Kaolinite ( ; also called kaolin) is a clay mineral, having the chemical composition Al2Si2O5(OH)4. 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 linked through hydrogen bonds on the other side.Clay Minerals - Thompson Rivers University
Kaolinite is a soft, earthy, usually white, mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminium silicate minerals like feldspar. It has a low shrink–swell capacity and a low cation-exchange capacity (1–15 meq/100 g).
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 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,'U.S. Geological Survey, Mineral Commodity Summaries, January 2022' USGS, 2022. with a total market value of US $4.24 billion.'Kaolin Market Size, Share & Trends Analysis Report By Application, By Region And Segment Forecasts, 2022 - 2030. Grand View Research, 2022
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 ("High Ridge") 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 , a combination of (，, "stone") and ("marl"). 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 (T) sheet composed of silicon and oxygen ions bonded to an octahedral (O) 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, thus there are no large cations (such as calcium, sodium, or potassium) 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 (OH−) are lost from the kaolinite crystal structure in the form of water: the material cannot now be plasticised by absorbing water.'Ceramics Are More Than Clay Alone - Raw Materials, Products, Applications' P. Bormans. Cambridge International Science Publishing, 2004. pg. 180 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 (but not strictly crystalline) 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 (in cooler climates) or smectite (in drier climates) 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 (INERA) classification system, soils in which the clay fraction is predominantly kaolinite are called kaolisol (from kaolin and soil).
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 (1978) 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 (1978) 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 (1971) 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 (1960) 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 (1971), in a review on the formation of kaolinite, raised the fundamental question how a disordered material (i.e., the amorphous fraction of tropical soils) 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 (and pressure).
Low-temperature synthesis of clay minerals (with kaolinite as an example) 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 (Caillère et al., 1957; Caillère and Hénin, 1960; Wey and Siffert, 1962; Millot, 1970). 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." (Iler, 1955, p. 182)