sedimentary

Siltstone

Siltstone (composed of microscopic silt-sized particles of quartz and feldspar)

AI-generated identification · may be incorrect

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

Identified April 9, 2026More sedimentary →

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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.