sedimentary
Shale
Fissile Mudstone (primarily composed of clay minerals and quartz grains)
Hardness: 3 on Mohs scale; Color: typically grey, black, or reddish-brown; Luster: dull/earthy; Structure: fine-grained clastic texture; Cleavage: exhibits fissility (splits into thin, flat layers).
- Hardness
- 3 on Mohs scale
- Color
- typically grey, black, or reddish-brown
- Luster
- dull/earthy
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Physical properties
Hardness: 3 on Mohs scale; Color: typically grey, black, or reddish-brown; Luster: dull/earthy; Structure: fine-grained clastic texture; Cleavage: exhibits fissility (splits into thin, flat layers).
Formation & geological history
Formed through the lithification of mud and silt particles, usually in low-energy environments like lake bottoms, river deltas, and deep ocean basins. The compaction occurs over millions of years under overlying pressure.
Uses & applications
Used as a major source of natural gas (shale gas) and oil. In construction, it is used for making bricks, tiles, and as a component in Portland cement.
Geological facts
Shale is the most common sedimentary rock on Earth. It serves as an excellent host for fossils because of the gentle deposition environment in which it forms. Famous deposits like the Burgess Shale contain some of the oldest well-preserved soft-bodied fossils.
Field identification & locations
Identify in the field by its tendency to break into thin sheets and its soft, muddy smell when wet. Commonly found in basins across the Appalachian Mountains, UK, and China. Collectors look for shale with visible plant or animal impressions.
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