sedimentary
Graywacke
Graywacke Sandstone (C6H10O5)x - predominantly quartz and feldspar
Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal; Luster: Dull/Earthy; Crystal Structure: Clastic (fine to medium grained); Cleavage: None (subconchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to charcoal
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal; Luster: Dull/Earthy; Crystal Structure: Clastic (fine to medium grained); Cleavage: None (subconchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the rapid deposition of poorly sorted sediment in deep marine environments, often associated with submarine avalanches or turbidity currents. Most specimens date from the Paleozoic or Mesozoic eras.
Uses & applications
Primarily used as high-quality road aggregate, railway ballast, and in heavy construction due to its hardness and durability. Occasionally used for decorative stone walls.
Geological facts
Graywacke is often called 'dirty sandstone' because of its high clay matrix content. It is a key indicator of ancient tectonic 'active margins' where mountains were rapidly eroding into deep ocean trenches.
Field identification & locations
Identify by its dark, muddy appearance and extreme hardness compared to other sandstones. It will not scratch easily with a steel knife. Commonly found in mountainous regions near old coastlines.
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