Kaolinite ( KAY-ə-lə-nyte, -lih-; also called kaolin) is a clay mineral, with the chemical composition Al2Si2O5(OH)4. It is a layered silicate mineral, with one "tetrahedral" sheet of silicate tetrahedra (SiO4) linked to one "octahedral" sheet of aluminate octahedrons (AlO2(OH)4) 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 (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 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 高嶺土, now romanized as gāolǐngtǔ 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 lithomarga, a combination of litho- (Ancient Greek: λίθος，líthos, "stone") and marga ("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 Al2Si2O5(OH)4, however, in ceramics applications the same formula is typically written in terms of oxides, thus giving Al2O3·2SiO2·2H2O.
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 and so 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. 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 900 °C (1,650 °F). 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 (SiO2) and alumina (Al2O3), but rather a complex amorphous structure that retains some longer-range order (but not strictly crystalline) due to stacking of its hexagonal layers.
Al2Si2O5(OH)4 → Al2Si2O7 + 2 H2O
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:
2 Al2Si2O7 → Si3Al4O12 + SiO2
Platelet mullite Upon calcination above 1050 °C, the spinel phase nucleates and transforms to platelet mullite and highly crystalline cristobalite:
3 Si3Al4O12 → 2 [ 3 Al2O3 · 2 SiO2 ] + 5 SiO2
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