Clay

Source: Wikipedia, the free encyclopedia.
For other uses, see Clay (disambiguation).

Gay Head Cliffs in Martha's Vineyard
consist almost entirely of clay.
A quaternary clay in Estonia

Clay is a type of fine-grained natural soil material containing clay minerals[1] (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). 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.[2][3]

Clays develop

filtering
. Between one-half and two-thirds of the world's population 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.[9] 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, silt and less than 40% clay are called loam. Soils high in swelling clays (expansive clay), which are clay minerals that readily expand in volume when they absorb water, are a major challenge in civil engineering.[1]

Properties

A 23,500 times magnified electron micrograph of smectite clay

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 plastic 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.[10] The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%.[11] 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.[10]

Clay has a high content of clay minerals that give it its plasticity. Clay minerals are

covalent bonding, which strengthens the material. The clay mineral kaolinite is transformed into a non-clay material, metakaolin, which remains rigid and hard if moistened again. Further firing through the stoneware and porcelain stages further recrystallizes the metakaolin into yet stronger minerals such as mullite.[6]

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.[12] The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility.[13]

Clay is a common component of

sedimentologists often use 4–5 μm, and colloid chemists use 1 μm.[4] Clay-size particles and clay minerals are not the same, despite a degree of overlap in their respective definitions. 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 silt particles as being larger. Mixtures of sand, silt 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.[1]

Formation

Italian and African-American clay miners in mine shaft, 1910

Clay minerals most commonly form by prolonged chemical

hydrothermal activity.[14] 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).)[15]

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. Smectite forms by weathering of igneous rock under alkaline conditions, while gibbsite forms by intense weathering of other clay minerals.[16]

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

sedimentary deposit.[17] Secondary clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins.[14]

Varieties

The main groups of clays include

kaolinite, montmorillonite-smectite, and illite. Chlorite, vermiculite,[18] talc, and pyrophyllite[19] 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.[20] Clay minerals in clays are most easily identified using X-ray diffraction rather than chemical or physical tests.[21]

Varve (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.[14]

Quick clay is a unique type of marine clay indigenous to the glaciated terrains of Norway, North America, Northern Ireland, and Sweden.[22] It is a highly sensitive clay, prone to liquefaction, and has been involved in several deadly landslides.[23]

Uses

Clay layers in a construction site in Auckland, New Zealand. Dry clay is normally much more stable than sand in excavations.
A 14th century bottle stopper made of fired clay

sculpting
. Clays are used for making
smoking pipes, and even musical instruments such as the ocarina
can all be shaped from clay before being fired.

Ancient peoples in

filtering.[26] Bentonite clay is widely used as a mold binder in the manufacture of sand castings.[27][28]

Mass bathing in liquid clay
as a type of relaxation
video icon Video (10 minutes) on
YouTube
Clay bath near lake Ahémé in Benin

Medicine

Traditional uses of

attapulgite have been used as anti-diarrheal medicines.[30]

Construction

South Estonia

Clay as the defining ingredient of

chimneys
and stone walls where protected from water.

Clay, relatively

impermeable to water, is also used where natural seals are needed, such as in pond linings, the cores of dams, or as a barrier in landfills against toxic seepage (lining the landfill, preferably in combination with geotextiles).[32] 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.[33][34]

See also

Notes

  1. ^ a b c Olive et al. 1989.
  2. ^ Klein & Hurlbut 1993, pp. 512–514.
  3. ^ Nesse 2000, pp. 252–257.
  4. ^ a b Guggenheim & Martin 1995, pp. 255–256.
  5. ^ Science Learning Hub 2010.
  6. ^ a b Breuer 2012.
  7. ^ a b Scarre 2005, p. 238.
  8. ^ a b Ebert 2011, p. 64.
  9. ^ a b Boggs 2006, p. 140.
  10. ^ a b Moreno-Maroto & Alonso-Azcárate 2018.
  11. ^ White 1949.
  12. ^ a b Bergaya, Theng & Lagaly 2006, pp. 1–18.
  13. ^ Hodges 2010.
  14. ^ a b c Foley 1999.
  15. ^ Leeder 2011, pp. 5–11.
  16. ^ Leeder 2011, pp. 10–11.
  17. ^ Murray 2002.
  18. ^ Nesse 2000, p. 253.
  19. ^ Klein & Hurlbut 1993, pp. 514–515.
  20. ^ Klein & Hurlbut 1993, p. 512.
  21. ^ Nesse 2000, p. 256.
  22. ^ Rankka et al. 2004.
  23. ^ Natural Resources Canada 2005.
  24. ^ "British Library". www.bl.uk. Archived from the original on 12 September 2022. Retrieved 9 May 2023.
  25. ^ Forouzan et al. 2012.
  26. ^ Nesse 2000, p. 257.
  27. ^ Boylu 2011.
  28. ^ Eisenhour & Brown 2009.
  29. ^ Diamond 1999.
  30. ^ Dadu et al. 2015.
  31. ^ Grim 2016.
  32. ^ Koçkar, Akgün & Aktürk 2005.
  33. ^ García-Sanchez, Alvarez-Ayuso & Rodriguez-Martin 2002.
  34. ^ Churchman et al. 2006.

References

External links

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