How Is A Clay Formed?
What is Clay?
Clay is a fine-grained natural rock or soil material that contains minerals such as silica, alumina, and water, which gives clay plasticity properties. The size of clay particles is less than 002 mm in diameter. Clays develop plasticity when mixed with a limited amount of water, allowing them to be molded into shapes that retain their form when dry.
The composition of clay varies, but the main elements are silica, alumina, and water. The silica and alumina combine to form aluminium phyllosilicates, a major component of clay minerals. Other elements present in clays include iron, magnesium, sodium, potassium, and calcium. The amount and combination of these elements affect the properties and classification of the clay.
Some key properties of clay include plasticity, shrinkage, cohesion, adsorption and absorption, and durability. The plasticity allows it to be shaped, while the cohesion holds the shape. Clay shrinks when dried and hardens when fired, making it ideal for pottery and ceramics. It also readily adsorbs molecules and ions onto its surface and absorbs water into the spaces between particles.
There are many different types of clays based on composition and properties, such as kaolin, montmorillonite-smectite, illite, and chlorite. Clays have a wide variety of uses, notably in construction, agriculture, ceramics, paper, cosmetics, medicine, drilling, and waste containment.
Sources:
https://www.britannica.com/science/clay-mineral
https://en.wikipedia.org/wiki/Clay
Origins of Clay
Clay is formed naturally from the weathering of rocks over long periods of time. The primary method of clay formation is through the chemical weathering and breakdown of feldspar rocks.
Feldspar is a mineral found in igneous and metamorphic rocks like granite, gneiss and mica schist. It contains aluminum and silicon, which give rise to clay minerals when the feldspar crystals break down. This breakdown occurs through hydrolysis, where feldspar reacts with carbonic acid formed from rainwater and carbon dioxide in the atmosphere (1).
As feldspar weathers, it releases ions like potassium, sodium, calcium and silicates. The freed silica then combines with water molecules to produce hydrated aluminum silicates, which make up the clay minerals kaolinite, illite and smectite (1).
In addition to chemical weathering, physical weathering processes like cycles of freezing and thawing as well as plant root action help to disintegrate feldspar crystals. Over time, these processes decompose the feldspar in igneous rocks into clay sized particles (2).
Therefore, clay formation requires the chemical breakdown of rocks over thousands to millions of years. The weathering of feldspar is the primary mechanism for the natural development of clays.
(1) https://en.wikipedia.org/wiki/Clay
(2) https://www.katherinefortnumceramics.com/post/where-does-clay-come-from
Types of Clay
There are several different types of clay that are commonly used in pottery and ceramics. The most common types include:
Kaolinite – This type of clay is also known as “China clay” or “kaolin”. Kaolinite clays are white firing and contain the mineral kaolinite as their main component. According to Soul Ceramics, kaolinite clays are very plastic and paler than other clays. They are often used for porcelain and whitewares.
Montmorillonite – Montmorillonite clays contain montmorillonite as their main mineral. They are very absorbent and can expand when wet. According to Wheel and Clay, montmorillonite clays are extremely plastic and make excellent throwing clays. However, they can also shrink considerably during drying and firing.
Illite – Illite clays contain the mineral illite and are smooth and plastic. Pottery Crafters notes that illite clays are darker firing than kaolinite clays but can still produce white wares. They are commonly mixed with other clays.
Chamosite – Chamosite clays contain the mineral chamosite and small amounts of titanium oxide. According to Wheel and Clay, these clays fire to a buff or cream color. They are plastic but not extremely sticky when wet.
Clay Formation Process
Clay is formed through the chemical and physical weathering of rocks over long periods of time. This weathering breaks rocks down into finer and finer particles. The clay formation process involves four main stages: chemical weathering, physical weathering, transportation, and deposition.
Chemical Weathering
Chemical weathering is the process by which rock material disintegrates and decomposes as a result of chemical reactions. Water is the main driver of chemical weathering. Rainwater is slightly acidic due to carbon dioxide dissolved in it, which reacts with minerals in rocks like feldspar to produce clays. Hydrolysis and oxidation reactions slowly dissolve the rocks and change their composition.
Physical Weathering
Physical weathering includes processes that mechanically break down rocks into smaller pieces without changing their chemical composition. Examples include frost wedging, thermal expansion, root wedging, and abrasion. These processes physically break apart rock material into smaller fragments.
Transportation
The weathered rock particles are then transported by wind, water, ice, and gravity to new locations. Streams, rivers, glaciers, waves, and wind carry the weathered particles along with them. The smaller clay particles are carried farther than larger particles.
Deposition
When the transporting agents lose energy, they deposit the weathered rock particles in new locations. Clay settles in lakes, rivers, deltas, floodplains, and oceans. Over time, these clay deposits accumulate into thicker layers. Compaction and cementation transform the loose clay into sedimentary rock layers.
Chemical Weathering
Chemical weathering is the process by which rocks break down through chemical reactions. There are several different types of chemical weathering reactions that can occur:
Reaction with carbonic acid – Carbon dioxide from the atmosphere dissolves in water to form carbonic acid, which reacts with rocks like limestone. This reaction dissolves the rock and forms soluble products like calcium bicarbonate.
Hydrolysis – This occurs when water reacts with minerals to produce new compounds. For example, feldspar is altered to clay minerals through hydrolysis. According to Physical Geology, the chemical reaction is:
2KAlSi3O8 + 2H+ + 9H2O → Al2Si2O5(OH)4 + 4H4SiO4 + 2K+
Here, feldspar reacts with water and acids to produce kaolinite clay and soluble silicic acid.
Oxidation – This involves the addition of oxygen to compounds, often changing the state of iron from Fe2+ to Fe3+. A common example is the rusting of iron.
Hydration – Water molecules bond directly with minerals to create new hydrated compounds. For instance, anhydrite (CaSO4) combines with water to form gypsum (CaSO4 • 2H2O).
Physical Weathering
Physical weathering occurs when rocks and minerals break down without changing their chemical composition. Physical processes like ice wedging, wetting and drying cycles, temperature changes, and plant roots physically break apart rock into smaller pieces over time, forming clay. Some examples of physical weathering processes that lead to clay formation include:
Ice wedging happens when water seeps into cracks and crevices in rock, then expands as it freezes, causing the cracks to widen. Over many freeze/thaw cycles, this can break rock into smaller fragments. According to the U.S. Geological Survey (https://pubs.usgs.gov/info/clays/), cycles of freezing and thawing are an important physical weathering process in cold regions that creates rocks and minerals like clays.
Wetting and drying can also break apart rock. As water is absorbed into crevices and pores, it expands, then contracts as it dries. This repetitive process can gradually break rock into smaller pieces over time. Plants and trees help facilitate wet/dry cycles as their roots take in water, then release it as the plants transpire.
Temperature changes cause expansion and contraction in rock, inducing strain that can cause cracks and breakage. Daily and seasonal temperature fluctuations slowly physically weather rock over time. According to Tulane University (https://www2.tulane.edu/~sanelson/eens211/Weathering_ClayMinerals.pdf), diurnal expansions and contractions contribute to physical weathering processes.
Plant roots grow into cracks and crevices in rock, slowly wedging them open wider as the roots expand. Plants can physically break down rock and minerals into smaller pieces, helping to create clays.
Transportation
Clay is transported from its original source to new locations through various natural processes. The main methods of clay transportation include moving water, wind, gravity, and glaciers. https://www.cargohandbook.com/Clays
Flowing water is one of the most significant means of clay transportation. Streams, rivers, and ocean currents can carry clay particles in suspension over long distances. The flow velocity of the water determines how large the clay particles can be to stay suspended. Faster currents can hold larger particles.
Wind is another key transportation method for clay. Strong winds can pick up and carry fine clay particles long distances in the air in a process known as suspension. The particles are held aloft by turbulence until the wind slows and they settle out by gravity.
Gravity transports clay material downslope as particles are dislodged and slide down mountains, cliffs, and slopes. Gravity-driven processes include mass movements like landslides, debris flows, rockfalls, and creep. Gravity transportation results in clay deposits at the base of slopes.
Glaciers can also transport clay as fine particles or larger fragments held in the ice. Clay material gets picked up as glaciers erode their beds and gets carried within or on top of the ice as the glacier moves. When the ice melts, the clay is deposited in the remaining sediments.
Deposition
Clay particles are deposited when the transporting agents, such as water, wind, ice, or gravity, can no longer carry them. Deposition occurs in still water such as lakes, swamps, lagoons, and seas through the process of sedimentation or settling from suspension. This allows clay particles to settle out based on differences in grain size and density (USGS).
Alluvial deposits occur when rivers deposit clay sediments on river beds, floodplains, and deltas. Over time, these deposits accumulate to form alluvial clay deposits (USGS). Lacustrine deposits form in lake bottoms and inland basins from the settling of clay and other sediments. These deposits can become thick clay beds over time.
Clays will continue to accumulate in depositional environments as long as weathering and erosion transports clay minerals and as long as still water bodies and conditions exist for the clays to settle out of suspension.
Clay Distribution
Clay is found all over the world, but its deposits tend to be concentrated in certain geographic regions and types of geologic settings. The highest concentrations of clay are found in tropical and subtropical regions. This is because warmer and wetter climates accelerate the chemical weathering processes that form clay minerals.
There are large deposits of kaolinite and smectite clays in tropical locations such as Brazil, central Africa, India, and Indonesia [1]. These regions have been coined “clay belts” due to the significant clay deposits. The Amazon basin is known for containing some of the largest kaolinite resources in the world.
Clay deposits have many industrial uses. Kaolinite is a key ingredient in paper, paint, rubber, and ceramics manufacturing. Bentonite has applications in drilling muds, foundry sands, iron ore pelletizing, and litter due to its absorbent properties. The availability of clay in a region often shapes local economies and industries.
Clay Uses
Clay is an incredibly useful material that has a wide variety of applications. Some of the most common uses of clay are in ceramics, construction materials, paper, absorbents, drilling, and medicine.
One of the best-known uses of clay is in ceramics. Different types of clay are essential for making pottery, both decorative and utilitarian. Clay is molded and fired into dishes, vases, bricks, tiles, and other objects. Examples include stoneware, porcelain, and earthenware pottery. Clay-based ceramics are valued for their durability, thermal properties, and aesthetic qualities.1
In construction, clay is a vital ingredient in materials like brick, cob, adobe, and rammed earth. Structures built from clay materials include houses, buildings, walls, floors, and more. Clay construction materials provide benefits like fireproofing, thermal mass, and sustainability.2
Clay is used in the paper industry as both a filler and a coating. Adding clay makes paper brighter, smoother, glossier and more opaque. It improves printability. Clay coatings allow high-quality printing with minimal ink.1
Clay absorbents have applications ranging from cat litter to oil-spill cleanups. The absorbent properties of clay materials make them ideal for soaking up messes and waste liquids.
In drilling, clays are important components of drilling mud. The mud lubricates drill bits, removes cuttings, and forms a cake to support well walls.
Medicinally, clays can be used externally and internally. Kaolin clay is the active ingredient in many skin care products. Bentonite clay has been used to treat gastrointestinal issues. Some clays have antimicrobial and toxin absorbing properties.
