sedimentary
Siltstone
Siltstone (fine-grained clastic sedimentary rock)
Hardness: 3-5 on Mohs scale; Color: Tan, brown, or grey; Luster: Dull/Earthly; Crystal Structure: Microcrystalline (clastic grains); Cleavage: None; Specific Gravity: 2.6-2.7.
- Hardness
- 3-5 on Mohs scale
- Color
- Tan, brown, or grey
- Luster
- Dull/Earthly
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Physical properties
Hardness: 3-5 on Mohs scale; Color: Tan, brown, or grey; Luster: Dull/Earthly; Crystal Structure: Microcrystalline (clastic grains); Cleavage: None; Specific Gravity: 2.6-2.7.
Formation & geological history
Formed from the accumulation and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy environments like river floodplains, deltas, or lakes. Geological age varies by outcrop location.
Uses & applications
Primarily used as low-grade construction fill, occasional flagstone for landscaping, and for educational geological study.
Geological facts
Siltstone is often mistaken for shale, but it lacks the 'fissility' or the ability to split into thin, flat sheets. It feels slightly grittier against the teeth than claystone due to the silt content.
Field identification & locations
Identify in the field by its fine-grained texture that feels slightly abrasive (like very fine sandpaper) and its lack of visible layers compared to shale. Found globally in sedimentary basins.
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