sedimentary
Graywacke
Graywacke
Hardness: 6-7 on Mohs scale; Color: Gray, greenish-gray, or brown; Luster: Dull/Earthy; Crystal structure: Clastic (sand-sized grains); Cleavage: None (conchoidal to irregular fracture); Specific gravity: 2.6-2.7
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, greenish-gray, or brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, greenish-gray, or brown; Luster: Dull/Earthy; Crystal structure: Clastic (sand-sized grains); Cleavage: None (conchoidal to irregular fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed from the rapid deposition of poorly sorted sediment in deep-marine environments, often via turbidity currents. They are typically found in Paleozoic to Mesozoic orogenic belts.
Uses & applications
Used primarily as aggregate for road construction, concrete, and railroad ballast due to its hardness and durability. Occasionally used for paving stones.
Geological facts
Often called 'dirty sandstone' because of its high clay and volcanic rock fragment content. It is a key indicator of ancient tectonic activity and deep-sea trench environments.
Field identification & locations
Identify by its dark color, high hardness (cannot be easily scratched by a steel knife), and poorly sorted grains (mixture of quartz, feldspar, and dark rock fragments). Common in mountain ranges like the Appalachians or the Alps.
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