Everything about Clays totally explained
Clay is a naturally occurring material composed primarily of fine-grained
minerals, which show
plasticity through a variable range of
water content, and which can be hardened when dried and/or fired. Clay deposits are mostly composed of
clay minerals (
phyllosilicate minerals), minerals which impart plasticity and harden when fired and/or dried, and variable amounts of water trapped in the mineral structure by
polar attraction.
Organic materials which don't impart plasticity may also be a part of clay deposits.
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, but thick deposits usually are formed as the result of a secondary
sedimentary deposition process after they've been eroded and transported from their original location of formation. Clay deposits are typically associated with very low energy depositional environments such as large lake and marine deposits.
Definition
Clays are distinguished from other fine-grained soils by various differences in composition.
Silts, which are fine-grained soils which don't include clay minerals, tend to have larger particle sizes than clays, but there's some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include both silts and clays. 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 colloid
chemists use 1 μm..
Grouping
Depending upon 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 thirty 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.
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
Clays exhibit
plasticity when mixed with water in certain proportions. When dry, clay 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 tablets were used as the first 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. Additionally, Clay is often used in the manufacture of pipes for smoking
tobacco and
marijuana.
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).
Recent studies have been carried out to investigate clay's
adsorption capacities in various applications, such as the removal of
heavy metals from waste water and air purification.
Medical
A recent article in
The Journal of Antimicrobial Chemotherapy found that certain iron-rich clay was effective in killing bacteria.
Further Information
Get more info on 'Clays'.
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