Clay is a general term including many combinations of one or more
clay minerals with traces of metal oxides and organic matter.
[1] Geologic clay
deposits are mostly composed of
phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.
Formation
Clay minerals are typically formed over long periods of time by the gradual chemical
weathering of rocks, usually silicate-bearing, by low concentrations of
carbonic acid and other diluted
solvents. These solvents, usually acidic, migrate through the weathering rock after
leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed by
hydrothermal activity. Clay deposits may be formed in place as residual deposits in soil, but thick deposits usually are formed as the result of a secondary
sedimentary deposition process after they have been eroded and transported from their original location of formation. Clay deposits are typically associated with very low energy
depositional environments such as large lakes and marine deposits.
Primary clays, also known as
kaolins, are located at the site of formation. Secondary clay deposits have been moved by
erosion and water from their primary location.
[2]
Definition
Clays are distinguished from other fine-grained soils by differences in size and mineralogy.
Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays, but there is some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include silts and also clay . The distinction between silt and clay varies by discipline.
Geologists and
soil scientists usually consider the separation to occur at a particle size of 2
µm (clays being finer than silts),
sedimentologists often use 4-5 μm, and
colloidchemists use 1 μm.
[1]Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils'
Atterberg Limits. ISO 14688 grades clay particles as being smaller than 2 μm and silts larger.
electron microscope photograph of smectite clay - magnification 23,500GroupingDepending on the academic source, there are three or four main groups of clays:
kaolinite,
montmorillonite-
smectite,
illite, and
chlorite. Chlorites are not always considered a clay, sometimes being classified as a separate group within the
phyllosilicates. There are approximately 30 different types of "pure" clays in these categories, but most "natural" clays are mixtures of these different types, along with other weathered minerals.
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. When glacial lakes are formed there is very little movement of the water that makes the lake, and these eroded soils settle on the lake bed. This allows such an even distribution on the different layers of clay.
[2]Quick clay is a unique type of
marine clay indigenous to the glaciated terrains of
Norway,
Canada,
Northern Ireland, and
Sweden. It is a highly sensitive clay, prone to
liquefaction, which has been involved in several deadly
landslides.
Quaternary clay in Estonia.Historical and modern usesClays exhibit
plasticity when mixed with water in certain proportions. When dry, clay becomes firm and when
fired in a
kiln, permanent physical and chemical changes occur. These reactions, among other changes, cause the clay to be converted into a
ceramic material. Because of these properties, clay is used for making
pottery items, both utilitarian and decorative. Different types of clay, when used with different
minerals and firing conditions, are used to produce
earthenware,
stoneware, and
porcelain. Prehistoric humans discovered the useful properties of clay, 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 tablets were used as the first known writing medium, inscribed with
cuneiform script through the use of a blunt
reed called a
stylus.
Clays
sintered in fire were the first form of
ceramic.
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. Clay is also often used in the manufacture of
pipes for smoking
tobacco. Until the late 20th century
bentonite clay was widely used as a mold binder in the manufacture of
sand castings.
Clay, being relatively
impermeable to water, is also used where natural seals are needed, such as in the cores of
dams, or as a barrier in
landfills against toxic seepage (lining the landfill, preferably in combination with
geotextiles).
[3]Recent studies have investigated clay's
absorption capacities in various applications, such as the removal of
heavy metals from waste water and air purification.
Medical and agricultural uses
A traditional use of
clay as medicine goes back to prehistoric times. An example is
Armenian bole, which is used to soothe an upset
stomach, similar to the way
parrots (and later, humans) in
South America originally used it.
[4] Kaolin clay and
attapulgite have been used as anti-diarrheal medicines.
Clay as a building material
Clay is one of the oldest building materials on Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood. Between one-half and two-thirds of the world's population, in traditional societies as well as developed countries, still live or work in a building made with clay as an essential part of its load-bearing structure. Also a primary ingredient in many
natural building techniques, clay is used to create
adobe,
cob,
cordwood, and
rammed earth structures.
See alsoFootnotes
References- Ehlers, Ernest G. and Blatt, Harvey (1982). 'Petrology, Igneous, Sedimentary, and Metamorphic' San Francisco: W.H. Freeman and Company. ISBN 0-7167-1279-2.
- Hillier S. (2003) Clay Mineralogy. pp 139–142 In: Middleton G.V., Church M.J., Coniglio M., Hardie L.A. and Longstaffe F.J.(Editors) Encyclopedia of sediments and sedimentary rocks. Kluwer Academic Publishers, Dordrecht.
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