sedimentary
Shale (with Iron Staining)
Argillaceous mudstone / Shale
Hardness: 3 (Mohs Scale); Color: Grey with yellow-orange limonite/hematite staining; Luster: Dull/Earthy; Crystal structure: Non-visible fine grains; Cleavage: Fissile (splits along flat planes)
- Hardness
- 3 (Mohs Scale)
- Color
- Grey with yellow-orange limonite/hematite staining
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3 (Mohs Scale); Color: Grey with yellow-orange limonite/hematite staining; Luster: Dull/Earthy; Crystal structure: Non-visible fine grains; Cleavage: Fissile (splits along flat planes)
Formation & geological history
Formed by the compaction of silt and clay-size mineral particles. The specimen shows evidence of 'slaty cleavage' or fissility typical of older sedimentary basins. The reddish staining indicates secondary iron oxide precipitation from groundwater.
Uses & applications
Used in the manufacture of tile, brick, and cement. Historically significant as a source rock for hydrocarbons (shale gas/oil), though primarily a common building material or fill rock.
Geological facts
Shale is the most common sedimentary rock in the Earth's crust. This specific specimen was found in the Bolivian Andes region (Cochabamba), known for its thick sequences of Devonian and Ordovician marine sedimentary rocks.
Field identification & locations
Identified in the field by its layered appearance, soft texture compared to silicate rocks, and tendency to break into thin plates. Very common in the Eastern Cordillera of Bolivia. Look for fossils like trilobites or brachiopods in similar layers.
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