is a type of fine-grained natural soil
material containing clay minerals
Clays develop plasticity
when wet, due to a molecular film of water surrounding the clay particles, but become hard, brittle and non–plastic upon drying or firing
Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide
Clay is the oldest known ceramic
material. Prehistoric humans discovered the useful properties of clay and used it for making pottery
. Some of the earliest pottery shards have been dated
to around 14,000 BC,
and clay tablets were the first known writing medium.
Clay is used in many modern industrial processes, such as paper
production, and chemical filtering
. Between one-half and two-thirds of the world's population still live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.
Clay is a very common substance. Shale
, formed largely from clay, is the most common sedimentary rock.
Although many naturally occurring deposits include both silts and clay, 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. Mixtures of sand
and less than 40% clay are called loam
. Soils high in swelling clays
, which are clay minerals that readily expand in volume when they absorb water, are a major challenge in civil engineering
The defining mechanical property of clay is its plasticity when wet and its ability to harden when dried or fired. Clays show a broad range of water content within which they are highly plastic, from a minimum water content (called the plasticity limit) where the clay is just moist enough to mould, to a maximum water content (called the liquid limit) where the moulded clay is just dry enough to hold its shape.
The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%.
High-quality clay is also tough, as measured by the amount of mechanical work required to roll a sample of clay flat. Its toughness reflects a high degree of internal cohesion.
Electron micrograph of smectite clay – magnification 23,500×
Clay has a high content of clay minerals that give it its plasticity. Clay minerals are hydrousaluminium phyllosilicate minerals
, composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen and hydroxyl
ions. These plates are tough but flexible, and in moist clay, they adhere to each other. The resulting aggregates give clay the cohesion that makes it plastic.
clay, the bonding between plates is provided by a film of water molecules that hydrogen bond
the plates together. The bonds are weak enough to allow the plates to slip past each other when the clay is being moulded, but strong enough to hold the plates in place and allow the moulded clay to retain its shape after it is moulded. When the clay is dried, most of the water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile. If the clay is moistened again, it will once more become plastic. When the clay is fired to the earthenware
stage, a dehydration reaction
removes additional water from the clay, causing clay plates to irreversibly adhere to each other via stronger covalent bonding
, which strengthens the material. The clay mineral, kaolin, is transformed into a non-clay material, metakaolin, which remains rigid and hard if moistened again. Further firing through the stoneware
stages further recrystallizes the metakaolin into yet stronger minerals such as mullite
The tiny size and plate form of clay particles gives clay minerals a high surface area. In some clay minerals, the plates carry a negative electrical charge that is balanced by a surrounding layer of positive ions (cations
), such as sodium, potassium, or calcium. If the clay is mixed with a solution containing other cations, these can swap places with the cations in the layer around the clay particles, which gives clays a high capacity for ion exchange
The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility.
Clay is a common component of sedimentary rock
is formed largely from clay and is the most common of sedimentary rocks.
However, most clay deposits are impure. Many naturally occurring deposits include both silts and clay. 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. There is, however, some overlap in particle size and other physical properties. 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. Geotechnical engineers
distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits
14688 grades clay particles as being smaller than 2 μm and silt particles as being larger. Mixtures of sand
and less than 40% clay are called loam
Some clay minerals (such as smectite
) are described as swelling clay minerals, because they have a great capacity to take up water, and they increase greatly in volume when they do so. When dried, they shrink back to their original volume. This produces distinctive textures, such as mudcracks
or "popcorn" texture, in clay deposits. Soils containing swelling clay minerals (such as bentonite
) pose a considerable challenge for civil engineering, because swelling clay can break foundations of buildings and ruin road beds.
Deforestation for clay extraction in Rio de Janeiro
. The picture is of Morro da Covanca, Jacarepaguá.
Clay minerals most commonly form by prolonged chemical weathering
of silicate-bearing rocks. They can also form locally from hydrothermal
Chemical weathering takes place largely by acid hydrolysis
due to low concentrations of carbonic acid
, dissolved in rainwater or released by plant roots. The acid breaks bonds between aluminium and oxygen, releasing other metal ions and silica (as a gel of orthosilicic acid
The clay minerals formed depend on the composition of the source rock and the climate. Acid weathering of feldspar
-rich rock, such as granite
, in warm climates tends to produce kaolin. Weathering of the same kind of rock under alkaline conditions produces illite
forms by weathering of igneous rock
under alkaline conditions, while gibbsite
forms by intense weathering of other clay minerals.
There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in soil and remain at the site of formation. Secondary clays are clays that have been transported from their original location by water erosion and deposited
in a new sedimentary
Secondary clay deposits are typically associated with very low energy depositional environments
such as large lakes and marine basins.
The main groups of clays include kaolinite
, and illite
, and pyrophyllite
are sometimes also classified as clay minerals. There are approximately 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals.
Clay minerals in clays are most easily identified using X-ray diffraction
rather than chemical or physical tests.
(or varved clay
) is clay with visible annual layers that are formed by seasonal deposition of those layers and are marked by differences in erosion
and organic content. This type of deposit is common in former glacial lakes
. When fine sediments are delivered into the calm waters of these glacial lake basins away from the shoreline, they settle to the lake bed. The resulting seasonal layering is preserved in an even distribution of clay sediment banding.
Historical and modern uses
Clay layers in a construction site in Auckland City
, New Zealand. Dry clay is normally much more stable than sand in excavations.
Clays are used for making pottery
, both utilitarian and decorative, and construction products, such as bricks, walls, and floor tiles. 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. Some of the earliest pottery shards recovered are from central Honshu
. They are associated with the Jōmon
culture, and recovered deposits have been dated
to around 14,000 BC.
Cooking pots, art objects, dishware, smoking pipes
, and even musical instruments
such as the ocarina
can all be shaped from clay before being fired.
Clay tablets were the first known writing medium.
Scribes wrote by inscribing them with cuneiform
script using a blunt reed
called a stylus
. Purpose-made clay balls were used as sling ammunition
Clay is used in many industrial processes, such as paper
production, and chemical filtering
clay is 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
Studies in the early 21st century have investigated clay's absorption
capacities in various applications, such as the removal of heavy metals
from waste water and air purification.
Traditional uses of clay as medicine
goes back to prehistoric times. An example is Armenian bole
, which is used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons. Kaolin clay
have been used as anti-diarrheal medicines.
As a building material
Clay building in southern Estonia
Clay as the defining ingredient of loam
is one of the oldest building materials
, 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 both traditional societies as well as developed countries, still live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.
Also a primary ingredient in many natural building
techniques, clay is used to create adobe
, and rammed earth
structures and building elements such as wattle and daub
, clay plaster, clay render case, clay floors and clay paints
and ceramic building material
. Clay was used as a mortar
in brick chimneys
and stone walls where protected from water.
Diamond clay tailings at the Lomonosov mine, Northwestern Russia 
The forecasted mass of clay minerals to be discharged into the tailings of ore processing makes up millions of tons. Worryingly, when macro- and micro-components are found in non-hazardous concentrations, fewer efforts are put into the environmental management of the tailings, though technogenic sediments offer prospects for reuse and valorization beyond their traditional disposal. Saponite is a demonstrative example of the tailings constituent that is often left unfairly mistreated. Electrochemical separation helps to obtain modified saponite-containing products with high smectite-group minerals concentrations, lower mineral particles size, more compact structure, and greater surface area. These characteristics open possibilities for the manufacture of high-quality ceramics and heavy-metal sorbents from saponite-containing products.
Furthermore, tail grinding occurs during the preparation of the raw material for ceramics; this waste reprocessing is of high importance for the use of clay pulp as a neutralizing agent, as fine particles are required for the reaction. Experiments on the Histosol
deacidification with the alkaline clay slurry demonstrated that neutralization with the average pH level of 7.1 is reached at 30% of the pulp added and an experimental site with perennial grasses proved the efficacy of the technique.
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