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[[Image:Gay head cliffs MV.JPG|right|thumb|250px|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]] [[Silicate minerals#Phyllosilicates|phyllosilicate]] (phyllosilicates being a subgroup of [[silicate minerals]]) [[mineral]]s (see [[clay minerals]]), that are typically less than 2 μm ([[micrometre]]s) in [[diameter]]. Clay consists of a variety of phyllosilicate minerals rich in [[silicon]] and [[aluminium]] [[oxide]]s and [[hydroxide]]s which include variable amounts of structural [[Water (molecule)|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 [[soil]]s 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: [[kaolin]]ite, [[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 [[phyllosilicate]]s). 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 ==
[[Image:Clay-ss-2005.jpg|thumb|[[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 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 [[mineral]]s and firing conditions, are used to produce [[earthenware]], [[stoneware]] and [[porcelain]]. An oven specifically designed for firing clay is called a [[kiln]]. 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 [[sintering|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. [[Brick]]s, 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 [[filter (chemistry)|filter]]ing.
== Some varieties of clay ==
[[Montmorillonite]], with a chemical formula of ([[Sodium|Na]],[[Calcium|Ca]])<sub>0.33</sub>([[Aluminium|Al]],[[Magnesium|Mg]])<sub>2</sub>[[Silicon|Si]]<sub>4</sub>[[Oxygen|O]]<sub>10</sub>([[Hydroxide|OH]])<sub>2</sub>'''·'''n[[Hydrogen|H]]<sub>2</sub>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 lake]]s 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]].
==See also==
* [[Ceramic]]
* [[Clay (industrial plasticine)]]
* [[Clay court]]
* [[Clay minerals]]
* [[Clay pit]]
* [[Grain size]]
* [[List of minerals]]
* [[London clay]]
* [[Modelling clay]]
* [[Paperclay]]
* [[Plasticine]]
* [[Pottery]]
* [[Graham Cairns-Smith]], proposer of the "clay theory" for [[abiogenesis]]
{{commonscat|Clay}}
[[category:ceramics]]
[[category:Ceramic materials]]
[[Category:Art materials]]
[[Category:Geotechnical engineering]]
[[Category:Natural materials]]
[[Category:Pedology]]
[[Category:Sedimentology]]
[[Category:Sediments]]
[[Category:Silicate minerals]]
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