sedimentary
Siltstone with Weathering Crust
Siltstone (primarily SiO2 with Al2O3)
Hardness: 3-4 (Mohs scale), Color: grayish-green with a white/orange weathered rind, Luster: dull to earthy, Crystal Structure: clastic (microscopic), Cleavage: none, Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 3-4 (Mohs scale), Color: grayish-green with a white/orange weathered rind, Luster: dull to earthy, Crystal Structure: clastic (microscopic), Cleavage: none, Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the lithification of silt-sized particles deposited in low-energy environments like lake beds or floodplains. The orange staining is due to iron oxidation during secondary weathering.
Uses & applications
Used primarily as fill material in construction, as a source for soil when weathered, and occasionally as minor ornamental stone if patterned.
Geological facts
Siltstones are finer-grained than sandstone but coarser than shale. This specific piece shows a distinct 'weathering rind' where the outer layer has chemically reacted with the environment.
Field identification & locations
Identify by its gritty feel on teeth but smoother than sandstone. Often found in layered beds or as river cobbles. Look for the distinct lack of visible grains and earthy smell when wet.
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