sedimentary
Siltstone
Siltstone (primarily composed of silica and clay minerals)
Hardness: 3-4 (Mohs scale), Color: Brown, reddish-brown, or grey, Luster: Dull/Earthy, Crystal Structure: Clastic (fine-grained), Cleavage: None/Irregular, Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Brown, reddish-brown, or grey, Luster: Dull/Earthy, Crystal Structure: Clastic (fine-grained), Cleavage: None/Irregular, Specific Gravity: 2.6-2.7
Formation & geological history
Formed by the deposition and lithification of silt-sized particles in low-energy environments like river floodplains, lakes, or deep marine basins. These rocks can range from Precambrian to Holocene in age.
Uses & applications
Used as low-quality construction fill, as a base material for roads, and occasionally for flagstones or tile if well-indurated. Rare varieties are used by collectors for fossil content.
Geological facts
Siltstone is intermediate between sandstone and shale; it has smaller grains than sandstone but lacks the fissility (layer-splitting) of shale. It often preserves ripples and fossil footprints.
Field identification & locations
Identify by its gritty feel on teeth (finer than sandstone) and lack of visible grains to the naked eye. Found in sedimentary basins worldwide. Often occurs in thick bedded sequences.
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