sedimentary
Hardened Gray Mudstone
Argillite / Silicified Mudstone (principally SiO2 and Al2Si2O5(OH)4)
Hardness: 5-6 (Mohs); Color: Slate gray to charcoal; Luster: Dull to earthy; Structure: Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: 2.5-2.7
- Hardness
- 5-6 (Mohs)
- Color
- Slate gray to charcoal
- Luster
- Dull to earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Slate gray to charcoal; Luster: Dull to earthy; Structure: Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: 2.5-2.7
Formation & geological history
Formed from the lithification of clay and silt particles over millions of years. This specimen shows significant rounding from water transport, likely in a river or glacial environment.
Uses & applications
Used as aggregate in construction, garden landscaping (river rock), and occasionally as a base material for road beds. Fine-grained variants are historically used for carving.
Geological facts
Unlike shale, mudstone does not split into thin layers (fissility). It represents quiet-water environments like lake bottoms or deep marine basins where fine particles can settle.
Field identification & locations
Identify by its smooth, uniform fine-grained texture and lack of visible crystals. Commonly found in riverbeds, glacial tills, and coastal areas.
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