sedimentary
Siltstone
Siltstone (primarily SiO2 with clay minerals)
Hardness: 3-4 (Mohs scale); Color: Brown, grey, yellowish; Luster: Dull/Earthy; Structure: Fine-grained clastic; Specific Gravity: 2.1-2.6
- Hardness
- 3-4 (Mohs scale)
- Color
- Brown, grey, yellowish
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Brown, grey, yellowish; Luster: Dull/Earthy; Structure: Fine-grained clastic; Specific Gravity: 2.1-2.6
Formation & geological history
Formed from the lithification of silt-sized particles deposited in low-energy environments like river floodplains, lakes, or marine basins. These deposits are typically millions of years old.
Uses & applications
Used primarily as low-grade construction fill, occasionally as flagstone or decorative garden stones, and an important study material for paleontology as it often contains fossils.
Geological facts
Siltstone is intermediate between sandstone and shale. Unlike shale, it lacks the 'fissility' or tendency to split into thin layers, and unlike sandstone, its grains are too small to be seen easily with the naked eye.
Field identification & locations
Identify by its gritty feel when rubbed against teeth (unlike smooth claystone) and its lack of visible grains. Common in sedimentary basins worldwide. Often found in outcrops along riverbanks.
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Arenite (if sandstone)
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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