Kaolinite crystals

Kaolinite crystals in the subsurface are submicron sized and exhibit a platelike morphology. They usually are found mixed with other layered structured minerals. In a comprehensive review, Dixon (1989) summarizes the structural properties of kaohnite. This mineral is composed of tetrahedral and octahedral sheets constituting a 0.7 mn layer in a triclinic unit cell. Two thirds of the octahedral positions are occupied by Al the tetrahedral positions are occupied by Si and Al, which are... 

Figure 12. Indexed SAD pattern (see inset) shows orientation of large platy kaolinite crystals of the type shown in the TEM micrograph...

Equally interesting was the clear observation (Neder et al. 1996b, 1999) of diffuse scattering from the kaolinite crystals studied at the ESRF. This has implications for clays and materials where diffuse scattering is important (Welberry and Butler 1995). 

There is no excess or deficiency of charge at the surface of kaolinite crystal plates because there is no excess or deficiency of charge within the crystal structure. 

Kaolinite can be neoformed from solutions of A1 and silica only at acid pH in alkaline solutions, the tetrahedrally coordinated A1 ion in Al(OH)4 is unable to form the gibbsitelike sheet that is the precursor of the 1 1 aluminosilicate layer. Even in acid solutions, however, kaolinite crystallization is very slow at room temperature, probably because Al(OH)3 is insoluble except at very low pH. Natural organic chelating agents, such as oxalate, increase the solubility of A1 at acidic pH, thereby increasing the rate of kaolinite neoformation. 

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. 

Figure 13.1 Kaolinite crystal with negatively charged faces and positively charged edges.

It can be seen from a number of the case studies reported that additional information on the mechanism of fabrication or functionality of nanoscale features can be found by using a number of these techniques simultaneously or consecutively. Examples of these can be seen in the work presented above from Pap et al. [6], Moghadam et al. [44] and Pisarek et al. [69]. Often one of these techniques is used to back up another. In the work of ClauseU et al., FESEM was used to back up XRD measurements when measuring kaolinite crystal thickness [73], hi another work by Huiqian et al., BET, EESEM, TEM and X-ray scattering were used to characterize tungsten powders. Although BET and SEM could not characterize the particle size of nanometre powders, they provided a method to allow the exclusion of non-nanometre powders [74],... 

Figure 2.3 Stack of kaolinite crystals found in granite...

Hobbs, J.D., R.T. Cygan, K.L. Nagy, P.A. Schultz, and M.P. Sears. 1997. All-atom ab initio energy minimization of the kaolinite crystal stracture. Am. Mineral. 82 657-662. 

A crystal of kaolinite is made of a series of these double layers or sheets stacked parallel to each other and form small flat plates that are typically less than 1 pim in diameter and nearly hexagonal. Figure 12.15 is an electron micrograph of kaolinite crystals at a high magnification, showing the hexagonal crystal plates, some of which are piled one on top of the other. 

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