Kaolin is a clay rock and part of the group of industrial minerals with the chemical composition (Al2Si205 (OH)4.
It is a layered silicate mineral with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet alumina i.e. structurally composed of silicate sheet (Si2O5) bonded to the aluminum oxide/hydroxide layer Al2 (OH)4 called gibbsite layers and repeating layer of the mineral are hydrogen bonded together. (Rost, 1992; Bish, 1993; Klein and Kuribut, 1993; Slivka, 2002).
Kaolin is a plastic raw material, particular consisting of clay mineral kaolinite. In systematic mineralogy, Kaolin ranks among phyllosilicates, which are stratified clay minerals formed by a net of tetrahedral and octahedral layers. Phyllosillicates are classified into the main groups according to the type of layers, inter-layer contents, charge of the layers and chemical formulae. Besides kaolinite groups, serpentine, halloysite, pyrofylite, mica and montmorillonite groups also rank among phylllosillicates. Group of kaolinites includes di-octahedral mineral with two layers and one silica (SiO4) tetrahedral layer and one aluminum (Al2(OH)4) octahedral layer. The layers are bondecd together by sharing oxygen anion between Al and Si together, these two layers are called platelets (Pauk,et al.,1962; Stejskal, 1971., Duda et al., and Hurlbut, 1993).
Kaolinite share the same chemistry as the mineral halloysite, dickeite and necrite. The four minerals are polymorphs as they have the same chemistry but different structures. All the minerals were derived from chemical alteration of aluminum-rich silicate minerals, such as feldspars. However, they could be found as sedimentary deposits as well as hydrothermal alteration product of rocks containing a high of alumino-silicate minerals.
Kaolin is formed under acidic conditions through weathering or hydrothermal change of feldspars, and to a lower extent also other weathered kaolin deposits, kaolin clay or maybe a compound of kaolinite, sandstones and olitic ironstones, and less frequently also of pegmatite and hydrothermal deposit. The most significant kaolin deposits were formed through intensive weathering of rock rich in feldspars (granite, arkoses, certain types of ortho-gneisses and migmatites).