sedimentary
Siltstone
Siltstone (primarily SiO2 with Al2O3)
Hardness: 3-5 (Mohs scale); Color: Gray, brown, or buff; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None/Planar bedding; Specific gravity: 2.3-2.6
- Hardness
- 3-5 (Mohs scale)
- Color
- Gray, brown, or buff
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-5 (Mohs scale); Color: Gray, brown, or buff; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None/Planar bedding; Specific gravity: 2.3-2.6
Formation & geological history
Formed through the lithification of silt-sized particles deposited in low-energy environments like river floodplains, lagoons, or deep-sea basins. Most specimens date from the Paleozoic to Cenozoic eras.
Uses & applications
Used primarily as low-quality construction fill, road base, and occasionally as flagstone for landscaping or interior flooring if specifically durable.
Geological facts
Siltstone is intermediate between sandstone and shale; it has smaller grains than sandstone but lacks the fine lamination and fissility of shale.
Field identification & locations
Identify by its gritty feel on teeth (the 'grit test') despite appearing smooth. Found in sedimentary basins worldwide. Often contains small trace fossils or ripple marks.
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