Clays and clay minerals

Clays are fine-grained materials with unique properties, such as plastic behavior when wet. They form by the weathering of silicate rocks at the earth’s surface, by diagenetic reactions, and by hydrothermal alteration. An understanding of clays is important to solving problems in petroleum geology, engineering, and environmental science.

Definition and Properties

The definition of clays varies depending on the scientific discipline or application. An engineer’s definition, which is based on particle size, differs from a mineralogist’s definition, which is based on crystal structure. In the broadest sense, clays are materials that have a very fine grain size (less than 0.002 millimeter) and behave plastically when wet. A more specific definition of a clay mineral is a hydrous aluminum phyllosilicate, or, more simply stated, a mineral that contains water, aluminum, and silicon, and has a layered structure. The term “clays” will be used for the broad definition, and “clay mineral” for the specific definition. Rock flour, or material that was ground to a fine powder by glaciers, would fit the definition of clays; however, it may contain minerals such as quartz and feldspars that do not fit the definition of a clay mineral. Mica is a hydrous aluminum phyllosilicate, but it often occurs as large crystals, so it does not fit the definition of clays. Certain minerals such as zeolites and hydroxides (goethite and gibbsite) have a very fine grain size and physical properties similar to clay minerals, so they are often included with the clay minerals. Unique properties of clays, including their plastic behavior when wet and ability to adsorb water and ions in solution, can be attributed to their small crystal size, large surface area, and unique crystal structure.

Clay Mineral Structure

There are two basic elements of the clay mineral structure: a tetrahedral sheet and an octahedral sheet. A silicon atom surrounded by four oxygen atoms forms the basic building block of all silicate minerals, the four-sided silica tetrahedron. In phyllosilicate minerals, the silica tetrahedra are linked together by sharing the three oxygen atoms at the corners of the tetrahedra, forming a continuous sheet. The tetrahedral sheet has a negative charge and a general chemical formula of Si₄O₁₀^4-. The octahedral sheet consists of a sandwich of positively charged atoms (cations), like magnesium, iron, or aluminum, between sheets of oxygen and hydroxyl anions, or negatively charged ions (OH^-). Each cation is surrounded by six anions that lie at the corners of an octahedron, a shape made of eight triangles. The octahedra are linked together to form a sheet by sharing the anions on the edges of the octahedra.

There are two types of octahedral sheets: trioctahedral and dioctahedral. The prefixes refer to the fraction of octahedral occupied by cations: 2/3 for dioctahedral and 3/3 for trioctahedral. The trioctahedral sheet is composed of divalent cations such as Mg²⁺ and Fe²⁺. For every six hydroxyl anions, the trioctahedral sheet contains three cations, resulting in a sheet where all the available octahedral sites contain a cation. The dioctahedral sheet contains trivalent cations such as Al³⁺ and Fe³⁺. The dioctahedral sheet has only two cations per six hydroxyl anions, resulting in only two-thirds of the available octahedral sites being filled with cations. The general chemical formulas for the trioctahedral and dioctahedral sheets are Mg₃OH₆ and Al₂OH₆, respectively.

Layers in clay minerals are made of combinations of tetrahedral and octahedral sheets. The unshared oxygen of the silica tetrahedra take the place of some of the hydroxyl anions in the octahedral sheet, resulting in neutralization of the negative charges on the tetrahedral sheet. There are two types of layers: a 1:1 or T-O layer made up of one tetrahedral sheet and one octahedral sheet, and a 2:1 or T-O-T layer, which contains one octahedral sheet sandwiched between two tetrahedral sheets. Not all minerals with these layered structures are considered clay minerals.

The prototype clays have neutral layer charges, with the layers held together by weak van der Waals bonds where positive portions of one sheet are attracted to negative portions of another sheet. Kaolinite and serpentine are prototype 1:1 minerals. Kaolinite is dioctahedral and serpentine is trioctahedral. Kaolinite, a pure white clay, is the major constituent of fine porcelain. Serpentine is not generally considered a clay mineral. It can occur as chrysotile asbestos, which was widely used as insulation material before it was recognized as a health hazard. Talc and pyrophyllite are the trioctahedral and dioctahedral 2:1 prototype minerals. The waxy or slippery feel of talc (also not a clay mineral) is the result of cleavage along the weak van der Waals bonds between layers.

Because of the various types of ionic substitutions in the tetrahedral and octahedral sheets, the layers may develop a negative charge, which needs to be balanced by interlayer materials. A lower-valence cation may substitute for a higher-valence cation in the tetrahedral or octahedral sheets. Layers can also develop a negative charge if some of the sites in a trioctahedral sheet are left vacant. In the mica group of minerals, which are also not considered clays, the charge on the 2:1 layer is -1 per repeat unit and is balanced by a positively charged potassium ion that occurs between the layers. Muscovite is a dioctahedral 2:1 phyllosilicate, and biotite is trioctahedral. The perfect cleavage of micas is in the direction parallel to the tetrahedral sheets and allows them to be peeled into paper-thin sheets. Micas usually occur as larger crystals and, therefore, are not considered true clay minerals; however, illite, a common clay mineral, has a structure and chemical formula similar to muscovite, a mica. The chlorite clay minerals have an extra octahedral sheet between the 2:1 layers to balance the excess negative charges. Two groups of clay minerals known as vermiculite and smectite have negatively charged layers that are balanced by hydrated cations between the layers; a hydrated cation is a positively charged ion, such as sodium, that is surrounded by water. In smectites, this water is held very loosely between the 2:1 layers and can be easily lost or gained depending on the humidity of the environment, causing these clays to shrink or swell.

Formation and Deposition

Clay minerals occur in soils, sediments, sedimentary rocks, and some metamorphic rocks. Sedimentary rocks cover approximately 80 percent of the earth’s surface, and shales are the most common type of sedimentary rock. Because shales are composed predominantly of clays, their abundance makes clays one of the most important constituents of the earth’s surface.

Most clays form through the breakdown and weathering of minerals rich in aluminum and silicon at the earth’s surface. Physical weathering is the breaking and fragmentation of rocks with no change in mineralogical or chemical composition. This process can form clay-sized particles, but it does not form clay minerals. Physical weathering, however, increases the surface area of minerals, which then favors chemical weathering. A change in the chemical and mineralogical composition of rocks by reaction with water at the earth’s surface is called chemical weathering. One chemical weathering reaction that results in the formation of clays is called hydrolysis. Water and carbonic acid, a weak acid, react with aluminum silicate minerals, resulting in the production of a clay mineral plus ions in solution. (The weak acid is called carbonic acid; it forms when carbon dioxide, a common gas in the atmosphere, is dissolved in rainwater.) The type and amount of clay that forms by this reaction depend on the nature of the rock being weathered (the parent material) and the intensity of weathering.

Clays produced by weathering are eventually eroded, transported, and deposited as sediments. Most clays are transported in suspension. The brown, muddy waters of rivers are a reflection of the clays being carried in suspension. Clays are deposited and accumulate in quiet water environments, where the energy is low enough to allow the clays to settle out of suspension. Several processes enhance the deposition of clays. When fresh river water mixes with salty ocean water, the negative charges on clay surfaces are neutralized, causing them to flocculate (clump together). Clay floccules—or aggregates of clay-sized particles that behave like larger silt- or sand-sized grains—rapidly settle out of suspension. Biodeposition is a process whereby organisms ingest clays with their food; the resultant fecal pellets settle to the bottom as sand-sized particles.

Types of Clay Minerals

There are two types of clays in sedimentary rocks: detrital and authigenic. Minerals that are transported and deposited as sediments are called detrital. Authigenic minerals form within the rocks during diagenesis, a process whereby sediments buried within the earth’s crust undergo compaction and cementation into sedimentary rocks. Water expelled from the sediments during this process may react with other minerals in the sediment to form clay minerals. Kaolinite, chlorite, illite, and smectite formed by diagenesis have been observed in sandstones. Diagenesis may also result in one clay mineral being converted into another clay mineral. One reaction that commonly occurs is the alteration of smectite to form illite as a result of an increase in temperature. This reaction is important to petroleum geologists because intermediate mixed-layered illite/smectite clays form at different temperatures. The clay mineralogy of shale can be used to determine the maximum burial temperature of a rock and, in turn, help to predict the chemistry of any petroleum encountered.

Clay minerals may also form in metamorphic rocks as the result of hydrothermal alteration. Hydrothermal fluids are hot, chemically active fluids that accompany igneous intrusions. In addition to forming clay minerals as they pass through a host rock, hydrothermal fluids are responsible for producing important ore deposits such as copper ores. Economic geologists may use the distribution of hydrothermal clay minerals to locate valuable ore deposits.

Identification and Analysis

Clay minerals are difficult to identify and analyze because of their small crystal size. Because they can be observed neither with the naked eye nor by standard petrographic microscopes, they require the use of sophisticated equipment for their identification. Analysis is further complicated by the fact that it is difficult to obtain pure samples of the clay minerals because they often occur as mixtures with other minerals. Before a clay mineral can be analyzed, it must be isolated from the sample by means of special physical and chemical techniques.

The tool a clay mineralogist uses most often is an X-ray diffractometer (XRD). This instrument focuses X rays onto the sample. The crystal structure of the minerals acts as a diffraction grating, and the instrument records X rays that are diffracted from the mineral. Mineralogists can determine the spacing between planes of atoms in the crystal structure by measuring the position of “reflections” produced by X rays diffracted by the mineral. Clay mineralogists prepare specially oriented samples that enhance the reflections between the layers of clay minerals called basal reflections. The basal reflections are used in determining the type of clay mineral present.

Other instruments that are used to investigate clay minerals include the scanning electron microscope (SEM) and the transmission electron microscope (TEM). The very high resolution of these microscopes allows the scientist to observe clay minerals at a very great magnification. The scientist is thus able to observe the outward crystal form of clay minerals and their texture or orientation with respect to other grains in the sample. The SEM is very helpful in distinguishing between detrital and authigenic clays in sedimentary rocks—that is, clays transported from elsewhere versus clay minerals that formed in place.

A property that is helpful in identifying clays and understanding their behavior is the cation exchange capacity (CEC). Because of their small size and unique crystal structure, clay surfaces are negatively charged and have the ability to adsorb, or attract, positive ions on their surfaces and within clays between the layers. These cations are easily exchanged with solutions. If clay that is saturated with cations is placed in a solution saturated with sodium ions, it will exchange its cations for the sodium ions. The ability of clay to adsorb and exchange cations is called the “cation exchange capacity,” and depends on the type of clay mineral. Kaolinite, chlorite, and illite have relatively low CECs; smectite has a relatively high CEC. This property is especially important to soil scientists, because it controls the availability of nutrients necessary for plant growth.

Industrial Applications

Clays have a variety of applications in industry. They are a readily available natural resource and are relatively inexpensive. As of 2020, more than 100 billion tons of materials were used in industrial projects worldwide, with half—approximately 50 million tons--were clay materials worth more than $1 billion. The petroleum industry uses kaolinite as a cracking catalyst in the refinement of petroleum, for triggering chemical reactions.

Sedimentary rocks that produce oil and gas by the heating of organic matter after it is buried are called source rocks. The source rock for most oil and gas is shale. By determining the type of diagenetic clay minerals present, the petroleum geologist can determine if the source rock has been heated to a temperature that is high enough to produce oil or gas. Rocks that contain oil or gas that can be easily extracted are called reservoir rocks. The best reservoir rocks are sandstones with a high porosity and permeability. Some sandstones contain clay minerals that occur between the sand grains as a cement or within the pores. Clay minerals have the potential to reduce the porosity and permeability of a reservoir. It is important to know the type and amount of clay minerals in a reservoir in order to evaluate its quality.

A knowledge of clays is also important to engineering because the concentration of clays in a soil determines it stability. Soils that contain high percentages of smectites could cause damage to the foundations of buildings because smectites swell when saturated with water and subsequently shrink when dried out.

Clays may be used as liners for sanitary landfills because the small grain size of clays allows them to be packed together very closely, forming an impermeable layer. The liner prevents toxic leachate (which forms when rainwater reacts with solid waste) from moving out of the landfill and contaminating surface water and groundwater supplies. Chemical engineers have developed what are called designer clays by altering the properties of naturally occurring clays. These clays act as catalysts in the breakdown of toxic substances to form less toxic products. Designer clays are helpful in the destruction and disposal of toxic wastes such as dioxin, and in the cleaning of existing toxic waste sites.

Clays are the basic raw material of the ceramics industry. When clays are mixed with water, they become plastic and are easily molded. A hard ceramic material is produced by firing the molded clay. In addition to the familiar pottery and dinnerware, fired clays are used in the production of brick, tiles, sewer pipes, sanitary ware pottery, kiln furniture, cement, and lightweight aggregates. Kaolinite is used as a coating on fine paper, in paints, and as a filler in plastics and rubber. Swelling clays such as smectite are used as binders in animal feed and iron ore pellets (taconite), drilling muds, industrial absorbents, and pet litter. Researchers are studying the use of clay in pharmaceutical products in the hope that clay can be used to stabilize drugs, meaning the clay will help them maintain the same properties over time.

Principal Terms

authigenic minerals: minerals that formed in place, usually by diagenetic processes

cation exchange capacity: the ability of a clay to adsorb and exchange cations, or positively charged ions, within its environment

chemical weathering: a change in the chemical and mineralogical composition of rocks by means of reaction with water at the earth’s surface

detrital minerals: minerals that have been eroded, transported, and deposited as sediments

diagenesis: the conversion of unconsolidated sediment into consolidated rock after burial by the processes of compaction, cementation, recrystallization, and replacement

hydrolysis: a chemical weathering process that produces clays by the reaction of carbonic acid with aluminosilicate minerals

phyllosilicate: a mineral with silica tetrahedra arranged in a sheet structure

shale: a sedimentary rock with a high concentration of clays

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