sedimentary
Siltstone
Siltstone (composed of microscopic silt-sized particles of quartz and feldspar)
Hardness: 3-4 (Mohs); Color: Gray, tan, or olive-drab; Luster: Dull/Earthy; Crystal structure: Clastic (grain size 0.0039 to 0.063 mm); Specific gravity: 2.3-2.6
- Hardness
- 3-4 (Mohs)
- Color
- Gray, tan, or olive-drab
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Gray, tan, or olive-drab; Luster: Dull/Earthy; Crystal structure: Clastic (grain size 0.0039 to 0.063 mm); Specific gravity: 2.3-2.6
Formation & geological history
Formed in low-energy environments like river floodplains, lagoons, or deep-water basins where fine particles settle. Can date back to almost any geological era from the Precambrian to present.
Uses & applications
Used primarily as low-quality fill material, soil conditioner when crushed, and occasionally as flagstone if it is fissile or well-indurated.
Geological facts
Siltstone is often confused with shale, but it lacks the 'fissility' (tendency to split into thin sheets) characteristic of shale. It represents a transitional grain size between sandstone and mudstone.
Field identification & locations
Identify by its gritty texture on the teeth (compared to smooth claystone) and its lack of visible grains to the naked eye. Often found in layered outcrops near water bodies or ancient lake beds.
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