Clay

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For other uses, see Clay (disambiguation).

Clay is a term used to describe a group of hydrous aluminium phyllosilicate (phyllosilicates being a subgroup of silicate minerals) minerals (see clay minerals), that are typically less than 2 μm (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and tendency toward high plasticity.

[edit] Definition

[edit] Grouping

Depending upon academic source, there are three or four main groups of clays: kaolinite, montmorillonite-smectite, illite, and chlorite (the latter group is not always considered a part of the clays and is sometimes classified as a separate group within the phyllosilicates). There are about thirty different types of "pure" clays in these categories but most "natural" clays are mixtures of these different types, along with other weathered minerals.

[edit] Some varieties

Montmorillonite, with a chemical formula of (Na,Ca)_0.33(Al,Mg)₂Si₄O₁₀(OH)₂·nH₂O, is typically formed as a weathering product of low silica rocks. Montmorillonite is a member of the smectite group and a major component of bentonite. This is also commonly called as nanoclay, and is an important product group in the emerging nanomaterials market.

Varve (or varved clay) is clay with visible annual layers, formed by seasonal differences in erosion and organic content. This type of deposit is common in former glacial lakes left over from an ice age.

Quick clay is a unique type of marine clay, indigenous to the glaciated terrains of Norway, Canada, and Sweden. It is a highly sensitive clay, prone to liquefaction which has been involved in several deadly landslides.

[edit] Historical and modern uses

Many clays exhibit plasticity when mixed with water. When dry, it becomes firm and when fired in a kiln, permanent physical and chemical reactions occur which, amongst other changes, causes the clay to be converted into a ceramic material. It is because of these properties that clay is used for making pottery items, both practical and decorative. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware and porcelain. Early humans discovered the useful properties of clay in prehistoric times, and one of the earliest artifacts ever uncovered is a drinking vessel made of sun-dried clay. Depending on the content of the soil, clay can appear in various colors, from a dull gray to a deep orange-red.

Clay was also used as the very first writing medium. Thousands of years BCE the cuneiform script was written in clay tablets.

Clays sintered in fire were the first ceramic, and remain one of the cheapest to produce and most widely used materials even in the present day. Bricks, cooking pots, art objects, dishware and even musical instruments such as the ocarina can all be shaped from clay before being fired. Clay is also used in many industrial processes, such as paper making, cement production and chemical filtering. Additionally, Clay is often used in the manufacture of pipes for smoking tobacco.

Clay, being almost impermeable to water, is also used where natural seals are needed, such as in the cores of dams, or to seal landfills against toxic seepage ('lining' the landfill, though often in combination with geotextiles).^[1]