sedimentary
Graywacke with Quartz Veins
Argillaceous Sandstone (Graywacke)
Hardness: 6-7 (Mohs); Color: Dark grey with white veins; Luster: Dull to earthy (matrix), vitreous (veins); Crystal structure: Clastic/microcrystalline; Cleavage: None/Conchoidal.
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey with white veins
- Luster
- Dull to earthy (matrix), vitreous (veins)
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey with white veins; Luster: Dull to earthy (matrix), vitreous (veins); Crystal structure: Clastic/microcrystalline; Cleavage: None/Conchoidal.
Formation & geological history
Formed in deep-water marine environments as turbidity currents (submarine landslides) deposited sand and silt. The white veins formed later when mineral-rich fluids (silica) filled fractures under tectonic pressure.
Uses & applications
Used primarily as high-quality aggregate for road construction, railroad ballast, and concrete due to its hardness and durability. Rarely used for decorative purposes.
Geological facts
Graywacke is often called 'dirty sandstone' because it contains a mix of poorly sorted grains and fine clay minerals. The white veins are actually 'fossilized' cracks that show the geological stress the rock underwent millions of years ago.
Field identification & locations
Identify by its dark, fine-grained matrix and the presence of hard, white crystalline veins that do not scratch with a knife. Found commonly in mountainous regions or ancient coastal orogenic belts.
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