sedimentary
Siltstone with iron staining
Siltstone (primarily SiO2 with FeO(OH) coatings)
Hardness: 3-5 on Mohs scale; Color: Grey to brown with dark dendritic or patchy iron/manganese staining; Luster: Dull or earthy; Crystal structure: Microcrystalline/Clastic grains; Cleavage: None, usually breaks in irregular blocks.
- Hardness
- 3-5 on Mohs scale
- Luster
- Dull or earthy
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Physical properties
Hardness: 3-5 on Mohs scale; Color: Grey to brown with dark dendritic or patchy iron/manganese staining; Luster: Dull or earthy; Crystal structure: Microcrystalline/Clastic grains; Cleavage: None, usually breaks in irregular blocks.
Formation & geological history
Formed through the accumulation and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy environments like lake beds or floodplains. The dark markings are likely iron or manganese oxides deposited by groundwater.
Uses & applications
Primarily used as fill material in construction or as road base. Some high-quality siltstone is used for flagstone or interior decorative stone.
Geological facts
Siltstone is intermediate between sandstone and shale. The dark patterns seen on the surface are often mistaken for fossils but are usually mineral precipitations called pseudofossils.
Field identification & locations
Identify in the field by its gritty feel (less smooth than shale) and lack of visible grains (unlike sandstone). Often found in riverbeds or eroded embankments.
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