Kaolinite has a distorted 1M sequence of layers with the octahedral vacancy said to be at the B site in every layer. The O..H vectors of the surface OH groups are quasinormal to (001), and that of the inner OH is parallel to (001) in space group C1. The major defects present in kaolinite are stacking mistakes that are related to the usual −a1/3 interlayer shifts by a pseudo-mirror plane along the diagonal of the unit cell base plus the presence of a few vacancies in the C sites. Single crystals are always twinned by ±120° rotations about the cleavage normal to create equal volumes of three domains of different orientations. These may form by random adoption of sites A,B,C as the vacancy during initial growth, followed by distortion of each domain to triclinic geometry. After distortion, the vacancy appears to be at the same site in each domain when using conventional triclinic axes, but with the domains rotated by 120°. The designation of B as the usual vacant site by recent authors is considered an error in nomenclature, with the vacancy really in the C site.
Dickite has the same 1M stacking sequence of layers as kaolinite, but the vacancy alternates regularly between B and C. Nacrite has the R sequence of layers. The octahedral vacancy alternates regularly between B and C, but every other octahedral sheet is rotated 180°. The pattern of vacancies reduces the symmetry to monoclinic and allows selection of an inclined Z axis along which there is a 2-layer repeat.
Halloysite has mainly irregular layer stacking but with a limited tendency for 2M1 stacking in small domains. The presence of interlayer water may be related to the presence of small amounts of Al(IV) that are balanced by exchangeable cations. Rolling of the layers into tubes as a result of lateral misfit of the component sheets is possible only if tetrahedral rotation is blocked by insertion of H2O or exchangeable cations into the ring openings or by dynamic disorder of the water molecules to attract the basal oxygens to rotate in different directions at the same time. Rotation may occur after dehydration and may explain the irreversibility of dehydration. Platy morphologies result when sufficient substitution of Fe3+ for Al increases the lateral dimensions of the octahedral sheet to fit those of the tetrahedral sheet. Spheroidal morphology is due to specific growth conditions.
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Kaolin is an important industrial mineral in several world markets including uses in paper coating and filling, ceramics, paint, plastics, rubber, ink, fiberglass, cracking catalysts and many other uses (Murray, 1991). The kaolin minerals kaolinite, halloysite, dickite, and nacrite have essentially similar chemical composition but each has important structural and stacking differences. The most common kaolin mineral and the one that is the most important industrially is kaolinite [Al2Si205(OH)4]. Kaolinite can be formed as a residual weathering product, by hydrothermal alteration, and as an authigenic sedimentary mineral. The residual and hydrothermal occurrences are classed as primary and the sedimentary occurrences as secondary. Primary kaolins are those that have formed in situ usually by the alteration of crystalline rocks such as granites and rhyolites. The alteration results from surface weathering, groundwater movement below the surface or action of hydrothermal fluids. Secondary kaolins are sedimentary which were eroded, transported and deposited as beds or lenses associated with other sedimentary rocks. Most kaolin deposits of secondary origin were formed by the deposition of kaolinite which had been formed elsewhere. Some secondary deposits were formed from arkosic sediments that were altered after deposition, primarily by groundwater. There are far more deposits of primary kaolins in the world than secondary kaolin deposits because special geologic conditions are necessary for both the deposition and preservation of secondary kaolins.