sedimentary
Argillite
Argillite (compact, clay-rich sedimentary rock)
Hardness of 3-4 on the Mohs scale, grayish-green or dark gray color, dull luster, very fine-grained microcrystalline structure, poor to moderate cleavage, specific gravity around 2.5-2.7
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Physical properties
Hardness of 3-4 on the Mohs scale, grayish-green or dark gray color, dull luster, very fine-grained microcrystalline structure, poor to moderate cleavage, specific gravity around 2.5-2.7
Formation & geological history
Formed through the lithification of mud, clay, and silt over millions of years, typically in quiet marine or lacustrine depositional environments.
Uses & applications
Used historically by indigenous peoples for carving tools and ceremonial items (such as Haida argillite carvings), and locally for decorative landscaping or crushed stone.
Geological facts
Argillite is an intermediate rock type between shale and slate, lacking the strong slaty cleavage of true slate but harder and more cemented than standard shale.
Field identification & locations
Found in sedimentary strata worldwide; can be identified in the field by its fine-grained texture, lack of fissility typical of shale, and relative softness compared to slate.
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