sedimentary
Graywacke
Lithic Sandstone (Graywacke)
Hardness: 6-7 (Mohs scale); Color: Gray to dark greenish-gray; Luster: Dull to earthy; Crystal structure: Clastic (poorly sorted mineral grains such as quartz, feldspars, and rock fragments in a clay matrix); Cleavage: None; Specific gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray to dark greenish-gray
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray to dark greenish-gray; Luster: Dull to earthy; Crystal structure: Clastic (poorly sorted mineral grains such as quartz, feldspars, and rock fragments in a clay matrix); Cleavage: None; Specific gravity: 2.6-2.7
Formation & geological history
Formed by rapid deposition of sediments in deep marine environments via turbidity currents (underwater landslides). Typical of geosynclinal accumulation, often dating back to the Paleozoic or Precambrian eras.
Uses & applications
Primarily used in construction as crushed stone for road base, concrete aggregate, and railway ballast. Due to its durability, it is also used for paving stones.
Geological facts
Graywacke is often called 'dirty sandstone' because of its poorly sorted nature and high clay content. It is a key indicator of ancient active tectonic plate margins.
Field identification & locations
Identify in the field by its dark, gritty appearance, extreme hardness compared to other sandstones, and the presence of angular rock fragments. It is commonly found in mountainous regions or ancient folded mountain belts.
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