sedimentary
Red Siltstone
Siltstone (composed mainly of SiO2 and Fe2O3)
Hardness: 3-4 (variable), Color: Reddish-brown to maroon, Luster: Dull/Earthy, Structure: Fine-grained clastic, Cleavage: None (prone to fissility or splitting along bedding planes), Specific Gravity: 2.6
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Physical properties
Hardness: 3-4 (variable), Color: Reddish-brown to maroon, Luster: Dull/Earthy, Structure: Fine-grained clastic, Cleavage: None (prone to fissility or splitting along bedding planes), Specific Gravity: 2.6
Formation & geological history
Formed through the accumulation and cementation of silt-sized particles in low-energy environments like floodplains or river deltas. Its red color indicates an oxygen-rich environment where iron minerals oxidized during lithification.
Uses & applications
Used primarily in construction as flagstone or fill material. Occasionally used for low-cost landscaping rocks or as a source of inorganic pigments if the iron content is high enough.
Geological facts
The red color is caused by minute amounts of hematite (iron oxide) coating the grains. It occupies the middle ground between sandstone (coarser) and shale (finer) in the classification of sedimentary rocks.
Field identification & locations
Identify in the field by its gritty texture (finer than sandpaper but coarser than mudstone) and distinct layers. Commonly found in the Colorado Plateau and Appalachian regions. Soft enough to be scratched by a steel nail.
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