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Clay

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Gay Head cliffs in Martha's Vineyard are made almost entirely of
   clay.
   Enlarge
   The Gay Head cliffs in Martha's Vineyard are made almost entirely of
   clay.

   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.

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.

Historical and modern uses of clay

   Quaternary clay in Estonia.
   Enlarge
   Quaternary clay in Estonia.

   Clay is plastic when wet, which means it can be easily shaped. When
   dry, it becomes firm and when subject to high temperature, known as
   firing, permanent physical and chemical reactions occur which, amongst
   other changes, causes the clay to be hardened. A fireplace or oven
   specifically designed for hardening clay is called a kiln. These
   properties make clay an ideal substance for making durable pottery
   items, both practical and decorative, with different types of clay and
   firing conditions giving 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.

   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 are all made with clay. Clay is also
   used in many industrial processes, such as paper making, cement
   production, pottery, and chemical filtering.

Some varieties of clay

   Montmorillonite, with a chemical formula of
   (Na,Ca)[0.33](Al,Mg)[2]Si[4]O[10]( OH)[2]·nH[2]O, 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.

   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 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.

   Retrieved from " http://en.wikipedia.org/wiki/Clay"
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   with only minor checks and changes (see www.wikipedia.org for details
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