The Kaolin Group

The kaolin group includes kaolinite, nacrite, dickite and halloysite halloysite can also exist in a hydrated form having the formula Al2Si205(0H)4.2H20. Of all these, by far the most important is kaolinite, since it is the principal constituent of china clay, ball clay, fireclays, and many brick clays the other kaolin minerals are somewhat rare in Britain. 

The average formula of the Si-0 rings is SijOa and that of the gibbsite layer is Al2(OH)402- If these rings are joined with the two O atoms forming a bridge , we arrive at the composite 

It is important to realize that the valencies of the oxygens of the Si20a layer (the so-called planar oxygens) are completely satisfied, since each O atom is joined to two Si atoms, and so it cannot be joined by direct chemical bonds to any other atoms— in other words, the Si203 layer has no tendency to extend itself perpendicularly. Horizontally, however, it can extend indefinitely precisely the same applies to the planar OH groups in the gibbsite layer. 

In the majority of British fireclays and ball clays, the clay mineral is usually kaolinite, but it is imperfectly crystalline, with some disorder in the stacking of the units. 

Once the student has grasped the structural features of the kaolin group, the structures of the other clay minerals and related groups will be readily understood. 

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Kaolins. The kaolin minerals include kaolinite. dickite. and nacrite which all have composition AUOi 2 SiO 2 FLO halloysite (7 At. AGO, 2 SiO, 2 H.O and halfoysile (10 A). Al 0, 2 SiO 4 H 0 The struclural formulas for kaolinite and halloysite tIO At. which are shown in Figure I, are AbSiaOiotOH)). and AljSi.iO, i(OH) -4 H,(). respectively. The so-called fire day mineral is a h-axis disordered kaolinite halloysite (7 A) and halloysite (10 A) are disordered along both the a- and h-Mcs Indeed, most variations in the kaolin group originate as structural polymorphs, related to variations in layer slacks. 

Youell, R.F., 1958. Isomorphous replacement in the kaolin group of minerals. Nature,... 

Minerals such as the kaolin group (l.T) and chlorite (2 2) do not exhibit significant expansion potential between layers (a layer is a mineral structure composed of combinations of sheets) (Table 3.5, Figs. 3.7 and 3.10). 

The structural unit of the kaolin group is formed by superimposing a tetrahedral sheet on an octahedral sheet. Such minerals are referred to as 1 1 layer silicates. The top oxygens of the tetrahedral sheet are shared by the octahedral sheet, forming a... 

So far, everything that has been stated with regard to structure applies equally to all the members of the kaolin group the distinguishing feature is the way in which the units are stacked one upon another. In Figure 13 the stacking is approximately that of... 

Like crystals of the kaolin group, a crystal of montmorillonite is many units thick, yet clearly the unit layers cannot be bonded chemically neither, indeed, can there be any hydroxyl bonding, since all the planar atoms on both sides of the units are oxygens. The unit layers are in fact held together by very weak residual forces, known as van der Waals forces, which are caused by slight Table 2 Empirical Formulae of some Montmorillonites... 

Kaolinite 1) A non-expandable clay mineral in the kaolin group with the formula Al2Si205 (0H>4, which does not exchange iron or magnesium. 2) Any kaolin clay mineral. 

The isomorphic substitution of some Al + for Si" " " in the tetrahedral sheet and some Mg + for Al + in the octahedral sheet (or to some local partial charges on Si-O-Si surfaces) was suggested to occur in the minerals of the kaolin group [63], which can lead to the generation of the surface cations and the presence of local charge defects [64]. The basal siloxane surfaces of kaolinite are believed by many [65-68] to carry a constant structural charge due to the isomorphous substitution of Si" + by Al + [69]. Natural kaolinites do contain very small amount of Mg [70] and Mg-rich kaolinite was synthesized [71]. 

Following Smith and Yoder [1956], Zvyagin [1967] has proposed symbols to represent the numbers of layers and symmetry of the kaolin group minerals. In the system proposed by Smith and Yoder, kaolin that contains one layer in a triclinic unit cell is symbolized as IT, and dickite that contains two layers in a monoclinic cell is symbolized as 2M. Bailey [1963], recognizing that the ordering of cations or the vacant octahedral position in the dioctahedral minerals changes the size, shape, and symmetry of the unit cell, has modified the Smith and Yoder system to indicate in one double symbol the basic crystallographic form and its ordered modification. 

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