sedimentary
Shale (or Slate / Coal / Jet)
Laminated fine-grained sedimentary rock (predominantly clay minerals)
Hardness: 1-3 (Mohs scale), Color: Dark gray to black, Luster: Dull to sub-vitreous, Crystal Structure: Microcrystalline, Cleavage/Fracture: Fissile (splits along parallel layers)
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Physical properties
Hardness: 1-3 (Mohs scale), Color: Dark gray to black, Luster: Dull to sub-vitreous, Crystal Structure: Microcrystalline, Cleavage/Fracture: Fissile (splits along parallel layers)
Formation & geological history
Formed from the compaction and lithification of silt and clay-sized mineral particles deposited in quiet water environments like lake beds, river deltas, and deep ocean basins.
Uses & applications
Used in the production of cement, brick manufacturing, and historically as writing tablets or roofing slates (when metamorphosed). Organic-rich black shale serves as a source rock for oil and natural gas.
Geological facts
Black shale gets its dark color from accumulated organic matter (carbon compounds) that decayed in low-oxygen aquatic environments millions of years ago.
Field identification & locations
Look for dark, fine-grained texture that easily splits along thin, flat layers (fissility). It feels smooth or slightly dusty to the touch and can leave a dark streak.
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