sedimentary
Greywhacke (River Pebble)
Graywacke (immature sandstone)
Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-grey; Luster: Dull/Earthy; Crystal structure: Clastic (fine to coarse grains); Cleavage: None; Specific gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to greenish-grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-grey; Luster: Dull/Earthy; Crystal structure: Clastic (fine to coarse grains); Cleavage: None; Specific gravity: 2.6-2.7
Formation & geological history
Formed from the rapid deposition of poorly sorted sediment in deep ocean trenches or near active mountain belts. Usually contains a mix of quartz, feldspar, and rock fragments in a clay-rich matrix.
Uses & applications
Commonly used as crushed stone for road construction, railroad ballast, and high-quality concrete aggregate due to its hardness and durability.
Geological facts
Graywacke is often referred to as 'dirty sandstone' because of its poorly sorted nature and high clay content. They are significant in understanding tectonic history and deep-sea turbidity currents.
Field identification & locations
Identify by its dark color, intense hardness (it will usually scratch glass), and fine-grained, 'gritty' texture. These pebbles are extremely common in riverbeds and on beaches in tectonically active areas.
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