Argillite
Indurated clay and silt (aluminosilicates)
Hardened, fine-grained mudrock intermediate between shale and slate, dense and non-fissile, often carved into ornaments.
- Mohs hardness
- 3-4
- Color
- Grey, green, red, brown, to black
- Type
- sedimentary
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Overview
Argillite is a compact, fine-grained sedimentary rock made mostly of consolidated clay and silt. It is harder and more thoroughly indurated than shale or mudstone but lacks the well-developed cleavage of slate, placing it in an intermediate position between unmetamorphosed mudrocks and true slate.
Argillite is dense and breaks with a blocky or splintery fracture rather than splitting into thin sheets. It comes in grey, green, red, brown, and black hues. Some argillites, such as the famous black argillite of Haida Gwaii, are prized for carving.
Formation & geology
Argillite forms from the burial and compaction of mud, the same fine clay and silt that produces shale and mudstone. With deeper burial, increased pressure, and slight heating, the sediment becomes more thoroughly cemented and hardened, expelling water and tightening its fabric.
This low-grade alteration sits at the boundary of diagenesis and very low-grade metamorphism. Unlike slate, argillite has not experienced enough directed pressure to develop slaty cleavage, so it stays massive and non-fissile. Many argillites formed in deep marine basins, deltas, and lakes where fine muds accumulated slowly over long periods.
How to identify it
Argillite is fine-grained, dull, and compact, breaking into blocky or splintery pieces rather than thin sheets. It is harder and tougher than shale (about 3-4) but can still be scratched by a knife. It usually does not fizz in acid unless calcareous.
Colors include grey, green, red, brown, and black. Streak is similar to the body color. It feels smooth and may show faint bedding.
Key distinctions: shale splits readily into thin layers and is softer and more crumbly; slate is harder, rings when struck, and cleaves into flat sheets. Argillite's lack of both fissility and slaty cleavage, combined with its toughness, is diagnostic.
Uses & significance
Argillite is used as a carving and ornamental stone, most famously the black argillite carved by Haida artists of the Pacific Northwest into sculptures, totems, and jewelry. Its fine, even texture takes detail and polish well.
It is also used locally as crushed aggregate, fill, and rough building stone. Some red and green argillites appear as decorative landscaping and dimension stone. Geologically, argillite layers are useful markers in stratigraphy and can host or seal mineral and hydrocarbon deposits.
Frequently asked questions
Is argillite a sedimentary or metamorphic rock?
It is essentially a hardened sedimentary mudrock at the boundary of very low-grade metamorphism, harder than shale but lacking slate's cleavage.
How is argillite different from shale?
Argillite is harder, denser, and breaks into blocky pieces, while shale is softer and splits easily into thin layers.
How is argillite different from slate?
Slate has well-developed cleavage and splits into flat sheets; argillite is massive and non-fissile.
What is argillite carved into?
It is famously carved into Haida sculptures and jewelry, as well as other ornaments, because it is fine-grained and takes polish well.
Argillite guides
In-depth guides for identifying, valuing, and understanding Argillite.
Argillite identified by the community
Real specimens identified with Rock Identifier.
