sedimentary

Siltstone with Iron Staining

Siltstone (primarily SiO2 with Fe2O3 staining)

Hardness: 3-4 on Mohs scale; Color: Greyish-brown with reddish-orange iron oxide staining; Luster: Dull/Earthy; Crystal structure: Clastic (fine-grained); Cleavage: None (conchoidal or irregular fracture); Specific gravity: 2.1-2.6

Hardness: 3-4 on Mohs scale
Color: Greyish-brown with reddish-orange iron oxide staining
Luster: Dull/Earthy

Identified June 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation and lithification of silt-sized particles in low-energy aquatic environments like river floodplains or lake beds. The reddish patches are caused by the oxidation of iron minerals within the rock.

Uses & applications

Primarily used as a low-grade construction aggregate, fill material, or occasionally for carving and flagstone if sufficiently durable.

Geological facts

Siltstone is intermediate between sandstone and shale. While it lacks the fissility of shale, it is much finer-grained than sandstone, often feeling slightly gritty against the teeth.

Field identification & locations

Identify in the field by its fine-grained texture that is grittier than claystone but smoother than sandstone. Commonly found in sedimentary basins and along riverbanks. This specimen shows a distinct weathering rind and oxidation.

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Sandstone or Siltstone

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Shale or Slaty Mudstone

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