sedimentary
Siltstone
Siltstone (primarily composed of SiO2, Al2O3, and phyllosilicates)
Hardness: 3-4 (Mohs); Color: Grey, brown, or tan; Luster: Dull or earthy; Structure: Clastic, fine-grained; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 2.1–2.6
- Hardness
- 3-4 (Mohs)
- Color
- Grey, brown, or tan
- Luster
- Dull or earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Grey, brown, or tan; Luster: Dull or earthy; Structure: Clastic, fine-grained; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 2.1–2.6
Formation & geological history
Siltstones form in low-energy depositional environments such as river floodplains, lake bottoms, or offshore marine shelves where fine-grained silt particles settle and undergo lithification. They can range from the Archean to Holocene in age.
Uses & applications
Used primarily in the construction industry as fill material, road base, and occasionally as dimension stone for flagstone or interior tiles if sufficiently hard.
Geological facts
Siltstone is distinct from shale because it lacks the property of fissility (the ability to split into thin layers). It is often found containing well-preserved trace fossils or small plant impressions.
Field identification & locations
Identify in the field by its gritty feel when rubbed against the teeth (unlike smooth claystone) and its lack of visible grains to the naked eye. Look for it in layered sedimentary sequences globally, often between sandstone and shale layers.
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