Kaolinite

Kaolinite is a clay mineral, with the chemical composition Al₂Si₂O₅₄. It is a layered silicate mineral, with one "tetrahedral" sheet of silicate tetrahedra linked to one "octahedral" sheet of aluminate octahedrons through oxygen atoms on one side, and another such sheet through hydrogen bonds on the other side.
Kaolinite is a soft, earthy, usually white, mineral, produced by the chemical weathering of aluminium silicate minerals like feldspar. It has a low shrink–swell capacity and a low cation-exchange capacity.
Rocks that are rich in kaolinite, and halloysite, are known as kaolin or china clay. In many parts of the world kaolin is colored pink-orange-red by iron oxide, giving it a distinct rust hue. Lower concentrations of iron oxide yield the white, yellow, or light orange colors of kaolin. Alternating lighter and darker layers are sometimes found, as at Providence Canyon State Park in Georgia, United States.
Kaolin is an important raw material in many industries and applications. Commercial grades of kaolin are supplied and transported as powder, lumps, semi-dried noodle or slurry. Global production of kaolin in 2021 was estimated to be 45 million tonnes, with a total market value of US $4.24 billion.

Names

The English name kaolin was borrowed in 1727 from François Xavier d'Entrecolles's 1712 French reports on the manufacture of Jingdezhen porcelain. D'Entrecolles was transcribing the Chinese term, now romanized as in pinyin, taken from the name of the village of Gaoling near Ehu in Fuliang County, now part of Jiangxi Province's Jingdezhen Prefecture. The area around the village had become the main source of Jingdezhen's kaolin over the course of the Qing dynasty. The mineralogical suffix -ite was later added to generalize the name to cover nearly identical minerals from other locations.
Kaolinite is also occasionally discussed under the archaic names lithomarge and lithomarga from Latin lithomarga, a combination of litho- and marga. In more proper modern use, lithomarge now refers specifically to a compacted and massive form of kaolin.

Chemistry

Notation

The chemical formula for kaolinite as written in mineralogy is, however, in ceramics applications the same formula is typically written in terms of oxides, thus giving.

Structure

Compared with other clay minerals, kaolinite is chemically and structurally simple. It is described as a 1:1 or TO clay mineral because its crystals consist of stacked TO layers. Each TO layer consists of a tetrahedral sheet composed of silicon and oxygen ions bonded to an octahedral sheet composed of oxygen, aluminium, and hydroxyl ions. The T sheet is so called because each silicon ion is surrounded by four oxygen ions forming a tetrahedron. The O sheet is so called because each aluminium ion is surrounded by six oxygen or hydroxyl ions arranged at the corners of an octahedron. The two sheets in each layer are strongly bonded together via shared oxygen ions, while layers are bonded via hydrogen bonding between oxygen on the outer face of the T sheet of one layer and hydroxyl on the outer face of the O sheet of the next layer.
A kaolinite layer has no net electrical charge and so there are no large cations between layers as with most other clay minerals. This accounts for kaolinite's relatively low ion exchange capacity. The close hydrogen bonding between layers also hinders water molecules from infiltrating between layers, accounting for kaolinite's nonswelling character.
When moistened, the tiny platelike crystals of kaolinite acquire a layer of water molecules that cause crystals to adhere to each other and give kaolin clay its cohesiveness. The bonds are weak enough to allow the plates to slip past each other when the clay is being molded, but strong enough to hold the plates in place and allow the molded clay to retain its shape. When the clay is dried, most of the water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile. If the clay is moistened again, it will once more become plastic.

Structural transformations

Kaolinite group clays undergo a series of phase transformations upon thermal treatment in air at atmospheric pressure.

Milling

High-energy milling of kaolin results in the formation of a mechanochemically amorphized phase similar to metakaolin, although the properties of this solid are quite different. The high-energy milling process is highly inefficient and consumes a large amount of energy.

Drying

Below 100 °C, exposure to low humidity air will result in the slow evaporation of any liquid water in the kaolin. At low moisture content the mass can be described leather dry, and at near 0% moisture it is referred to as bone dry.
Above 100 °C any remaining free water is lost. Above around 400 °C hydroxyl ions are lost from the kaolinite crystal structure in the form of water: the material cannot now be plasticised by absorbing water. This is irreversible, as are subsequent transformations; this is referred to as calcination.

Metakaolin

Endothermic dehydration of kaolinite begins at 550–600 °C producing disordered metakaolin, but continuous hydroxyl loss is observed up to. Although historically there was much disagreement concerning the nature of the metakaolin phase, extensive research has led to a general consensus that metakaolin is not a simple mixture of amorphous silica and alumina, but rather a complex amorphous structure that retains some longer-range order due to stacking of its hexagonal layers.

Spinel

Further heating to 925–950 °C converts metakaolin to an aluminium-silicon spinel which is sometimes also referred to as a gamma-alumina type structure:

Platelet mullite

Upon calcination above 1050 °C, the spinel phase nucleates and transforms to platelet mullite and highly crystalline cristobalite:

Needle mullite

Finally, at 1400 °C the "needle" form of mullite appears, offering substantial increases in structural strength and heat resistance. This is a structural but not chemical transformation. See stoneware for more information on this form.

Occurrence

Kaolinite is one of the most common minerals; it is mined, as kaolin, in Australia, Brazil, Bulgaria, China, Czech Republic, France, Germany, India, Iran, Malaysia, South Africa, South Korea, Spain, Tanzania, Thailand, United Kingdom, United States and Vietnam.
Mantles of kaolinite are common in Western and Northern Europe. The ages of these mantles are Mesozoic to Early Cenozoic.
Kaolinite clay occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates; for example in tropical rainforest areas. Comparing soils along a gradient towards progressively cooler or drier climates, the proportion of kaolinite decreases, while the proportion of other clay minerals such as illite or smectite increases. Such climatically related differences in clay mineral content are often used to infer changes in climates in the geological past, where ancient soils have been buried and preserved.
In the National Institute of Agronomic Studies and Research classification system, soils in which the clay fraction is predominantly kaolinite are called kaolisol.
In the United States, the main kaolin deposits are found in central Georgia, on a stretch of the Atlantic Seaboard fall line between Augusta and Macon. This area of thirteen counties is called the "white gold" belt; Sandersville is known as the "Kaolin Capital of the World" due to its abundance of kaolin. In the late 1800s, an active kaolin surface-mining industry existed in the extreme southeast corner of Pennsylvania, near the towns of Landenberg and Kaolin, and in what is present-day White Clay Creek Preserve. The product was brought by train to Newark, Delaware, on the Newark-Pomeroy line, along which can still be seen many open-pit clay mines. The deposits were formed between the late Cretaceous and early Paleogene, about 100 to 45 million years ago, in sediments derived from weathered igneous and metakaolin rocks. Kaolin production in the United States during 2011 was 5.5 million tons.
During the Paleocene–Eocene Thermal Maximum sediments deposited in the Espluga Freda area of Spain were enriched with kaolinite from a detrital source due to denudation.

Synthesis and genesis

Difficulties are encountered when trying to explain kaolinite formation under atmospheric conditions by extrapolation of thermodynamic data from the more successful high-temperature syntheses. La Iglesia and Van Oosterwijk-Gastuche thought that the conditions under which kaolinite will nucleate can be deduced from stability diagrams, based as they are on dissolution data. Because of a lack of convincing results in their own experiments, La Iglesia and Van Oosterwijk-Gastuche had to conclude, however, that there were other, still unknown, factors involved in the low-temperature nucleation of kaolinite. Because of the observed very slow crystallization rates of kaolinite from solution at room temperature Fripiat and Herbillon postulated the existence of high activation energies in the low-temperature nucleation of kaolinite.
At high temperatures, equilibrium thermodynamic models appear to be satisfactory for the description of kaolinite dissolution and nucleation, because the thermal energy suffices to overcome the energy barriers involved in the nucleation process. The importance of syntheses at ambient temperature and atmospheric pressure towards the understanding of the mechanism involved in the nucleation of clay minerals lies in overcoming these energy barriers. As indicated by Caillère and Hénin the processes involved will have to be studied in well-defined experiments, because it is virtually impossible to isolate the factors involved by mere deduction from complex natural physico-chemical systems such as the soil environment.
Fripiat and Herbillon, in a review on the formation of kaolinite, raised the fundamental question how a disordered material could ever be transformed into a corresponding ordered structure. This transformation seems to take place in soils without major changes in the environment, in a relatively short period of time, and at ambient temperature.
Low-temperature synthesis of clay minerals has several aspects. In the first place the silicic acid to be supplied to the growing crystal must be in a monomeric form, i.e., silica should be present in very dilute solution. In order to prevent the formation of amorphous silica gels precipitating from supersaturated solutions without reacting with the aluminium or magnesium cations to form crystalline silicates, the silicic acid must be present in concentrations below the maximum solubility of amorphous silica. The principle behind this prerequisite can be found in structural chemistry: "Since the polysilicate ions are not of uniform size, they cannot arrange themselves along with the metal ions into a regular crystal lattice."
The second aspect of the low-temperature synthesis of kaolinite is that the aluminium cations must be hexacoordinated with respect to oxygen. Gastuche et al. and Caillère and Hénin have concluded that kaolinite can only ever be formed when the aluminium hydroxide is in the form of gibbsite. Otherwise, the precipitate formed will be a "mixed alumino-silicic gel". If it were the only requirement, large amounts of kaolinite could be harvested simply by adding gibbsite powder to a silica solution. Undoubtedly a marked degree of adsorption of the silica in solution by the gibbsite surfaces will take place, but, as stated before, mere adsorption does not create the layer lattice typical of kaolinite crystals.
The third aspect is that these two initial components must be incorporated into one mixed crystal with a layer structure. From the following equation for kaolinite formation
it can be seen that five molecules of water must be removed from the reaction for every molecule of kaolinite formed. Field evidence illustrating the importance of the removal of water from the kaolinite reaction has been supplied by Gastuche and DeKimpe. While studying soil formation on a basaltic rock in Kivu, they noted how the occurrence of kaolinite depended on the "degrée de drainage" of the area involved. A clear distinction was found between areas with good drainage and those areas with poor drainage. Kaolinite was only found in the areas with distinct seasonal alternations between wet and dry. The possible significance of alternating wet and dry conditions on the transition of allophane into kaolinite has been stressed by Tamura and Jackson. The role of alternations between wetting and drying on the formation of kaolinite has also been noted by Moore.