Tetrahedral-substituted layered silicates

Layered silicates dispersed as a reinforcing phase in polymer matrix are one of the most important forms of hybrid organic-inorganic nanocomposites. MMT, hectorite, and saponite are the most commonly used layered silicates. Layered silicates have two types of structure tetrahedral-substituted and octahedral substituted figure 6. In the case of tetrahedrally substituted layered silicates the negative charge is located on the surface of silicate layers, and hence, the polymer matrices can react interact more readily with these than with octahedrally-substituted material. 

The mineral phase. Mineral colloids are composed of layered silicates and amorphous metal hydroxides. The two basic building layers of the silicates are (i) a tetrahedral silicon dioxide layer modified by occasional substitution by Al and (ii) an octahedral A1 oxyhydroxide layer with occasional substitution by Mg2+, or... 

These diagrams indicate that when the total layer charge is less than 0.7, A1 will be the dominant cation when the seat of the charge is largely in the octahedral sheet as the predominant charge shifts to the tetrahedral sheet the larger Fe ion substitutes for A1 in the octahedral sheet. Radoslovich (1962) found that montmorillonite was the only layer silicate in which tetrahedral A1 caused the layer to increase in size in the b direction. He explains this by suggesting ... 

An important feature of the smectites, vermiculites and other 2 1 layer silicates is that isomorphous substitutions can occur in both the tetrahedral and octahedral sheets. Thus, substitution of Si by A1 occurs in the tetrahedral sheet, together with replacement of A1 by Mg, Fe, Li or other small atoms in the octahedral sheet. The substitutions lead to a deficit of positive charge, which is compensated by the presence of exchangeable, interlayer cations. 

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. 

NMR spectra, and chemical analysis by X-ray fluorescence, the five samples El to E5 are dioctahedral 2 1 layer silicates, mainly constituted of Si and Al as framework-forming elements, containing Al for Si substitutions in tetrahedral layers and fluorine for hydroxyl groups substitutions. 29Si MAS NMR spectroscopy gives rise to their (Si/AI)iv tetrahedral ratio with a good reliability (Table 10-5). The well-controlled and well-known hydrogels composition indicates that only dioctahedral layer silicates, with two Al and one vacancy per... 

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