sedimentary
Siltstone
Siltstone (composed of clastic quartz and feldspar)
Hardness: 3-4 on Mohs scale; Color: Reddish-brown to tan; Luster: Dull or earthy; Structure: Clastic, fine-grained; Cleavage: None (conchoidal or irregular fracture); Texture: Slightly gritty to the touch.
- Hardness
- 3-4 on Mohs scale
- Color
- Reddish-brown to tan
- Luster
- Dull or earthy
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Physical properties
Hardness: 3-4 on Mohs scale; Color: Reddish-brown to tan; Luster: Dull or earthy; Structure: Clastic, fine-grained; Cleavage: None (conchoidal or irregular fracture); Texture: Slightly gritty to the touch.
Formation & geological history
Formed from the accumulation and lithification of silt-sized particles (smaller than sand, larger than clay) in low-energy aquatic environments like floodplains, lagoons, or river Deltas over millions of years.
Uses & applications
Primarily used in construction for road fill and foundation material; fine-grained varieties are sometimes used for low-grade ornamental stones or flagstone.
Geological facts
Siltstone is often mistaken for shale, but it lacks the 'fissility' (tendency to split into thin sheets) that defines true shale. It bridges the gap between sandstone and mudstone.
Field identification & locations
Check for a gritty feel on the teeth or fingers compared to mudstone. It is found globally in sedimentary basins. Collectors often look for siltstones that contain fossils or ripple marks.
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Sandstone or Siltstone
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Shale or Slaty Mudstone
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