sedimentary
Shale (possibly Carbonaceous or Oil Shale)
Argillaceous Sedimentary Rock
Mohs Hardness: 2-3; Color: Black to dark grey; Luster: Dull/Earthy; Crystal Structure: Very fine-grained particles of clay minerals; Cleavage: Fissile (splits easily along layers); Specific Gravity: 2.3–2.8.
- Hardness
- 2-3
- Color
- Black to dark grey
- Luster
- Dull/Earthy
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Physical properties
Mohs Hardness: 2-3; Color: Black to dark grey; Luster: Dull/Earthy; Crystal Structure: Very fine-grained particles of clay minerals; Cleavage: Fissile (splits easily along layers); Specific Gravity: 2.3–2.8.
Formation & geological history
Formed by the compaction of silt and clay-size mineral particles. These dark varieties often form in stagnant, low-oxygen water where organic matter is preserved. Dating back to various eras from Paleozoic to more recent.
Uses & applications
Used as a source for oil and natural gas (fracking), in the manufacturing of cement, brick, and tile. It can also be a precursor to metamorphic rocks like slate.
Geological facts
Black shale is one of the most common sources of hydrocarbons in the world. It is also a key rock for finding fossils, as the low-oxygen formation environment prevents rapid decay.
Field identification & locations
Identify by its fine-grained texture and its tendency to split into thin flat pieces. Found globally in basins; look for it in river cutbanks or road cuts where flat, dark, brittle layers are visible.
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