sedimentary
Limonite-coated Shale
Ferruginous Siliciclastic Shale
Hardness: 3-5 (shale) with 4-5.5 (limonite coating); Color: Dark grey to black with reddish-brown to mustard-yellow oxidation; Luster: Dull to earthy; Structure: Fissile, platy cleavage.
- Hardness
- 3-5 (shale) with 4-5
- Luster
- Dull to earthy
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Physical properties
Hardness: 3-5 (shale) with 4-5.5 (limonite coating); Color: Dark grey to black with reddish-brown to mustard-yellow oxidation; Luster: Dull to earthy; Structure: Fissile, platy cleavage.
Formation & geological history
Formed through the lithification of clay and silt particles in low-energy environments like lake beds or deep ocean basins. The reddish-brown coating is Limonite, formed by the oxidation of iron-bearing minerals (like pyrite) within the rock.
Uses & applications
Shale is used in the production of bricks, cement, and as a source of natural gas (shale gas). Specimens with distinct iron staining are prized by amateur collectors for their color contrast.
Geological facts
Shale is the most common sedimentary rock on Earth, making up about 70 percent of the rocks in the Earth's crust. The 'rust' seen here is actually a mixture of hydrated iron oxides.
Field identification & locations
Identified by its ability to split into thin flat pieces (fissility) and the characteristic earthy smell when wet. Found in sedimentary basins worldwide. Look for flat, crumbly outcrops in road cuts.
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