Editing Clay (section)

== Properties ==
[[File:Clay magnified.jpg|thumb|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 [[Atterberg limits|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.{{sfn|Moreno-Maroto|Alonso-Azcárate|2018}} The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%.{{sfn|White|1949}} 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.{{sfn|Moreno-Maroto|Alonso-Azcárate|2018}}

Clay has a high content of clay minerals that give it its plasticity. Clay minerals are [[hydrate|hydrous]] [[aluminium]] [[Silicate minerals#Phyllosilicates|phyllosilicate minerals]], composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen and [[hydroxide]] 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.{{sfn|Bergaya|Theng|Lagaly|2006|pp=1-18}} In [[kaolinite]] 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 form direct hydrogen bonds with each other, making the dried clay 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 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]].{{sfn|Breuer|2012}}

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 ([[cation]]s), 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]].{{sfn|Bergaya|Theng|Lagaly|2006|pp=1-18}} The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility.{{sfn|Hodges|2010}}

Clay is a common component of [[sedimentary rock]]. [[Shale]] is formed largely from clay and is the most common of sedimentary rocks.{{sfn|Boggs|2006|p=140}} 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. [[Silt]]s, 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. [[Geologist]]s and [[soil scientist]]s usually consider the separation to occur at a particle size of 2 [[Micrometre|μm]] (clays being finer than silts), [[sedimentologist]]s often use 4–5 μm, and [[colloid]] [[chemist]]s use 1 μm.{{sfn|Guggenheim|Martin|1995|pp=255–256}} Clay-size particles and clay minerals are not the same, despite a degree of overlap in their respective definitions. [[Geotechnical engineering|Geotechnical engineers]] distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' [[Atterberg limits]]. [[International Organization for Standardization|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 [[mudcrack]]s 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.{{sfn|Olive|Chleborad|Frahme|Shlocker|1989}}

=== Agriculture ===
Clay is generally considered undesirable for agriculture, although some amount of clay is a necessary component of good soil. Compared to other soils, clay soils are less suitable for crops due to their tendency to retain water, and require artificial [[drainage]] and [[tillage]] to make suitable for planting. However, clay soils are often more fertile and can hold onto nutrients better due to their higher [[cation-exchange capacity]], allowing more land to remain in production rather than being left [[fallow]]. As clay tends to retain nutrients for longer before leaching them, this also means plants may require more fertilizer in clay soils.<ref name="v874"/><ref name="x742"/>