Silica tetrahedral sheets

The three-sheet or 2 1 layer lattice silicates consist of two silica tetrahedral sheets between which is an octahedral sheet. These three sheets form a layer approximately 10 A thick. The oxygens at the tips of the tetrahedra point towards the center octahedral sheet and substitute for two-thirds of the octahedrally coordinated hydroxyls. The 2 1 clay minerals include the mica and smectite groups which are by far the most abundant of the clay minerals. The pure end members of this type are talc, a hydrous magnesium silicate pyrophyllite, a hydrous aluminum silicate and minnesotaite, a hydrous iron silicate. 

It would seem that when the occupied octahedral sites are more than 65% occupied by A1 or by Mg the layer can adjust to compensate for the internal strain and can grow in two dimensions. The minerals which form larger sheets generally have a larger proportion of their occupied sites filled with A1 or Mg than the smaller, clay minerals. The occupancy value can be less than 65% when the smaller Fe3+ is substituted for Al. When these conditions are not satisfied, the internal strain is such that growth is in only one direction. The width of the sheet is restricted to five octahedral sites. Sufficient strain accumulates within this five-site interval that the silica tetrahedral sheet is forced to invert to accommodate the strain. 

Figure 6 Structures of common clay minerals I I Silica tetrahedral sheet I I alumina octahedral sheet I I brucite sheet.

Crystalline solids that contain only silica tetrahedral sheets do not exist in soils, but carbonate, oxide, oxyhydroxide, and hydroxide solids that have metal cations in octahedral coordination are widespread. The ratio of the radius of the common metal cations in soil clays to that of bivalent oxygen usually ranges between 0.4 and 0.7 (Tabl 1.1), which means that, according to the Pauling Rules, octahedral coordination of the metal cations with O(-II) is preferred. Because of their great abundance in the lithosphere and their low solubility in the normal range of soil pH values, aluminum, iron, and manganese form the most important oxide,... 

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... 

Fig. 3. Schematic of a montmorillonite, layered clay mineral with a triple-sheet sandwich structure consisting of a central, hydrous alumina octahedral sheet (O), bonded to two silica tetrahedral sheets (T) by shared oxygens.

The most widely used layered silicates for polymer nanocon sites are the smectite clays such as montmorillonite. They offer a high aspect ratio and a high surface area. Sodium montmorillonite is 2 1 phyllosilicate, constructed of repeating triple-layers with two silica tetrahedral sheets fused to an edge-shared octahedral sheet of alumina. The physical dimensions for these silicate sheets are approximately one hundred to several hundred nanometers in lateral... 

Kaolinite is a clay mineral with the chemical formula Al2(0H)4Si205 [5] or (OH)8Si4Al40io [6]. The basic unit of kaolinite consists of a single silica tetrahedral sheet and a single alumina octahedral sheet such that the oxygen atoms at the tips of the silica tetrahedrons and one of the oxygen atoms of the alumina octahedral sheet form a common layer. 

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). 

The crystal lattice of clay minerals consist of two basic structural units hydrated alumina and hydrated silica sheets. The alumina sheet consists of oxygens or hydroxyls in octahedral configuration around aluminum, iron, and/or magnesium. The silicon sheet consists of a central silicon with four oxygens in a tetrahedral coordination. The smectite clays consist of two silica tetrahedral sheets with one central octahedral sheet (Fig. 31). 

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. 

The layers are characterized by a thickness of about 1 nm, and the other dimensions vary from 30 mn to several micrometres or more. Several layers are stacked in clay particles, kept together by weak van der Waals forces. The performance of polymer-clay nanocomposites strongly depends on the breaking-up of clay particles in the polymer matrix. However, the most frequently used layered silicate, MMT, is a naturally occurring 2 1 phyllosilicate, which has the same layered and crystalline structure as talc and mica but a different layer charge. A central octahedral sheet of alumina fused between two external silica tetrahedral sheets (the oxygens from the octahedral sheet also belong to the silica tetrahedral) forms the layers of the... 

---