Isomorphous substitution kaolins

The ideal constitution of the kaolin layer represents an electrically neutral unit, with rarely any isomorphous substitution of cations of different charges within the lattice. Consequently, kaolinite and related minerals would not be expected to show a large cation exchange capacity, and indeed this is usually the case. That a small but varying exchange capacity does occur may be attributed to two principal causes. 

Kaolinite is a 1 1 (T-O) phyllosilicate. The fundamental unit of its structure is an extended sheet of two constituents a silica-type layer of composition (Si4O10)4- and a gibbsite-type layer of composition (0H)6A14(0H)204 (see schematic representation in Fig. 10). Ideally, kaolinite crystals are not permanently charged. However, due to isomorphic substitution of Si by Al at the siloxane surface, kaolinite platelets carry a small, permanently negative charge (Van Olphen, 1977). Lim et al. (1980) and Talibudeen (1984) postulate that the permanent charge of kaolins is caused by contamination with small amounts of 2 1 phyllosilicates rather than a consequence of isomorphic substitution. 

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]. 

The dioctahedral kaolin minerals have the generali2ed chemical formula Al2Si205 (0H)4- H20. In kaolinite, dickite, and nacrite, n=0 in hydrated halloysite, n=2 and in other halloysites, n varies between 0 and 2. A small amount of isomorphous substitution of A1 and Fe for Si in the tetrahedral sheets and Fe and Mg for A1 in the octahedral sheets might occur in halloysites, but has not been suggested for other dioctahedral minerals. 

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