uses & applications
Primary source of natural gas and petroleum (shale gas); used in the manufacturing of cement, brick, and tile. Historically used as a source for oil shale processing.
Geological facts
Shale is the most common sedimentary rock on Earth, making up over 50% of the sedimentary rocks in the Earth's crust. It often preserves fossils more effectively than coarser-grained rocks.
Field identification & locations
Identify by its 'fissility' - the way it breaks into flat, thin sheets. It is commonly found in river basins and ancient sea beds. For collectors, it is best found in road cuts through sedimentary basins.
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