Where is Argillite found?
Argillite
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Open the appArgillite occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Argillite
Haida Gwaii (Queen Charlotte Islands), British Columbia, Canada
Famous for the 'black argillite' used by the Haida people for carving. This argillite is a specific type, often referred to as 'Haida Gwaii Argillite' or 'Skidegate Argillite', known for its fine grain and uniform black color.
Appalachian Mountains, Eastern North America
Extensive occurrences of argillite are found in various formations throughout the Appalachian orogenic belt, representing metamorphosed shales and mudstones.
Wales, United Kingdom
Regions known for slate production often have associated argillite layers, representing less metamorphosed portions of the same protoliths.
Australia
Found in various sedimentary basins that have experienced low-grade metamorphism, particularly in older geological terrains.
Various Orogenic Belts Worldwide
Commonly found in mountain ranges and areas that have undergone significant tectonic deformation and associated low-grade metamorphism of sedimentary sequences.
Collecting tips
- Look for Transitional Zones. Argillite often occurs in geological transitions between unmetamorphosed shales/mudstones and true slates. Look for areas where sedimentary rocks show increasing induration and a subtle change in fracture pattern.
- Examine Outcrops in Tectonically Active Areas. Regions that have experienced mountain building (orogeny) are prime locations for finding argillite, as these processes provide the necessary pressure and temperature for its formation.
- Check Road Cuts and River Banks. Freshly exposed surfaces in road cuts, quarries, and river banks can reveal the characteristic texture and fracture of argillite. Look for fine-grained, dark rocks that are harder than shale but lack the perfect cleavage of slate.
- Distinguish from Shale and Slate. Carry a hand lens and a small hammer. Shale will be softer and more easily broken along bedding planes. Slate will exhibit a very strong, planar cleavage. Argillite will be harder than shale but lack the distinct cleavage of slate, often breaking into blocky fragments.
Quick Check
- Color:
- Variable (dark gray, black, green, red, purple)
- Luster:
- Dull to earthy, sometimes slightly waxy
- Streak:
- White, gray, or color of the rock (if dark)
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