Octahedral alumina

Alumina. Octahedral and tetrahedral Al sites have been observed by Al n.m.r. in various oxides.92 The effect of temperature on the distribution of these... 

Figure 3.5. Compositional arrangement of silica tetrahedral and alumina octahedral (from Schulze, 1989, with permission).

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

A 2 1 clay is a silicate clay that has two silica tetrahedral layers that sandwich an alumina octahedral layer between them. There are two general groups of 2 1 clays, and they differ by where the isomorphic substitution occurs. Soils that have isomorphic substitution in the alumina octahedral layer are known to have high CEC and high shrink-swell capabilities. These soils expand when wet and shrink when... 

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 basic smectite structure if formed by an alumina octahedral layer sandwiched between two siUca tetrahedral layers. These layers share the apical oxygen atoms... 

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). 

Silica (Tetrahedral layer)-Alumina (Octahedral layer)-Interlayer Space -... 

Smectites are water swellable clays that have a platy structure. Smectite is the mineralogical term for a group of clays, which includes montmorillonite, hectorite, and saponite. Most smectites are more commonly known under the geological term bentonite. By convention, bentonite is understood to be an ore or product with a substantial smectite content. The range of possible chemical variations in the basic smectite trilayer lattice starts with montmorillonite, the high-aluminum end member. Montmorillonite is composed of a central alumina octahedral layer sandwiched between tetrahedral silica layers. This is identical to the dioctahedral pyrophyllite structure except for small... 

According to Wada [1967], this water is held within the structure of allophane, which consists of long chains of silica tetrahedra bonded to alumina octahedral chains, the combined chains being very loosely packed. 

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