Alumina octahedra

The ideal layered silicate is the mineral montmorrillonite, which is main component of the clay bentonite (fig 6). Montmorrillonite is a so called 2 1 layered silicate. Each layer consists of two sheets of silica tetrahedrae which are sandwiched by an alumina octahedrae sheet. The layers are only weakly bound, often by hydrogen bonding from water. The structure has been extensively studied by transmission electron diffraction especially by Z5wagin and coworkers [4]. 

Currently, the only biological barrier registered as a biochemical pest control agent is kaolin, a clay mineral [47]. Kaolin is a ubiquitous clay substance found in soils worldwide and consists of a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra [Al2Si205(0H)4]. 

The principal class of swelling clays is smectite. This type of mineral comprises beidellite, hectorite, fluorhectorite, saponite, sauconite, montmorillonite, and nontronite. Smectites can be described merely on the basis of layers enclosing two sheets of silica sandwiching a layer of octahedral A1 or Mg, that is, 2 1 layered clays (see Figure 2.24) where the layer of alumina octahedra is inserted into the two layers of silica tetrahedra. 

At present, rearrangement of silica tetrahedra and alumina octahedra into zeolite-type tetrahedra in the basal plane cannot be excluded. 

Weed, 1977). These minerals typically take the form of extremely fine particles, often with diameters of less than 2 /itM, and having a high relative surface area. Clays are highly organized, often forming stacked layers of parallel planes made up of silica tetrahedra and alumina octahedra. The ability of the planar arrays to move relative to one another, especially in the presence of water, accounts for the slipperiness and pliability of these materials. 

Kaolinite is a 1 1 layered mineral based on alumina octahedra and silica tetrahedra, with platelets of approximate size 1 pm by 1.5 pm by 50 nm. 

Kaolinite - When a layer of sihca rings is joined to a layer of alumina octahedra through shared oxygens, as shown in Figure 8, the mineral kaolinite is formed. Kaolinite is the sole or dominant constituent of what is known as kaolin clay or simply kaolin. 

The principal class of swelling clays is termed smectites. These clays characteristically have alumina octahedra sandwiched between silica tetrahedra as shown in Figure 1. The smectites are distinguished by the type and location of cations in the layered framework. In a unit cell formed from twenty oxygens and four hydroxyl groups, there are eight tetrahedral sites and six octahedral sites. Idealized formulas are given in Table I. 

The principal class of swelling clays is termed smectites. The smectites characteristically have alumina octahedra sandwiched between layers of silicate tetrahedra as shown in Fig. 1. 

Figure 3 Structure of (a) silica tetrahedron and (h) alumina octahedron...

The cis isomer is defined as that compound in which the three methyl keto groups are on adjacent corners of the octahedral structure the trans isomer has two of the methyl keto groups on opposite apexes of the octahedron. Both the cis and trans isomers should be di-racemates, but their resolution has not been reported. The structures of the isomers are assigned on the basis of their different chromatographic adsorption (the cis isomer adheres more strongly to alumina) and the ratio of isolated products (the trans isomer predominates) by analogy with the work of Fay and Piper on tris complexes of unsymmetrical 1,3-diketones. 

This is a reasonable result considering that the twist angle of the oxygen octahedron is 12.5° in beryl, which is much greater than that in alumina... 

The crystal field optical transitions of Cr + in an octahedral site are shown in Fig. 26. The splitting of the upper energy levels by the weak trigonal distortion ( 350 cm ) (67,168-170) of the octahedron cannot be resolved in powder samples of chromia-alumina. The optical reflectance spectra shown in Fig. 27 show a gradual shift of the absorption maxima near 17,000 and 23,000 cm (15,39,171) due to the lattice expansion that results when chromia is added to an alumina lattice. The optical spectra conform to the Tanabe and Sugano (168,172-174) theoretical calculations. The Racah parameter varied with the chromium content (15,171), Variable temperature (169,175) and variable pressure (176,177) optical spectra have been obtained for ruby samples. 

While the silicon in silica is always surrounded by four oxygen atoms in a tetrahedral configuration, the aluminum in alumina can be located in the center of either a tetrahedron or an octahedron. The commonly used form of alumina is y-alumina, whose surface can be formulated as frdlows (20) ... 

The commercially available HPLC-grade aluminas consist latgdy of y-alumina, a form prepared by heat treatment between S(X) and 8(XPC. TIm structure of y-alumina is spinel, where the unit cell contains 32 oxygen atoms arranged in 64 tetrahedrons and 32 octahedrons (19). The aluminum atoms are heterogeneously distributed over the tetrahedrons and octahedrons of the unit cell. This leads to a formation of Lewis base sites for the vacant positions and Lewis acid sites for the occupied positions. Like the surface of silka, the surface of alumina is covered by hydroxy groups. However, the number of hydroxy groups and their coordination is different (11) ... 

Illite [(K,H30)(AlMg,Fe) (Si,Al)p ((0H), (Hp))] is a non-expanding micaceous mineral. It occurs as aggregates of small monoclinic grey to white crystals. It is layered alumina-silicate consisting of poorly hydrated potassium Cation, which is responsible for the poor swelling behavior. Its structure consists of repetition of tetrahedron- octahedron - tetrahedron (TOT) layers. The cation exchange capacity (CEC) of illite is comparatively better than kaolinite (20-30 meq/100 g). 

Two crystalline polymorphs of monohydrated alumina or aluminum oxihydroxide [A10(OH) = AI3O3.H3O], where the [AlO/ j octahedrons share one edge boe/imite [y-AIO(OH)] ... 

MMT nanoclays are also known as 2 1 phyllosilicates and possess a layer of octahedral alumina and two linked tetrahedral silicate layers, illustrated in Rgure 6.2. A hexagonal symmetry is formed from one silicate atom and four joint oxygen atoms on the outer tetrahedral layer. Meanwhile, each aluminum atom is bonded with six oxygen atoms in the shape of an octahedron (i.e., a polyhedron with eight faces) on the... 

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