sedimentary

Argillite

Argillite (Fine-grained sedimentary rock composed mostly of clay-sized particles)

Hardness: 3-4 on the Mohs scale. Color: Brownish-red to dark brown. Luster: Dull or earthy. Crystal structure: Microcrystalline, amorphous clay minerals. Cleavage: Poor or none, breaks with a splintery or conchoidal fracture. Specific gravity: 2.5-2.7.

Hardness: 3-4 on the Mohs scale
Color: Brownish-red to dark brown
Luster: Dull or earthy

Identified August 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the compaction and lithification of argillaceous sediments, mainly mud, silt, and clay. Typically deposited in quiet marine, lacustrine, or deltaic environments over millions of years.

Uses & applications

Used in educational geology collections, sometimes carved into ornamental objects or native art (such as Haida argillite carvings), and as a raw material in ceramics.

Geological facts

Argillite is intermediate in terms of metamorphism between shale and slate; it lacks the pronounced slaty cleavage of true slate.

Field identification & locations

Can be identified in the field by its fine-grained texture, earthy appearance, lack of fissility typical of shale, and dull luster. Common worldwide in sedimentary basins.

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Banded Siltstone / Mudstone Pebble

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