What does argillite mean?
Argillite is a fine-grained sedimentary rock composed predominantly of clay minerals and silt-sized particles that have been compacted and hardened without undergoing metamorphism. It occupies a middle ground in the clay-rock family: harder and more consolidated than shale or mudstone, yet lacking the perfect cleavage and sheen that characterize slate, its metamorphosed counterpart. Geologists encounter argillite in layered sequences worldwide, where it commonly forms durable beds that resist erosion and split along bedding planes. The word also carries a distinctive cultural meaning: the Haida people of the Pacific Northwest carve monumental sculptures, pipes, and poles from a rare, lustrous black argillite quarried from a single mountain source in British Columbia. Because it bridges sedimentary and low-grade metamorphic categories, argillite is a useful term for anyone studying rock classification, and its role in Indigenous Northwest Coast art gives it a significance well beyond geology.
nounA fine-grained sedimentary rock composed largely of clay minerals and silt, intermediate in character between shale and slate but not yet metamorphosed into true slate.
- A hard, fine-grained sedimentary rock composed mainly of clay-sized particles, intermediate between shale and slate.
- (In Haida art) A dense black argillite from Slatechuck Creek, British Columbia, traditionally used for carving pipes, poles, and sculpture.
"The quarry's argillite beds split cleanly along their bedding planes, making them ideal for flagstones."
"Geologists identified the layer as argillite rather than slate because it showed no evidence of metamorphic cleavage."
"The museum's collection includes a nineteenth-century Haida pipe carved from black argillite."
The plural is used mainly in geological writing when referring to multiple occurrences, beds, or varieties of the rock.
"The region's argillites vary in colour from dark grey to greenish black."
The Haida of the Pacific Northwest carved their famous totemic pipes and sculptures from a single deposit of black argillite on Slatechuck Mountain — a stone found almost nowhere else on Earth.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of argillite
Argillite derives from the Latin word argilla, meaning 'clay' or 'white clay', combined with the suffix -ite used for rocks and minerals. The same Latin root gave rise to related English terms such as argil (a poetic word for clay) and argillaceous (clay-bearing). The term entered English geological usage in the late eighteenth century as mineralogists sought precise names for clay-based rocks.
How argillite is actually used
Primarily a technical term used in geology, petrology, archaeology, and art history; it rarely appears in everyday speech. In North American contexts it often refers specifically to the black Haida carving stone of British Columbia.
Easily confused with argillite
Slate is a metamorphic rock formed when argillite or shale undergoes low-grade metamorphism, giving it the perfect cleavage that unaltered argillite lacks.
Shale is a softer, more fissile clay-rich rock that splits readily into thin layers, whereas argillite is harder and more indurated without being metamorphosed.