'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.
Definition
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.
Some varieties
Montmorillonite, with a chemical formula of (
Na,
Ca)
0.33(
Al,
Mg)
2Si4O10(
OH)
2'·'n
H2O, 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.
Historical and modern uses
Clay layers in a construction site. Clay is normally much more stable than sand.
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 with a blunt
reed called a
stylus.
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]
See also
References
★
Clay mineral nomenclature ''American Mineralogist''.
1. ''Preliminary evaluation of a compacted bentonite / sand mixture as a landfill liner material'' (Abstract) - Koçkar, Mustafa K.; Akgün, Haluk; Aktürk, Özgür; Department of Geological Engineering, Middle East Technical University, Ankara, Turkey
External links
★
''Bentonite, kaolin, and selected clay minerals'' (
PDF) (Volume 231, ''Environmental Health Criteria'',
World Health Organisation, 2005)
★
Historical information about the clays of North Staffordshire, UK
★
Information about clays used in the UK pottery industry
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