Montmorillonite octahedral sheet, composition

When the modal values are considered, there is relatively little overlap in the composition of the octahedral sheets of the three Fe3+-rich clays whereas, for the Al-rich clays, the illite and mixed-layer illite-montmorillonite fields fall within the montmorillonite-beidellite field. 

Montmorillonite is the name given to day found near MontmoriUonin in France, whereit was identified by Knight in 1896 (Utracki, 2004). Montmorillonite is a 2 1 layered hydrated aluminosilicate, with a triple-sheet sandwich structure consisting of a central, hydrous alumina octahedral sheet, bonded to two silica tetrahedral sheets by shared oxygen ions (Fig. 3). The unit cell of this ideal structure has a composition [Al2(0H)2(Si205)2]2 with a molar... 

Clay minerals are layered. As shown in Figure 2.1, montmorillonite (Mt), the most used clay mineral for the preparation of rubber composites, is made of layers 1 nm thick, with lateral dimensions between 100 and 1000 nm, with two tetrahedrally coordinated silicon atoms fused to an edge-shared octahedral sheet of either aluminium or magnesium hydroxide. 

The natural montmorillonite clays consist of several hundred individual platelike particles of dimensions 1 jum x 1 jum x 1 nm, held together by electrostatic forces with a gap of approximately 0.3 nm between two adjacent particles. The structure at the atomic level is shown in Figure 13.28 (77). The sodium montmorillonite layer is a crystalline 2 1 layered clay mineral in which a central alumina octahedral sheet is sandwiched between two silica tetrahedral sheets. These structures are sometimes called smectite clays, because of their layered structure see Figure 7.1. Note that this clay mineral comprises silicate layers in which the fundamental unit is planar. In the gap between the silicate layers are sodium ions. The gap is widely known as a gallery or an interlayer. The density of montmorillonite clays vary slightly with composition, but is generally near 2.5 g/crn (78). 

Although LDHs have a layered structure like the conventional layered silicate type of clays, these two materials are quite different from each other. While LDH has positively charged layers with anionic interlayer species (so they are called anionic clay), the layered silicates are of exactly opposite nature (hence called cationic clay). In terms of compositions, geometry, and layer thickness, the LDHs are vastly different from layered silicates. In LDH, as described earlier, each crystal layer is composed of a single octahedral metal hydroxide sheet whereas in layered silicates it is a sandwiched structure of two or more sheets of metal oxides. For example, a montmorillonite crystal layer is made up of three sheets one octahedral sheet containing Fe, Al, Mg, etc. remaining sandwiched between two silica tetrahedral sheets. This difference in layer structure results in much lower crystal layer thickness and rigidity in LDH-type clays. 

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