sedimentary
Greywacke
Greywacke (Lithofeldspathic Sandstone)
Hardness: 6-7 (Mohs scale); Color: Dark gray, green, or blackish; Luster: Dull/Earthy; Crystal Structure: Clastic (composed of poorly sorted sand-sized fragments in a clay matrix); Cleavage: None (fractures roughly); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark gray, green, or blackish
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark gray, green, or blackish; Luster: Dull/Earthy; Crystal Structure: Clastic (composed of poorly sorted sand-sized fragments in a clay matrix); Cleavage: None (fractures roughly); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through rapid deposition in deep-sea environments (turbidity currents) near active continental margins or island arcs. Typically found in Paleozoic or Mesozoic era formations where tectonic activity was high.
Uses & applications
Primary use is as crushed stone for road construction, concrete aggregate, and railroad ballast. Due to its durability, it is occasionally used for architectural facings or paving stones.
Geological facts
Greywacke is often called 'dirty sandstone' because of its high clay and silt content (matrix). It is a key indicator for geologists to identify ancient subduction zones or deep-water marine trenches.
Field identification & locations
Identify in the field by its dark color, 'salt and pepper' appearance of grains, and extreme hardness compared to many other sandstones. It will not scratch easily with a knife. Found commonly in the Appalachian Mountains, New Zealand, and parts of the UK.
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