sedimentary

Shale

Fissile Mudstone (primarily composed of phyllosilicate minerals like kaolinite, montmorillonite, and illite)

Hardness: 3 on Mohs scale; Color: typically gray or tan, can be black, red, or green; Luster: dull/earthy; Structure: fine-grained clastic; Cleavage: none, but exhibits fissility (splits into thin layers); Specific Gravity: 2.4 - 2.8

Hardness: 3 on Mohs scale
Color: typically gray or tan, can be black, red, or green
Luster: dull/earthy

Identified March 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the lithification (compaction and cementation) of mud, silt, and clay particles. Typically forms in slow-moving water environments such as lake beds, river deltas, and floodplains over millions of years.

Uses & applications

Used in the production of cement and brick; historic source of oil and natural gas (shale gas/fracking); crushed for use as sub-base in road construction.

Geological facts

Shale is the most common sedimentary rock on Earth, making up approximately 70% of the sedimentary rock in the Earth's crust. It is the primary rock type where fossils of soft-bodied organisms are found, like in the Burgess Shale.

Field identification & locations

Identify in the field by its tendency to break along flat, thin planes (fissility). It will not fizz with acid (unlike limestone) and can often be scratched with a fingernail or copper penny. Found globally in basins and mountainous regions.

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