What Is Considered Clay?

Definition of Clay

Clay is a fine-grained natural rock or soil material that combines minerals with traces of metal oxides and organic matter (Britannica). It is made up of tiny crystalline grains with diameters less than 0.005 millimeters (mm). These microscopic clay particles pack together to form clay, a plastic and malleable material when wet but can be hardened through heating or drying (Wikipedia).

Clay Composition

Clay is composed primarily of phyllosilicate minerals, which have a sheet-like structure. The main phyllosilicate minerals in clay are kaolinite, montmorillonite-smectite, illite, and chlorite (Ouyang et al., 2021).

Kaolinite is the most abundant clay mineral, making up 50-65% of many clay deposits. It has a 1:1 layered structure consisting of silica tetrahedron and alumina octahedron sheets. Montmorillonite-smectite has a 2:1 layered structure with two silica tetrahedron sheets sandwiching an alumina octahedron sheet. The montmorillonite-smectite group includes sodium and calcium montmorillonites. Illite also has a 2:1 layered structure but contains potassium ions between its layers. Chlorite has a more complex 2:1:1 layered structure with an additional brucite-like sheet (Sturz, 2000).

The specific clay mineral composition depends on the parent rock material and weathering conditions during formation. For example, highly weathered tropical soils tend to be dominated by kaolinite while less weathered soils contain more smectite and illite (Ouyang et al., 2021).

Clay Formation

Clay is formed over long periods by the gradual chemical weathering of rocks, usually silicate-bearing rocks like granite (USGS). The process starts when rocks are exposed to air, water, acids, or other weathering agents. These erode the rocks, releasing their constituent chemicals. The released silica then combines with water and other minerals to start forming clays. Over time, clays accumulate in deposits through processes like sedimentation from erosion and runoff. Ultimately, given enough time (geological time spans of hundreds to thousands of years), large clay deposits form from the gradual chemical weathering of rocks (Glendale).

Physical Properties

Some key physical properties of clay include plasticity, cohesiveness, shrinkage, hardness, and flexibility when dry. Plasticity refers to the ability of clay to be molded or shaped without cracking or crumbling (“Properties of clay”). The clay particles bond together when wet, allowing the clay to be shaped into various forms. Cohesiveness describes how well clay holds together and resists cracking and crumbling. The cohesive nature of clay also makes it sticky or gluey when wet.

Clay tends to shrink when it dries due to the loss of water between the clay particles. The shrinkage capacity depends on the clay composition. Shrinkage can result in cracks in clay structures. When dry, clay is quite hard, but still flexible if bent or impacted. The flexibility prevents the clay from shattering when bent or hit (Mukherjee). The hardness provides structure, while the flexibility when dry prevents brittleness and cracking.

Chemical Properties

Clay minerals have some unique chemical properties that differentiate them from other soil components. Three of the most notable chemical properties are cation exchange capacity, hydration, and absorption.

Cation exchange capacity (CEC) refers to the ability of clays to adsorb cations, which are positively charged ions. Clays have a net negative surface charge, allowing them to attract and hold positively charged nutrient cations like calcium, magnesium, and potassium ^[1]. This provides nutrients for plants and also prevents cations from being leached away by water.

Clays also readily absorb water molecules into their crystal structure through hydration. The layers in clay minerals can absorb water in between the sheets, causing them to expand. This allows clays to hold water and nutrients. Hydration also facilitates cation exchange ^[2].

In addition to hydration, clays are able to absorb other polar molecules like proteins, dyes, and drugs. Their charged surfaces give clays excellent absorption capabilities. This property allows clays to be used as industrial absorbents and molecular sieves.

Clay Classifications

There are four main types of clay minerals that constitute the majority of clay deposits. These include kaolinite, smectite (including montmorillonite and bentonite), illite, and chlorite.

Kaolin is one of the most common clay minerals. It is relatively pure clay formed by chemical weathering of aluminium silicate minerals like feldspar. Kaolin clays are characterized by low plasticity and also used in the production of porcelain. Some key properties of kaolin clays include high brightness and whiteness, fine particle size, and chemical inertness.

Smectites like montmorillonite and bentonite consist of hydrated aluminum silicates. They have high cation exchange capacity and can absorb large amounts of water, leading to significant swelling behavior. Smectites are commonly used as drilling muds and foundry sand binders. Montmorillonite in particular readily adsorbs ions and polar organic compounds.

Illite is a nonexpanding dioctahedral mica-type layered silicate with a micaceous structure. The particle size is very small, giving illite significant surface area. Common uses include in ceramics, as a filler, and as a constituent in drilling muds. Illite clay can be transformed into smectite through ion exchange reactions.

Ball clays are kaolinitic sedimentary clays with high plasticity. Their name refers to forming a ball when moist. Ball clays are composed mainly of kaolinite but with some smectite as well. They are valued for their whiteness and high strength when fired.

Fire clays are refractory kaolinitic clays that can withstand very high temperatures before deforming. Their refractory properties make them ideal for applications like firebricks and ceramic kiln linings.

Uses of Clay

Clay is an extremely versatile material that has been used by humankind for thousands of years. Some of the most common uses of clay minerals include:

Ceramics and Pottery: Clay is the primary material used in ceramics and pottery. Different clay minerals provide plasticity and harden at high firing temperatures to produce ceramics that are strong, durable, and impermeable. According to Geology Science, clay is essential for making porcelain and china tableware, sanitaryware, and ceramic tiles and pipes [1].

Bricks: Clay bricks have been used extensively in construction for millennia and continue to be a popular building material today. Bricks made from clays rich in kaolinite are ideal because of their high alumina content which prevents shrinkage and cracking when fired [1].

Cement: Clay is one of the raw materials used in the production of Portland cement. The alumina content of clay controls the rate of hardening and strength development in the cement [1].

Paper: Kaolinite clay is used as a filler and coating pigment in the production of high quality paper. It improves opacity, printability, and brightness [1].

Cat Litter: Clays such as bentonite are excellent absorbents and are widely used as cat litter.

Skin Care: Clays have been used topically for skin care due to their absorbing and adsorbing properties. According to MDedge Dermatology, the most commonly used clays in cosmetics and dermatologic products are kaolin and bentonite [2].

Drilling Fluids: Smectite clays such as bentonite help control viscosity and gel strength in drilling muds used to lubricate drill bits and transport cuttings up boreholes [1].

Clay vs Silt

Clay and silt represent the smallest particle sizes found in soil. However, clay particles are significantly smaller than silt particles.

Clay particles measure less than 0.002 mm in size, while silt particles range from 0.002 mm to 0.05 mm. This means that clay particles are over 20 times smaller than the largest silt particles.

The tiny size of clay particles gives clay unique properties compared to silt. Clay feels sticky and malleable when wet, while silt feels smooth. Clay also holds more water than silt.

The small size of clay particles also means clay has a much larger specific surface area than silt. This surface area difference allows clay to adsorb and hold onto more chemical compounds and nutrients than silt.

Due to the difference in particle size, clay and silt behave differently when mixed with water. Clay forms a colloid, while silt simply settles out. This settling property allows silt particles to be physically separated from clay using sedimentation techniques.

Overall, the smaller size of clay particles leads to very different physical and chemical properties compared to silt. This size contrast makes it possible to distinguish clay from silt using simple tests.

Clay vs Soil

Clay is a specific component of soil. Soil is made up of a mixture of materials including clay, sand, silt, organic matter, air and water. The percentage of clay present in soil can vary greatly depending on the location. Clay particles are the smallest of the three main soil components. Clay particles are less than 0.002 mm in size, which allows them to pack tightly together to form dense, heavy, cohesive soils.

Some key differences between clay and soil:

• Clay contains very small mineral particles that give it plasticity when wet and hardness when dry.
• Soil is a mixture of materials including clay, sand, silt, organic matter, air and water.
• Clay holds nutrients and water much better than sandy soils due to the small size of its particles.
• Soil can vary greatly in its composition and properties based on environmental factors.
• Clay is smooth and sticky when wet, while soil can be loose and crumbly when dry.

In summary, clay is an important and unique component of soil due to its fine texture, mineral composition and ability to retain moisture and nutrients. The amount of clay present has a significant impact on the overall properties and fertility of a soil.

Sources:
https://hinative.com/questions/14479081
https://m.facebook.com/reel/1003379157337976/?locale=mk_MK

Interesting Facts

Clay has been an important material since the beginnings of human civilization. Here are some fascinating facts about the history and uses of clay:

Early humans discovered that clay could be molded into pottery vessels over 14,000 years ago. Some of the earliest pottery artifacts made from clay have been found in East Asia, dating back 18,000-20,000 years. https://pottery-nyc.com/fun-facts-about-pottery-and-ceramics

Clay tablets were one of the earliest forms of writing, used as early as 3200 BC in ancient Mesopotamia. Important writings like the Epic of Gilgamesh were recorded on clay tablets. https://justfunfacts.com/interesting-facts-about-clay/

Clay is beneficial for skin and health because of its excellent absorption properties. It can draw out toxins, absorb excess oil, and help purify skin. Many natural skin care products today contain clay. https://www.ehow.com/info_8100576_interesting-clay-soil.html