An argillite ( /ˈɑrdʒɨlaɪt/) is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts of silt-sized particles. The argillites grade into shale when the fissile layering typical of shale is developed. Another name for poorly lithified argillites is mudstone. These rocks, although variable in composition, are typically high in aluminium and silica with variable alkali and alkaline earth cations. The term pelitic or pelite is often applied to these sediments and rocks. Metamorphism of argillites produces slate, phyllite, and pelitic schist.
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The Belt Supergroup of rocks is composed mainly of argillite, as well as other metamorphosed or semi-metamorphosed mudstones. This supergroup is mainly exposed in Western Montana, including the Bitterroot Valley and Bitterroot Mountains, Missoula area, Flathead Lake, and Glacier National Park, and is also exposed in parts of Northern Idaho, and British Columbia. Belt Supergroup rocks are also thought to be present in what is now Korea, Northeast China, and Eastern Siberia as well as possibly Australia. Glacier National Park in northwestern Montana and Wolf Creek Canyon along the US Interstate 15 in west-central Montana are well-known for their exposed deep purple, wine red, red, blue, turquoise, and green argillites.
The Haida carvings of (Haida Gwaii) along the coast of British Columbia are notable aboriginal art treasures created from a type of a hard, fine black silt argillite, sometimes called "black slate". The black slate occurs only at a quarry on a Slatechuck Mountain in the upper basin of Slatechuck Creek, near the town of Skidegate on Graham Island. At one time, around 1900, it was shipped to Victoria for manufacturing; today the Haida have a monopoly on use of the argillite. This artwork has been of high quality and prized around the world since the Haida first began carving it to sell to sailors around 1800; modern Haida carvers continue the tradition.