sedimentary
Greywacke
Lithic Sandstone
Hardness: 6-7 (Mohs); Color: Dark grey to greenish-grey; Luster: Dull or earthy; Structure: Clastic with poorly sorted grains; Cleavage: None (interrupts clean break); Specific Gravity: 2.6–2.7
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey to greenish-grey
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to greenish-grey; Luster: Dull or earthy; Structure: Clastic with poorly sorted grains; Cleavage: None (interrupts clean break); Specific Gravity: 2.6–2.7
Formation & geological history
Formed from turbidity currents in deep marine environments during periods of high tectonic activity. It contains a mix of muddy matrix (more than 15%) and varied grains from different sources.
Uses & applications
Commonly used as crushed stone for road construction, concrete aggregate, railway ballast, and shoreline protection (riprap).
Geological facts
Greywacke is often called 'dirty sandstone' because of its high clay content. It is extremely tough and resists erosion and weathering better than most other sandstones.
Field identification & locations
Identify by its dark gray color and 'salt-and-pepper' appearance. It is very hard and will not scratch easily with a steel blade. Found extensively in mountainous regions like New Zealand, parts of the UK, and the USA coastlines.
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