An argillite is a fine-grained sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

 composed predominantly of indurated clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts of silt

Silt

Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

-sized particles. The argillites grade into shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

 when the fissile

Fissility (geology)

Fissility refers to the property of rocks to split along planes of weakness into thin sheets. This is commonly observed in shales, which are sedimentary rocks, and in slates and phyllites, which are foliated metamorphic rocks. The fissility in these rocks is caused by the preferred alignment of...

 layering typical of shale is developed. Another name for poorly lithified argillites is mudstone

Mudstone

Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

. These rocks, although variable in composition, are typically high in aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 and silica with variable alkali

Alkali

In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...

 and alkaline earth cations. The term pelitic or pelite

Pelite

Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...

 is often applied to these sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s and rocks. Metamorphism

Metamorphism

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

 of argillites produces slate

Slate

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

, phyllite

Phyllite

Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...

, and pelitic schist

Schist

The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...

.

Belt Supergroup

The Belt Supergroup

Belt Supergroup

The Belt Supergroup, is an assemblage of Mesoproterozoic sedimentary rocks which outcrop chiefly in western Montana, but also exposed in Idaho, Washington, Wyoming, and British Columbia. It is most famous as the formation that makes up Glacier National Park in northwest Montana...

 of rocks is composed mainly of argillite, as well as other metamorphosed or semi-metamorphosed mudstones. This supergroup is mainly exposed in Western Montana

Montana

Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

, including the Bitterroot Valley

Bitterroot Valley

The Bitterroot Valley is located in southwestern Montana in the northwestern United States. It extends over 100 miles from remote Horse Creek Pass north to a point near the city of Missoula...

 and Bitterroot Mountains

Bitterroot Mountains

The Northern and Central Bitterroot Range, collectively the Bitterroot Mountains, is the largest portion of the Bitterroot Range, part of the Rocky Mountains, located in the panhandle of Idaho and westernmost Montana in the Western United States...

, Missoula area, Flathead Lake

Flathead Lake

Flathead Lake is the largest natural freshwater lake in the western part of the contiguous United States. With a surface area of between and , it is slightly larger than Lake Tahoe. The lake is a remnant of the ancient inland sea, Lake Missoula of the era of the last interglacial. Flathead Lake...

, and Glacier National Park

Glacier National Park

Glacier National Park may refer to:*Glacier National Park in British Columbia, Canada*Glacier National Park in Montana, USA-See also:*Glacier Bay National Park, in Alaska, USA*Los Glaciares National Park, in Argentina...

, and is also exposed in parts of Northern Idaho

Idaho

Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....

, and British Columbia

British Columbia

British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

. Belt Supergroup rocks are also thought to be present in what is now Korea

Korea

Korea ) is an East Asian geographic region that is currently divided into two separate sovereign states — North Korea and South Korea. Located on the Korean Peninsula, Korea is bordered by the People's Republic of China to the northwest, Russia to the northeast, and is separated from Japan to the...

, Manchuria

Manchuria

Manchuria is a historical name given to a large geographic region in northeast Asia. Depending on the definition of its extent, Manchuria usually falls entirely within the People's Republic of China, or is sometimes divided between China and Russia. The region is commonly referred to as Northeast...

, and Eastern Siberia

Siberia

Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 as well as possibly Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. 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.

"Black slate"

The Haida carving

Northwest Coast art

Northwest Coast art is the term commonly applied to a style of art created primarily by artists from Tlingit, Haida, Tsimshian, Kwakwaka'wakw, Nuu-chah-nulth and other First Nations and Native American tribes of the Northwest Coast of North America, from pre-European-contact times up to the...

s of (Haida Gwaii) along the coast of British Columbia

British Columbia

British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

 are notable aboriginal art treasures created from a type of a hard, fine black silt argillite, sometimes called "black slate

Slate

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

". The black slate occurs only at a quarry on a Slatechuck Mountain

Slatechuck Mountain

Slatechuck Mountain, known as Kaagan in the Haida language, is a summit on south-central Graham Island in the Queen Charlotte Islands of the North Coast of British Columbia, Canada...

 in the upper basin of Slatechuck Creek

Slatechuck Creek

Slatechuck Creek is a short but culturally important stream on Graham Island in the Queen Charlotte Islands of the North Coast of British Columbia, Canada. Only 6.7 km in length , it is located on the south flank of the small range including Slatechuck Mountain, to the north of which is...

, near the town of Skidegate on Graham Island

Graham Island

Graham Island is the largest island in the Haida Gwaii archipelago , lying off the coast of British Columbia, Canada. It is separated only by a narrow channel from the other principal island of the group, Moresby Island Graham Island is the largest island in the Haida Gwaii archipelago (formerly...

. 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.