sedimentary
Siltstone or Mudstone
Siltstone (composed primarily of silt-sized particles)
Hardness: 3-4 (Mohs scale); Color: Brown, tan, or grey; Luster: Dull/Earthy; Structure: Fine-grained, often exhibiting thin layering; Cleavage: None, usually breaks with uneven or sub-conchoidal fracture
- Hardness
- 3-4 (Mohs scale)
- Color
- Brown, tan, or grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Brown, tan, or grey; Luster: Dull/Earthy; Structure: Fine-grained, often exhibiting thin layering; Cleavage: None, usually breaks with uneven or sub-conchoidal fracture
Formation & geological history
Formed from the accumulation of fine-grained particles of clay and silt in low-energy environments like river floodplains, lakes, or deep marine basins. These sediments are lithified over millions of years through compaction and cementation.
Uses & applications
Used primarily as fill material in construction, in the manufacture of cement, and occasionally as decorative landscaping stones or stepping stones.
Geological facts
Siltstone is often mistaken for shale, but it lacks the 'fissility' or tendency to split into thin, flat sheets. It represents a transition between the coarser sandstone and the finer claystone.
Field identification & locations
Identified in the field by its gritty feel on the teeth (if licked) compared to smooth claystone. It is common in sedimentary basins worldwide. For collectors, it is a basic sedimentary reference specimen.
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