sedimentary
Siltstone
Siltstone (composed of clastic quartz and feldspar grains)
Hardness: 3-4 (Mohs scale); Color: Brown, reddish-brown; Luster: Dull, earthy; Crystal structure: Amorphous/fine-grained; Cleavage: None; Specific gravity: 2.3-2.6
- Hardness
- 3-4 (Mohs scale)
- Color
- Brown, reddish-brown
- Luster
- Dull, earthy
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Brown, reddish-brown; Luster: Dull, earthy; Crystal structure: Amorphous/fine-grained; Cleavage: None; Specific gravity: 2.3-2.6
Formation & geological history
Formed by the lithification (compaction and cementation) of silt-sized particles. These particles were likely deposited by water, wind, or ice in low-energy environments like river floodplains or lake beds.
Uses & applications
Used as a base material in road construction, as fill material, and occasionally in the manufacture of cement. Fine-grained varieties are sometimes used for sharpening stones.
Geological facts
Siltstone is distinguished from sandstone by its smaller grain size and from shale by its lack of fissility (the ability to split into thin layers). It often feels slightly gritty when rubbed against the teeth.
Field identification & locations
Identified in the field by its fine-grained texture that is grittier than clay but smoother than sand. Commonly found in sedimentary basins and exposed cliff faces. Collectors should look for preserved ripple marks or fossils within the layers.
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