sedimentary
Siltstone with Weathering Rind
Siltstone (composed of microscopic quartz and clay minerals)
Hardness: 3-4 on Mohs scale; Color: Dark grey exterior with tan/yellowish weathered core; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained); Cleavage: None
- Hardness
- 3-4 on Mohs scale
- Color
- Dark grey exterior with tan/yellowish weathered core
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 on Mohs scale; Color: Dark grey exterior with tan/yellowish weathered core; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained); Cleavage: None
Formation & geological history
Formed from the accumulation and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy environments like river floodplains or lake beds. The tan core represents a weathering rind where minerals have oxidized.
Uses & applications
Commonly used as fill material in construction, as a soil conditioner when crushed, or as flagstone if it splits into flat plates.
Geological facts
Siltstone is intermediate between sandstone and shale. When it is extremely fine-grained and fissile, it transitions into shale; when more massive and coarse, it becomes sandstone.
Field identification & locations
Identify in the field by its gritty 'toothy' feel on the teeth (unlike smooth clay/shale) and lack of visible individual sand grains. Often found in layered outcrop sequences.
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