sedimentary
Siltstone
Siltstone (siliciclastic sedimentary rock)
Hardness: 3-4 (Mohs), Color: Reddish-brown to grey, Luster: Dull/Earthy, Grain size: Very fine, Crystal structure: Not visible to the naked eye, Cleavage: None/Planar bedding commonly observed
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Physical properties
Hardness: 3-4 (Mohs), Color: Reddish-brown to grey, Luster: Dull/Earthy, Grain size: Very fine, Crystal structure: Not visible to the naked eye, Cleavage: None/Planar bedding commonly observed
Formation & geological history
Formed through the accumulation and lithification of silt-sized particles in quiet water environments like river floodplains, lakes, or deep marine basins. The reddish hue usually indicates the presence of iron oxides.
Uses & applications
Primarily used in construction for fill material or as a building stone for interior decorative purposes. Sometimes used for low-cost landscaping stones or flagstones.
Geological facts
Siltstone is intermediate between sandstone and shale. While it feels gritty like sandstone, the grains are too small to be seen individually without magnification.
Field identification & locations
Identify in the field by its fine-grained texture that feels slightly gritty on the teeth (unlike smooth clay-rich shale). Often found in areas with ancient alluvial or deltaic deposits.
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