What is a clay mineral?

A clay mineral is a hydrous aluminum silicate with a layered structure and small particle size. They form through weathering and hydrothermal and diagenetic actions. The nature of the parent rock, climate, topography, vegetation, and time influence their formation through weathering. They may also contain iron, alkali metals, or alkaline earths. Clays consist of fine mineral fragments or particles, mostly hydrous-layer silicates of aluminum, but can contain magnesium and iron. They are classified into nine groups based on chemical composition and atomic structure variations. Clay materials are plastic when wet and coherent when dry.

Where are clay minerals commonly found in rocks?

Clay minerals are commonly found in various types of rocks, especially sedimentary rocks like mudrocks and shales, where they can constitute a significant portion of the rock's composition. They can be either detrital, formed from the weathering of pre-existing rocks, or authigenic, forming in place through geochemical processes. Clay minerals also occur as alteration products in hydrothermal deposits, such as around hot springs, geysers, and metalliferous ore deposits, often displaying a zonal arrangement depending on the parent rock and hydrothermal solution type. For example, kaolinite zones can be found around tin-tungsten mines, while smectites occur as alteration products of tuff and rhyolite. Additionally, the formation of clay minerals is influenced by factors such as the parent rock's nature, climate, topography, vegetation, and the duration of these conditions.

How are clay minerals formed in rocks?

Clay minerals in rocks form through neoformation and transformation, which are induced by weathering, hydrothermal activity, and diagenetic action. Weathering The formation of clay minerals through weathering depends on the parent rock, climate, topography, vegetation, and time. For example, the partial decomposition of minerals like feldspar at a weathering site can produce clay minerals. These minerals are then transported as detrital particles. Hydrothermal processes In hydrothermal processes, the presence of alkalies and alkaline earths affects the resulting products. Near-neutral hydrothermal solutions cause rock alteration, leading to the formation of illite, chlorite, and smectite, while acidic hydrothermal solutions result in the formation of kaolinite. Diagenesis Diagenesis, which includes physicochemical, biochemical, and physical processes, modifies sediments after deposition. Lithification, a result of diagenesis, converts sediment into solid sedimentary rock.

What are some different types of clay minerals found in rocks?

Clay minerals, hydrous-layer silicates of aluminum, are common in sedimentary rocks, especially mudrocks. They can be detrital, formed by weathering and transported as clasts, or authigenic, forming in place through geochemical processes. Common clay minerals include: * Kaolinite : A common clay mineral * Halloysite : Another frequently occurring clay mineral * Montmorillonite : A clay mineral named after Montmorillon, France * Illite : The most abundant clay mineral * Vermiculite : Notable for its worm-like exfoliation upon heating * Chlorite : Another significant clay mineral These minerals' formation is influenced by parent rock, climate, and water movement, with hydrothermal processes also playing a role.

How do clay minerals affect the properties of rocks?

Clay minerals significantly influence the properties of rocks due to their fine particle size and composition. They affect soil's capacity to hold water, air, and nutrients, with clay particles defined as less than 0.002 mm in diameter. Clay minerals, formed through weathering and hydrothermal activities, consist of hydrous-layer silicates of aluminum, sometimes with magnesium and iron. Their presence enhances porosity but reduces permeability in soil. They also bind soil grains, creating macropores that facilitate water movement. The plasticity index, influenced by the type of exchangeable cations and salt concentration, measures the flow properties of clays, impacting soil structure and various engineering applications.

Why are clay minerals important to geologists and soil scientists?

Clay minerals are hydrous-layer silicates with very fine-sized particles. They are essential to geologists and soil scientists for several reasons: Understanding Earth's Composition Clay minerals are fundamental components of sedimentary rocks like shales, which constitute a large portion of the Earth’s sedimentary rock column. Analyzing Soil Properties Clay minerals significantly influence soil properties such as porosity, aeration, water retention, and nutrient reservoir. They are critical in soil formation, affecting soil evolution and characteristics over time. Reconstructing Past Environments Certain clay minerals are sensitive to temperature and pressure, making them useful indicators of diagenesis stages in sediments. The type of clay mineral found in soil can also indicate the age and weathering stage of the soil. Versatile Industrial Applications Clay minerals have diverse applications, including ceramics, paper production, construction, and environmental remediation. For example, kaolinite is used in porcelain and paper coatings, while smectitic clays are used in drilling muds. Formation Clay minerals can form through weathering, hydrothermal processes, and diagenetic actions. The study of clay minerals involves various methods, including X-ray diffraction, infrared spectroscopy, and electron microscopy.

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History

General considerations

The term clay is generally applied to (1) a natural material with plastic properties, (2) particles of very fine size, customarily those defined as particles smaller than two micrometres (7.9 × 10⁻⁵ inch), and (3) very fine mineral fragments or particles composed mostly of hydrous-layer silicates of aluminum, though occasionally containing magnesium and iron. Although, in a broader sense, clay minerals can include virtually any mineral of the above-cited particle size, the definition adapted here is restricted to represent hydrous-layer silicates and some related short-range ordered aluminosilicates, both of which occur either exclusively or frequently in very fine-size grades.

The development of X-ray diffraction techniques in the 1920s and the subsequent improvement of microscopic and thermal procedures enabled investigators to establish that clays are composed of a few groups of crystalline minerals. The introduction of electron microscopic methods proved very useful in determining the characteristic shape and size of clay minerals. More recent analytical techniques such as infrared spectroscopy, neutron diffraction analysis, Mössbauer spectroscopy, and nuclear magnetic resonance spectroscopy have helped advance scientific knowledge of the crystal chemistry of these minerals.

Clay minerals are composed essentially of silica, alumina or magnesia or both, and water, but iron substitutes for aluminum and magnesium in varying degrees, and appreciable quantities of potassium, sodium, and calcium are frequently present as well. Some clay minerals may be expressed using ideal chemical formulas as the following: 2SiO₂·Al₂O₃·2H₂O (kaolinite), 4SiO₂·Al₂O₃·H₂O (pyrophyllite), 4SiO₂·3MgO·H₂O (talc), and 3SiO₂·Al₂O₃·5FeO·4H₂O (chamosite). The SiO₂ ratio in a formula is the key factor determining clay mineral types. These minerals can be classified on the basis of variations of chemical composition and atomic structure into nine groups: (1) kaolin-serpentine (kaolinite, halloysite, lizardite, chrysotile), (2) pyrophyllite-talc, (3) mica (illite, glauconite, celadonite), (4) vermiculite, (5) smectite (montmorillonite, nontronite, saponite), (6) chlorite (sudoite, clinochlore, chamosite), (7) sepiolite-palygorskite, (8) interstratified clay minerals (e.g., rectorite, corrensite, tosudite), and (9) allophane-imogolite. Information and structural diagrams for these groups are given below.

Kaolinite is derived from the commonly used name kaolin, which is a corruption of the Chinese Gaoling (Pinyin; Wade-Giles romanization Kao-ling), meaning “high ridge,” the name of a hill near Jingdezhen where the occurrence of the mineral is known as early as the 2nd century bce. Montmorillonite and nontronite are named after the localities Montmorillon and Nontron, respectively, in France, where these minerals were first reported. Celadonite is from the French céladon (meaning grayish yellow-green) in allusion to its colour. Because sepiolite is a light and porous material, its name is based on the Greek word for cuttlefish, the bone of which is similar in nature. The name saponite is derived from the Latin sapon (meaning soap), because of its appearance and cleaning ability. Vermiculite is from the Latin vermiculari (“to breed worms”), because of its physical characteristic of exfoliation upon heating, which causes the mineral to exhibit a spectacular volume change from small grains to long wormlike threads. Baileychlore, brindleyite, corrensite, sudoite, and tosudite are examples of clay minerals that were named after distinguished clay mineralogists—Sturges W. Bailey, George W. Brindley, Carl W. Correns, and Toshio Sudō, respectively.

Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.

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Structure

General features

The structure of clay minerals has been determined largely by X-ray diffraction methods. The essential features of hydrous-layer silicates were revealed by various scientists including Charles Mauguin, Linus C. Pauling, W.W. Jackson, J. West, and John W. Gruner through the late 1920s to mid-1930s. These features are continuous two-dimensional tetrahedral sheets of composition Si₂O₅, with SiO₄ tetrahedrons (Figure 1) linked by the sharing of three corners of each tetrahedron to form a hexagonal mesh pattern (Figure 2A). Frequently, silicon atoms of the tetrahedrons are partially substituted for by aluminum and, to a lesser extent, ferric iron. The apical oxygen at the fourth corner of the tetrahedrons, which is usually directed normal to the sheet, forms part of an adjacent octahedral sheet in which octahedrons are linked by sharing edges (Figure 3). The junction plane between tetrahedral and octahedral sheets consists of the shared apical oxygen atoms of the tetrahedrons and unshared hydroxyls that lie at the centre of each hexagonal ring of tetrahedrons and at the same level as the shared apical oxygen atoms (Figure 4). Common cations that coordinate the octahedral sheets are Al, Mg, Fe³⁺, and Fe²⁺; occasionally Li, V, Cr, Mn, Ni, Cu, and Zn substitute in considerable amounts. If divalent cations (M²⁺) are in the octahedral sheets, the composition is M2+/3 (OH)₂O₄ and all the octahedrons are occupied. If there are trivalent cations (M³⁺), the composition is M3+/2 (OH)₂O₄ and two-thirds of the octahedrons are occupied, with the absence of the third octahedron. The former type of octahedral sheet is called trioctahedral, and the latter dioctahedral. If all the anion groups are hydroxyl ions in the compositions of octahedral sheets, the resulting sheets may be expressed by M²⁺(OH)₂ and M³⁺(OH)₃, respectively. Such sheets, called hydroxide sheets, occur singly, alternating with silicate layers in some clay minerals. Brucite, Mg(OH)₂, and gibbsite, Al(OH)₃, are typical examples of minerals having similar structures. There are two major types for the structural “backbones” of clay minerals called silicate layers. The unit silicate layer formed by aligning one octahedral sheet to one tetrahedral sheet is referred to as a 1:1 silicate layer, and the exposed surface of the octahedral sheet consists of hydroxyls. In another type, the unit silicate layer consists of one octahedral sheet sandwiched by two tetrahedral sheets that are oriented in opposite directions and is termed a 2:1 silicate layer (Figure 5). These structural features, however, are limited to idealized geometric arrangements.

Real structures of clay minerals contain substantial crystal strains and distortions, which produce irregularities such as deformed octahedrons and tetrahedrons rather than polyhedrons with equilateral triangle faces, ditrigonal symmetry modified from the ideal hexagonal surface symmetry, and puckered surfaces instead of the flat planes made up by the basal oxygen atoms of the tetrahedral sheet. One of the major causes of such distortions is dimensional “misfits” between the tetrahedral and octahedral sheets. If the tetrahedral sheet contains only silicon in the cationic site and has an ideal hexagonal symmetry, the longer unit dimension within the basal plane is 9.15 Å, which lies between the corresponding dimensions 8.6 Å of gibbsite and 9.4 Å of brucite. To fit the tetrahedral sheet into the dimension of the octahedral sheet, alternate SiO₄ tetrahedrons rotate (up to a theoretical maximum of 30°) in opposite directions to distort the ideal hexagonal array into a doubly triangular (ditrigonal) array (Figure 2B). By this distortion mechanism, tetrahedral and octahedral sheets of a wide range of compositions resulting from ionic substitutions can link together and maintain silicate layers. Among ionic substitutions, those between ions of distinctly different sizes most significantly affect geometric configurations of silicate layers.

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Another significant feature of layer silicates, owing to their similarity in sheet structures and hexagonal or near-hexagonal symmetry, is that the structures allow various ways to stack up atomic planes, sheets, and layers, which may be explained by crystallographic operations such as translation or shifting and rotation, thereby distinguishing them from polymorphs (e.g., diamond-graphite and calcite-aragonite). The former involves one-dimensional variations, but the latter generally three-dimensional ones. The variety of structures resulting from different stacking sequences of a fixed chemical composition are termed polytypes. If such a variety is caused by ionic substitutions that are minor but consistent, they are called polytypoids.

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Weathering

Hydrothermal processes

Diagenesis

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Structure

General features