Clay minerals are often defined according to idealized chemical compositions.[1]​ The amount of silica (SiO) in the formula is a key determinant in the classification of clay minerals.[1]​.

Within the granulometric classification of soil particles, clays occupy the following place:

Notwithstanding the above, the SUCS classification that is commonly used in engineering uses maximum size limits of 4.75 mm for sands and 0.075 mm for clays and silts.

Clay minerals are hydrated aluminum phyllosilicates sometimes with varying amounts of iron, magnesium, alkali metals, alkaline earths and other cations.[1]​[2]​ Clay minerals are generally microscopic.[2]​ In nature clay minerals are important components of shale and soils.[2]​ They originate from the weathering or hydrothermal alteration of feldspars, pyroxenes and micas.[2] Some plastic materials and particles with a size equal to or less than 2 micrometers are also called clay, "Micrometer (unit of length)"), which is the only or most common size of all clay minerals.[1]​.

Knowledge of the nature of clay became better in the 1920s and 1930s with the improvement in microscope technology, necessary to analyze the tiny particles.[1]​[4]​ The standardization of terminology during that period was good with special attention to similar and confusing terms, such as sheet and plane.[4]​.

Clay has plastic properties, which means that when moistened it can be easily modeled. When it dries, it becomes firm and when it is subjected to high temperatures, chemical reactions appear that, among other changes, cause the clay to become a permanently rigid material, called ceramic.

Due to these properties, clay is used to make pottery objects, for everyday or decorative use. Different types of clay, when mixed with different minerals and under various conditions, are used to produce earthenware, stoneware and porcelain. Depending on the mineral content of the soil, clay can appear in various colors, from a pale gray to a dark orange-red. A kiln designed specifically for firing clay is called a potter's kiln.

Humanity discovered the useful properties of clay in prehistoric times, and the oldest vessels discovered are vessels made from clay. It was also used, since prehistory, to build buildings of rammed earth, adobe and later brick, a construction element whose use still persists and is the most used to make walls in the modern world. Clay was also used in ancient times as a writing medium. Thousands of years before Christ, by the Sumerians in the Mesopotamian region, cuneiform writing was inscribed on clay tablets.

Fire-fired clay, ceramics, is one of the cheapest means of producing everyday objects, and one of the raw materials used profusely, even today. Bricks, vessels, plates, art objects, and even sarcophagi or musical instruments, such as the ocarina, were and are modeled with clay. Clay is also used in many industrial processes, such as cement production, papermaking, and obtaining filter substances.

Archaeologists use the magnetic characteristics of fired clay found in the bases of bonfires, ovens, etc., to date clay elements that have remained in the same orientation, and compare them with other historical periods.

Ball clay is called a clay with a lot of plasticity and little calcium that was extracted manually, at the end of the century, from the fields of southern England. Its name comes from the balls that miners made to facilitate their extraction.

Clay minerals are most commonly formed by prolonged chemical weathering of silicate-bearing rocks. They can also form locally from hydrothermal activity.[5] Chemical weathering occurs largely by acid hydrolysis due to low concentrations of carbonic acid, dissolved in rainwater or released by plant roots. The acid breaks the bonds between aluminum and oxygen, releasing other metal ions and silica (like an orthosilicic acid gel).[6]​.

The clay minerals formed depend on the composition of the parent rock and the climate. Acid weathering of feldspar-rich rocks, such as granite, in hot climates tends to produce kaolin. Weathering of the same type of rock under alkaline conditions produces illite. Smectite is formed by weathering of igneous rock under alkaline conditions, while gibbsite is formed by intense weathering of other clay minerals.[7]​.

There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in the 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.

The main clay groups include kaolinite, montmorillonite-smectite and illite. Chlorite), vermiculite,[9] talc, and pyrophyllite[10] are also sometimes 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.[11] Clay minerals in clays are most easily identified using X-ray diffraction of clay minerals) instead. of chemical or physical tests.[12]​.

A varve (or varved clay) is clay with visible annual layers that form by the seasonal deposition of those layers and are marked by differences in erosion and organic content. This type of deposit is common in ancient glacial lakes. When fine sediment reaches the still waters of these glacial lake basins far from the coast, they settle on the lake bed. The resulting seasonal stratification is preserved in a uniform distribution of bands of clayey sediments.[5]​.

Quick clay") is a unique type of marine clay indigenous to the glaciated terrains of Norway, North America, Northern Ireland and Sweden.[13] It is a very sensitive clay, prone to soil liquefaction, and has been involved in several deadly landslides.[14]

Clay minerals are often defined according to idealized chemical compositions.[1]​ The amount of silica (SiO) in the formula is a key determinant in the classification of clay minerals.[1]​.

Within the granulometric classification of soil particles, clays occupy the following place:

Notwithstanding the above, the SUCS classification that is commonly used in engineering uses maximum size limits of 4.75 mm for sands and 0.075 mm for clays and silts.

Clay minerals are hydrated aluminum phyllosilicates sometimes with varying amounts of iron, magnesium, alkali metals, alkaline earths and other cations.[1]​[2]​ Clay minerals are generally microscopic.[2]​ In nature clay minerals are important components of shale and soils.[2]​ They originate from the weathering or hydrothermal alteration of feldspars, pyroxenes and micas.[2] Some plastic materials and particles with a size equal to or less than 2 micrometers are also called clay, "Micrometer (unit of length)"), which is the only or most common size of all clay minerals.[1]​.

Knowledge of the nature of clay became better in the 1920s and 1930s with the improvement in microscope technology, necessary to analyze the tiny particles.[1]​[4]​ The standardization of terminology during that period was good with special attention to similar and confusing terms, such as sheet and plane.[4]​.

Clay has plastic properties, which means that when moistened it can be easily modeled. When it dries, it becomes firm and when it is subjected to high temperatures, chemical reactions appear that, among other changes, cause the clay to become a permanently rigid material, called ceramic.

Due to these properties, clay is used to make pottery objects, for everyday or decorative use. Different types of clay, when mixed with different minerals and under various conditions, are used to produce earthenware, stoneware and porcelain. Depending on the mineral content of the soil, clay can appear in various colors, from a pale gray to a dark orange-red. A kiln designed specifically for firing clay is called a potter's kiln.

Humanity discovered the useful properties of clay in prehistoric times, and the oldest vessels discovered are vessels made from clay. It was also used, since prehistory, to build buildings of rammed earth, adobe and later brick, a construction element whose use still persists and is the most used to make walls in the modern world. Clay was also used in ancient times as a writing medium. Thousands of years before Christ, by the Sumerians in the Mesopotamian region, cuneiform writing was inscribed on clay tablets.

Fire-fired clay, ceramics, is one of the cheapest means of producing everyday objects, and one of the raw materials used profusely, even today. Bricks, vessels, plates, art objects, and even sarcophagi or musical instruments, such as the ocarina, were and are modeled with clay. Clay is also used in many industrial processes, such as cement production, papermaking, and obtaining filter substances.

Archaeologists use the magnetic characteristics of fired clay found in the bases of bonfires, ovens, etc., to date clay elements that have remained in the same orientation, and compare them with other historical periods.

Ball clay is called a clay with a lot of plasticity and little calcium that was extracted manually, at the end of the century, from the fields of southern England. Its name comes from the balls that miners made to facilitate their extraction.

Clay minerals are most commonly formed by prolonged chemical weathering of silicate-bearing rocks. They can also form locally from hydrothermal activity.[5] Chemical weathering occurs largely by acid hydrolysis due to low concentrations of carbonic acid, dissolved in rainwater or released by plant roots. The acid breaks the bonds between aluminum and oxygen, releasing other metal ions and silica (like an orthosilicic acid gel).[6]​.

The clay minerals formed depend on the composition of the parent rock and the climate. Acid weathering of feldspar-rich rocks, such as granite, in hot climates tends to produce kaolin. Weathering of the same type of rock under alkaline conditions produces illite. Smectite is formed by weathering of igneous rock under alkaline conditions, while gibbsite is formed by intense weathering of other clay minerals.[7]​.

There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in the 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.

The main clay groups include kaolinite, montmorillonite-smectite and illite. Chlorite), vermiculite,[9] talc, and pyrophyllite[10] are also sometimes 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.[11] Clay minerals in clays are most easily identified using X-ray diffraction of clay minerals) instead. of chemical or physical tests.[12]​.

A varve (or varved clay) is clay with visible annual layers that form by the seasonal deposition of those layers and are marked by differences in erosion and organic content. This type of deposit is common in ancient glacial lakes. When fine sediment reaches the still waters of these glacial lake basins far from the coast, they settle on the lake bed. The resulting seasonal stratification is preserved in a uniform distribution of bands of clayey sediments.[5]​.

Quick clay") is a unique type of marine clay indigenous to the glaciated terrains of Norway, North America, Northern Ireland and Sweden.[13] It is a very sensitive clay, prone to soil liquefaction, and has been involved in several deadly landslides.[14]