Kaolinite

Kaolinite is a common 1:1 dioctahedral phyllosilicate (clay) mineral found throughout the world in highly-weathered environments. Being a 1:1 mineral, it has one silica tetrahedral layer and one aluminum octahedral layer. Individual layers are held together in a crystal by O - H - O bonds between the octahedral layer of one crystal and the tetrahedral layer of the one above or below.

The crystallography of kaolinite played an important role in Linus Pauling's formulation of the nature of the chemical bond, although for 15 years kaolinite was thought to be monoclinic (crystallographic axes all equal to 90°) instead of triclinic. Crystallographic axes: alpha, 91.8°; beta 104.7°; gamma, 90°).

The crystal structure displayed to the left, including all H atoms, is based on low-temperature (1.5° K) neutron powder diffraction data (Bish, 1993) instead of the more common x-ray diffraction data that has been used since the time of Linus Pauling's original determination of the crystal structure of kaolinite in 1930.

Highlighting Features

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Plane of Si atoms
Single silica tetrahedron
Silica tetrahedral layer
Plane of Aluminum (Al) atoms
Single aluminum octahedron
Aluminum octahedral layer
Single unit cell
Show all atoms

Kaolinite normally is present as stacks of kaolinite sheets, usually with an overall hexagonal appearance. View a larger, 3-sheet crystal of kaolinite.

Distribution

Kaolinite is widely distributed in nature. Pure deposits are mined for porcelain, both bathroom fixtures and fine China cups. Most soils of the world contain kaolinite in the clay size fraction (<2µm). In highly weathered soils, such as those of Southeastern U.S. and tropical regions of Africa, Asia, and S. America, kaolinite is usually the dominant clay mineral because of its relative resistance to chemical weathering.

Crystallographic Data