sedimentary
Siltstone
Siltstone (primarily composed of quartz and clay minerals)
Hardness: 3-4 on Mohs scale; Color: Tan to light brown; Luster: Dull/Earthy; Crystal Structure: Clastic (grain size between sandstone and shale); Specific Gravity: 2.1-2.6
- Hardness
- 3-4 on Mohs scale
- Color
- Tan to light brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 on Mohs scale; Color: Tan to light brown; Luster: Dull/Earthy; Crystal Structure: Clastic (grain size between sandstone and shale); Specific Gravity: 2.1-2.6
Formation & geological history
Formed through the accumulation and lithification of silt-sized particles in low-energy environments like river floodplains, lakes, or marine basins. These often date back millions of years depending on the stratigraphic layer.
Uses & applications
Used primarily as low-quality fill material in construction or as a source of soil when weathered. Sometimes used as a flagstone if thin-bedded.
Geological facts
Siltstone is distinct from shale because it lacks fissility (the ability to split into thin layers). It feels slightly gritty against the teeth, whereas shale feels smooth.
Field identification & locations
Identify in the field by its fine-grained texture that is grittier than clay but smoother than sand. It is commonly found in sedimentary basins worldwide.
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