Structure of Phyllosilicates

From the discussion above, it can be seen how the atomic structure of phyllosilicate clays plays a key role in determining the final state of clay particles in aqueous media. The presence of structural charges, neutralizing cations, and the capacity of forming hydrogen bonds between different layers produces a system that can be completely delaminated, completely flocculated, or in an intermediate state having floes mixed with isolated layers. Whether the more stable situation corresponds to isolated layers, floes, or a mixture depends on the type of clay, its concentration, pH, concentration and type of supporting electrolyte, and so on. 

Silicate Si04" tetrahedron is the basic structural unit of all silicate minerals in which each Si " is surrounded by four 0 ions in tetrahedral coordination. Phyllosilicate In the structure of phyllosilicate, each Si04 tetrahedron is linked to three adjacent tetrahedra to form an infinite sheet of tetrahedra. Each tetrahedron in the sheet thus shares three (out of fom) apical oxygens, having the basic structiual imit Si205. The micas, clay minerals, chlorite, talc and serpentine minerals are examples of phyllosilicates. 

Some general aspects of the internal structures of phyllosilicates are described below ... 

There are several aspect of the overall crystal structure of phyllosilicates which are important in determining the properties of these materiak and for their nomenclature. To begin with one distinguishes a lane of atoms (e.g., the plane of oxygens forming the surface of a mica layer), a sheet as in the octahedral sheet of a mica layer, and finally a layer. Of primary importance is the nature of the layer itself the manner in which the layers are stacked is of secondary importance. 

Fujii and coworkers reported the synthesis and detailed structural analyses of alkylammonium/magnesium phyllosilicate hybrids [88], which were prepared by hydrothermal reaction from a mixture ofoctadecyldimethyl(3-trimethoxysilylpropyl)-ammonium chloride, silica sol, and magnesium hydroxide Mg(OH)2. The structure of the hybrid compound was studied by XRD, TEM, electron diffraction, high-resolution solid-state NMR, TG-DTA/MS, and elemental analysis. The resulting analytical information confirmed the unit structure, which consists of a 2 1... 

Fig. 8.2 PXRD pattern of ethlyenediamine-functionalized magnesium phyllosilicate showing reflections indexed according to the 2 1 trioctahedral phyl losi I icate structure of talc.

The clay minerals can now be discussed in terms of their relationship with the phyllosilicates (sheet silicates). It is important to keep clearly in mind here the difference between clay - the material which is dug out of the ground, and which may be a mixture of different clay minerals, together with various nonclay minerals (such as quartz, pyrite, etc), as well as unaltered rock fragments and incorporated organic material (Grim, 1968) - and the clay minerals themselves, which are crystalline compounds of specified stoichiometry and structure. At this stage, we are only considering the structure of the clay minerals. 

Brown, G. (1984). Crystal structure of clay minerals and related phyllosilicates. In Clay Minerals Their Structure, Behaviour and Uses, ed. Fowden, L., Barrer, R.M. and Tinker, P.B., Royal Society, London, pp. 1 20. 

It should be possible, through a unification of chemical and mineral structure data and the results of experimental studies on silicate phase equilibria, to develop a general picture of clay mineralogy based upon the known chemical behavior of phyllosilicates under various physical conditions. The major elements for such a study are presently available in a rough outline. It is fact the purpose of this essay to summarize the available information and create a general outline of clay mineral petrology. It is hoped that such an attempt meets with some success and, more important in the long run, that such an attempt will interest others in similar exercises, especially those of precision and revision. 

Figure 2.1. Basic structures of 1 1 and 2 1 phyllosilicates. Adapted from Schulze (1989). Reprinted with permission from the Soil Society of America.

VUId Silicates with layered anions (the so-called phyllosilicates) Figure 7.11 represents a way to link Si04 tetrahedrons in one layer. The repetitive unit in the structure of figure 7.11 is represented in between the dotted lines and has the formula Si404-. The anion described here for example occurs in the minerals ... 

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).

What are the long-range superexchange pathways which give rise to the magnetic structures of iron bearing phyllosilicates ... 

White, G.N. and Zelazni, L., Analysis and implications of the edge structures of dioctahedral phyllosilicates, Clays Clay Miner., 36, 141, 1988. 

The structural distortions of phyllosilicate minerals have been investigated by solid-state Si nuclear magnetic resonance spectroscopy (NMR) [24], In trioctahedral layer silicates such as talc, the wSi chemical-shift value is related to the amount of >VA1 substitution and/or distortion of the tetrahedral sheet structure. 

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