sedimentary
Siltstone or Mudstone
Argillaceous sedimentary rock
Hardness: 3-4 (Mohs); Color: Reddish-brown to tan/grey; Luster: Dull or earthy; Crystal structure: Clastic (microscopic grains); Cleavage: None (may exhibit bedding planes); Specific gravity: 2.3-2.6
- Hardness
- 3-4 (Mohs)
- Color
- Reddish-brown to tan/grey
- Luster
- Dull or earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Reddish-brown to tan/grey; Luster: Dull or earthy; Crystal structure: Clastic (microscopic grains); Cleavage: None (may exhibit bedding planes); Specific gravity: 2.3-2.6
Formation & geological history
Formed through the lithification of fine-grained silt or clay deposited in low-energy environments like river floodplains, lagoons, or deep sea beds. This specimen appears water-worn, likely from a river or beach environment.
Uses & applications
Generally used as fill material or in the manufacture of cement and bricks; less frequently used as a minor decorative stone if it exhibits unique coloration.
Geological facts
Siltstone is intermediate between sandstone and shale. While it lacks the distinct fissility (layering) of shale, it is much smoother to the touch than sandstone because the grains are too small to be seen without a lens.
Field identification & locations
Identify by its fine-grained texture and dull appearance. If you rub it against your teeth, siltstone will feel gritty, whereas clay-heavy mudstone will feel smooth. Commonly found in sedimentary basins worldwide.
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