2:1 clay structures generalized structure

There are a number of different clays, and some of the most common are listed in Table 55. The composition of each clay can be expressed by a formula that differs slightly from the general formula given above. Chemical composition alone, however, is not sufficient for characterizing clays their crystal structure provides the best way of characterizing any type of clay (see Textbox 21). In many clays, for example, the atoms are grouped in... 

Neither carbonates nor sulfates are considered nor have the various more rarely occurring salts. In general the elements in these minerals either do not enter into clay structures in appreciable quantities (Ca,... 

Structures of Clay Minerals. The term "clay minerals" generally refers to fine-grained (< lpm) sheet silicates. Many detailed discussions of the structures and compositions of clay minerals exist (e.g., 14-16). and their interesting chemical properties have been reviewed previously (H). However, a short introduction to their structures is necessary to understand their HRTEM images. 

Figure 1. Generalized structure of a 2 1 phyllosilicate (e.g., smectite clay). There are two different octahedral sites corresponding to cis-M04(0H)2 and trans-MO OH. (Modified from Grimm, R.E. "Clay Mineralogy" McGraw-Hill, 1968).

Clays are generally insensitive to such approaches. They don t react readily with acids or bases, and they don t go into solution when reacted with water. Their reactions are characterized as being slow and incomplete. Important information about clay structure was not available until 1930 with the introduction of X rays (Dixon and Weed, 1989). 

Hitherto the most successful pillaring has been carried out on smectite type clays. The original research on pillaring included several series of unsuccessful experiments on mica, vermiculite and the sheet silicic acid minerals magadiite, silhydrite and kenyaite. Recently renewed attention has focused on these and other alternative sheet structures. They include rectorite, zirconium phosphates, tetrasilicic micas, hydrotalcites and silicic acids, the general structural features of which are shown in Figure 4. 

Regarding their use as cracking and isomerization catalysts, bulk oxides such as clays and amorphous silica-aluminas have been widely displaced by molecular sieve compounds (e.g., zeolites, aluminophosphates), whose well-defined pore structures generally offer higher selectivity and flexibility. Nevertheless, bulk oxides continue to be used for various cracking and isomerization applications in the petroleum industry. 

General structure of smectite clays and their properties... 

So far we have discussed molecular modeling of clay minerals and their surface chemistry (either clay mineral s structural features or the clay mineral-water interface), different molecular modeling techniques (either classical mechanics or quantum mechanics in general), the interplay between spectroscopic methods and molecular simulations, and the visualization techniques of the results from... 

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