sedimentary
Banded Siltstone or Argillite
Siltstone (primarily SiO2 with Al and Fe oxides)
Hardness: 3-6 Mohs. Color: Gray with lighter horizontal bands. Luster: Dull/Earthy. Structure: Clastic, fine-grained. Cleavage: Poor to none, often splits along bedding planes. SG: 2.6-2.7.
- Hardness
- 3-6 Mohs
- Color
- Gray with lighter horizontal bands
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-6 Mohs. Color: Gray with lighter horizontal bands. Luster: Dull/Earthy. Structure: Clastic, fine-grained. Cleavage: Poor to none, often splits along bedding planes. SG: 2.6-2.7.
Formation & geological history
Formed from the lithification of silt-sized particles deposited in low-energy aquatic environments like lake beds or deep ocean floors. These layers represent seasonal or cyclic depositional events.
Uses & applications
Used primarily in construction aggregate, landscaping pebbles, and occasionally in decorative lapidary work or for making whetstones.
Geological facts
The distinct bands are called 'varves' if they represent annual cycles. Rock like this is common in glacial lake deposits and is a record of ancient water movement and climate cycles.
Field identification & locations
Identify by its very fine-grained texture (smoother than sandstone) and visible horizontal layering or bedding. Common in riverbeds and coastal regions with ancient sedimentary basins.
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