Clays are a group of clay minerals that are generated by processes of erosion, digenesis, and weathering, and are therefore the main components of fine-grained sedimentary rocks called mudstones (including shales, siltstones, and shale).
In fact, clay minerals make up about 40% of the components in sedimentary rocks. In addition, clay minerals are the main component of soils.
Therefore, understanding clays is important from an engineering point of view, as some minerals expand significantly when exposed to water.
It is also important in the area of agronomy because they contain a large amount of nutrients and also play an important role in the transmission of water in the soil.
Furthermore, clay minerals are widely used in the ceramic industry and are therefore important economic minerals.
Clay characteristics
The term clay is somewhat ambiguous unless specifically defined, because it is used in three ways:
- They are a diverse group of fine-grained minerals.
- Sometimes taken as a rock term
- And also clays it is said as a size of clay
As a rock term, clay is difficult to define due to the great variety of materials that compose it; therefore, the definition must be general.
Grim (1962) defined clay as a natural, earthy, loamy, fine-grained material.
The particle size of clays is very fine and is generally considered by most clay scientists to be about 2 μm or less.
These minerals are hydrated silicates composed primarily of silica, alumina, and water.
Several of these minerals also contain appreciable amounts of magnesium, iron, alkalis, and alkaline earths.
Many definitions state that a clay is plastic when wet. Most clay materials have this property, but some clays do not have plastic behaviour, such as halloysite and flint clay.
As a particle size term, clays are used for the category that includes the smallest particles.
The maximum size particles in the clay size grade are defined differently at various grade scales.
Soil researchers and mineralogists generally use 2 μm (micrometers) as the maximum size, while the scale widely used by engineers (Wentworth 1922) defines clay as a material finer than about 4 μm (micrometers).
Clays are an abundant natural raw material and have an amazing variety of uses and properties, which are discussed in this article.
Most important clay minerals
In this section we show you the most common clay minerals:
| clay minerals |
| Kaolin |
| illite |
| smectite |
| Bentonite |
| palygorskite |
| sepiolite |
| Chlorites |
| kaolinite |
| Montmorillonite |
| dickite |
| hematite |
| goethite |
Classification and types of clays
The clays They are classified according to their usefulness and composition in:
- ball clays
- Bentonite
- Common clays and shales
- Kaolin
- Paligorskite and sepiolite (hormites)
- fuller lands
- refractory clays
According to their color the most common are:
- white clays
- green clays
- red clays
Physical and chemical properties of clays.
The common features of all clay minerals are derived from their chemical composition, structure, and size.
The clay minerals they have a great affinity for water. Some swell easily and can double in thickness when wet.
Most have the ability to absorb ions (electrically charged atoms and molecules) from a solution and release the ions later when conditions change.
Water molecules are strongly attracted to clay mineral surfaces.
When a little clay is added to water, a slurry is formed because the clay is evenly distributed throughout the water.
The paint industry uses this property of clay to disperse the pigment (color) evenly throughout the paint.
Without clays to act as a carrier, it would be difficult to evenly mix the paint base and color pigment.
A mixture of a lot of clay and a little water results in a mud that can be molded and dried to form a relatively rigid solid.
Potters and the ceramics industry exploit this property to produce plates, cups, bowls, pipes, etc.
Environmental industries use these two properties to produce homogeneous liners for waste containment.
The process by which some clay minerals swell when they absorb water is reversible.
Expansive clays expand or contract in response to changes in environmental factors (wet and dry conditions, temperature).
Hydration and dehydration can change the thickness of a single clay particle by almost 100 percent.
Homes, offices, schools, and factories built on soils containing the expansive clays may be subject to structural damage caused by seasonal swelling of the clay portion of the soil.
Another important property of clay minerals is the ability to exchange ions, and it is related to the charged surface of clay minerals.
Ions can be attracted to the surface of a clay particle or absorbed within the structure of these minerals.
The property of clay minerals that causes ions in solution to bind to clay surfaces or within internal sites applies to all types of ions, including organic molecules such as pesticides.
Clays can be an important vehicle for transporting and widely dispersing pollutants from one area to another.
How and where clay deposits form
Clay occur in a fairly limited range of geological conditions.
Formation environments include soil horizons, continental and marine sediments, geothermal fields, volcanic deposits, and weathered rock formations.
Most clay minerals form where rocks are in contact with water, air, or steam.
Examples of these situations include eroded rocks on a hillside, sediments on the sea or lake bottom, deeply buried sediments containing pore water, and rocks in contact with water heated by magma (molten rock).
All of these environments can cause clay minerals to form from pre-existing minerals.
Extensive alteration of rocks to clay minerals can produce relatively pure clay deposits that are of economic interest (for example, bentonites, mainly montmorillonites, used for drilling muds and clays used in ceramics).
| Erosion | The transport and deposition of clay minerals produced by eroding older continental and marine rocks and soils are important parts of the cycle that forms sedimentary rocks. The ancient sedimentary rock record is composed of about 70 percent shales (containing about 50 percent clay-size fragments) and siltstones (which are thicker than shales but may contain clay-size particles) ( Blatt et al., 1980). Today, sedimentary environments containing muds and clays cover about 60 percent of continental sea shelves and 40 percent of deep ocean basins. Inland aquatic environments such as lakes, rivers, estuaries, and deltas also contain high proportions of fine-grained sediments. Clearly, clays are critical components of ancient and modern sedimentary environments. |
| Diagenesis | Diagenesis is the in situ alteration of a mineral to more stable forms, excluding surface alteration (which is weathering). Diagenesis occurs, for example, when stable minerals in one depositional environment are exposed to another by burial and compaction. Common silicate materials such as quartz, feldspars, and volcanic glasses, as well as carbonates, non-crystalline iron oxides, and primary clay minerals, are transformed during diagenesis into more stable clay minerals, primarily by dissolution and recrystallization. The formation of bentonite (beds containing clay minerals of the smectite group, including montmorillonite) and fuller’s earth (a type of clay mineral deposit that has a high capacity to absorb water) can occur mainly by diagenesis, although some Deposits can also be formed by hydrothermal processes. Bentonite beds generally form from disturbed volcanic ash, but other rock types can also serve as sources. The absorption properties of bentonites and fuller’s earth make them ideal for uses as diverse as drilling mud; foundry-sand bond; binder for pelletizing iron ores and bleaching fluids; absorbents for oils, greases, and animal waste; and carriers of pesticides and fertilizers. Bentonite is also used as a floor coating for environmental containment applications and with polyacrylamide to make paper. |
| weathering | Rock and soil weathering is the primary way that clay minerals are formed on Earth’s surface today. The weathering process involves physical breakdown and chemical decomposition that changes the original minerals to clay minerals. Weathering is uneven, and many stages of decomposition can be found in the same clay sample. Factors governing rock weathering and soil formation include initial rock type, water to rock ratio, temperature, presence of organisms and organic material, and amount of time. The types of clay minerals found in weathered rocks strongly control how weathered rock behaves under various climatic conditions (such as humid, tropical, tropical, and temperate conditions). Kaolinite is found in most weathered zones and soil profiles. Chemically more complex than kaolinites, montmorillonites are common in the lower parts of weathering profiles, closer to the rock, where chemistry exerts a strong control over mineralogy. Complex mixed-layer clay minerals (such as illite-smectites) are abundant in clay assemblages that develop from mica-bearing parent rocks, such as granite plutons found in temperate regions of the Northeastern United States. For example, a large component of soils formed by the weathering of granites may consist of metastable muscovite, biotite, and chlorite. These minerals will progressively alter to clay minerals. |