sedimentary
Siltstone
Siltstone (primarily composed of SiO2)
Hardness: 3-4 on Mohs scale; Color: Grey to brownish-grey; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained); Cleavage: None, usually breaks into irregular fragments.
- Hardness
- 3-4 on Mohs scale
- Color
- Grey to brownish-grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 on Mohs scale; Color: Grey to brownish-grey; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained); Cleavage: None, usually breaks into irregular fragments.
Formation & geological history
Formed from the lithification of silt-sized particles. These particles are typically deposited in low-energy environments like river floodplains, lagoons, or deep ocean basins. Age can range from modern to billions of years old depending on the strata.
Uses & applications
Primarily used as fill material, in the manufacture of cement, or occasionally as low-quality building stone. It is not generally used in jewelry due to its softness and lack of aesthetic appeal.
Geological facts
Siltstone is finer than sandstone but coarser than shale. Unlike shale, it lacks fissility (the ability to split into thin layers easily). It often preserves ripple marks or fossils within its fine layers.
Field identification & locations
Identify in the field by its gritty 'toothy' feel when rubbed against the teeth (unlike clay) and its lack of visible grains to the naked eye (unlike sand). It is common in sedimentary basins worldwide.
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