sedimentary
Shale with Siltstone layering
Sedimentary Clastic Rock (Mudstone/Shale)
Hardness: 3 (Mohs scale); Color: charcoal grey to dark brown; Luster: dull/earthy; Crystal structure: fine-grained/microcrystalline; Cleavage: fissile (splits in layers); Specific Gravity: 2.4 - 2.8
- Hardness
- 3 (Mohs scale)
- Color
- charcoal grey to dark brown
- Luster
- dull/earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: charcoal grey to dark brown; Luster: dull/earthy; Crystal structure: fine-grained/microcrystalline; Cleavage: fissile (splits in layers); Specific Gravity: 2.4 - 2.8
Formation & geological history
Formed from the compaction of silt and clay-sized mineral particles over millions of years, typically in low-energy environments like lake beds or deep ocean floors. Can originate from the Paleozoic or Mesozoic eras.
Uses & applications
Used in the production of cement, bricks, and tiles. Historically used as a source for oil and natural gas (shale gas) via hydraulic fracturing.
Geological facts
Shale is the most common sedimentary rock on Earth, making up an estimated 70 percent of all sedimentary rocks in the Earth's crust. It often preserves fossils of soft-bodied organisms that other rocks can't.
Field identification & locations
Identify by its thin-layered (fissile) structure and soft texture that can often be scratched with a penny. Commonly found in river basins and plateau regions. Collectors should look for impressions on flat faces which may indicate fossils.
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