sedimentary
Siltstone
Siltstone (primarily composed of SiO2 with various clay minerals)
Hardness: 3-4 (Mohs); Color: Brown, gray, or reddish-brown; Luster: Dull or earthy; Crystal structure: Clastic (grain size 1/256 to 1/16 mm); Cleavage: None (non-fissile); Specific Gravity: 2.6-2.7
- Hardness
- 3-4 (Mohs)
- Color
- Brown, gray, or reddish-brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Brown, gray, or reddish-brown; Luster: Dull or earthy; Crystal structure: Clastic (grain size 1/256 to 1/16 mm); Cleavage: None (non-fissile); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation and lithification of silt-sized particles, usually in low-energy environments like lake beds, floodplains, or marine basins. These specimens can range from recent to hundreds of millions of years old.
Uses & applications
Mainly used as fill material in construction, and occasionally for flagstones or decorative landscaping if durable enough.
Geological facts
Siltstone is often confused with shale, but it lacks the 'fissility' (tendency to split into thin layers) that characterizes true shale. It feels grittier to the teeth than claystone but smoother than sandstone.
Field identification & locations
Identify in the field by its fine-grained texture that feels slightly abrasive (sandpaper-like but much finer) compared to mudstone. It is commonly found in sedimentary basins worldwide, often interbedded with sandstone and shale.
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