Clay, Clǣġ or Kaolin "glue, to stick together, high hill" Chemical Formula: Varying composition Discovery: ~26,000 BCE, Czech Republic (Earliest Known)

Category: Hydrous aluminium phyllosilicates Crystal System: Monoclinic or triclinic Crystal Habit: Compact Luster: Pearly to dull and earthy Mohs Scale: 1-2.5

Clay is a type of fine-grained natural soil material containing clay minerals, or hydrous aluminium phyllosilicates (e.g. kaolin, Al2Si2O5(OH)4). Clay minerals form in the presence of water and have been important to life, and many theories of abiogenesis involve them. Clay develops plasticity when wet, but becomes 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 one of the oldest building materials on Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood, and has been used by humans since ancient times in agriculture and manufacturing. Clay minerals are important constituents of soils, and their chemistry and capacity to retain nutrient cations such as potassium and ammonium is crucially important to soil fertility.

Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2Si2O5(OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. 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.

The composing clay minerals are what give clay its plasticity. These minerals are 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. 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 is transformed into a non-clay material which remains rigid and hard if moistened again. Further firing through the stoneware and porcelain stages further recrystallizes the material into yet stronger minerals.

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.

Clay is a very common substance. Shale, formed largely from clay, is the most common sedimentary rock. 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 deposit. Secondary clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins.

Clay minerals are most commonly form by prolonged chemical weathering of silicate-bearing rocks, but can also be formed locally from hydrothermal activity. 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.

Most clay deposits are impure, and many naturally occurring deposits include both silts and clay. 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. There is, however, some overlap in particle size and other physical properties, and the distinction between silt and clay varies by discipline. Mixtures of sand, silt and less than 40% clay are called loam.

The main groups of clays include kaolinite, montmorillonite-smectite, and illite (green clay). Chlorite, vermiculite, talc, 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.

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.

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

Clay is the oldest known ceramic material. Prehistoric humans discovered the useful properties of clay and began using it as early as 26,000 BCE, evidenced by a baked clay human figurine found in an Upper Paleolithic site in the Czech Republic. The world's oldest ceramic containers date to around 15,500 BCE, evidenced by pottery sherds recovered from Japan and Southern China. By 8,500 BCE, ceramic vessel traditions had reached the Near East and Anatolia. Clay tablets were the first known writing medium in Eurasia. Beginning in Bronze Age Mesopotamia, c.3300 BCE, scribes wrote by inscribing tablets with cuneiform script using a blunt reed called a stylus. Clay tablets were the preferred medium for writing up until the beginning of the Common Era.

Modelling clay is mainly used in art and handicraft for sculpting. Clays are used for making pottery, both utilitarian and decorative, and construction products, such as bricks, walls, and floor tiles. Artware, sculpture, smoking pipes, and even musical instruments such as the ocarina can all be shaped from clay before being fired. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware, and porcelain. Purpose-made clay balls have historically been used as sling ammunition. Clay is used in many modern industrial processes, such as paper making, cement production, and chemical filtering. Bentonite clay is widely used as a mold binder in the manufacture of sand castings. 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.

Baked clay bricks have historically been one of the most common materials from structural construction. Clay is also a primary ingredient in many other construction materials, used to create adobe, cob, cordwood, and structural elements like wattle and daub, clay plaster, clay render case, clay floors and clay paints and ceramic building material. Clay has been used as a mortar in brick chimneys and stone walls where protected from water. Between one-half and two-thirds of the world's population live or work in buildings made with clay or clay aggregate materials as an essential part of its load-bearing structure.

Traditional uses of clay as medicine goes back to prehistoric times. Certain clays are currently consumed in some nations to as a method for treating digestive problems. An example is the Armenian bole, a reddish clay powder that can be mixed with water to form a thick drinkable paste, used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons. However, moderation is key as ingesting too much clay can cause constipation. Clay has also been used to make topical poultices for external use. Clay compresses have been used to treat burns and inflammation. Iron rich clay has antibacterial properties, and can be applied to open wounds to prevent infection. However, only 5%–10% of natural clays have antibacterial properties; some clays may even encourage bacterial growth. A key factor in identifying anti-bacterial clay is the colour: blue and green clays contain reduced iron, a less positively charged counterpart to oxidized iron, which is a known bacteria-fighting ingredient.

Given the requirement of water, clay minerals are relatively rare in the Solar System, though they occur extensively on Earth where water has interacted with other minerals and organic matter. Clay minerals have been detected at several locations on Mars, including Echus Chasma, Mawrth Vallis, the Memnonia quadrangle and the Elysium quadrangle. Spectrography has confirmed their presence on asteroids including the dwarf planet Ceres and Tempel 1, as well as Jupiter's moon Europa.

The English word clay comes from Middle English cley/clay, from Old English clǣġ, from Proto-West Germanic *klaij, from Proto-Germanic *klajjaz, from Proto-Indo-European *gley- (“to glue, paste, stick together”). Cognate with Dutch klei, German Klei, Danish klæg; compare Ancient Greek γλία (glía), Latin glūten (“glue”) (whence ultimately English glue), Russian глина (glina, “clay”). The English name for kaolin, a type of fine clay rich in kaolinite, comes from an orthographic borrowing, from French kaolin, from the Wade–Giles romanization of Mandarin 高嶺／高岭 (Gāolǐng, “high hill”), in Jingdezhen, Jiangxi province, China, the location where this clay was first found.