Sheets silicic tetrahedral

The appearance of a flake of clay reflects its internal structure, which is something like an untidy stack of papers (Fig. 14.45). Sheets of tetrahedral silicate units or octahedral units of aluminum or magnesium oxides are separated by layers of water molecules that serve to bind the layers of the flake together. Each flake of clay is surrounded by a double layer of ions that separates the... 

We alluded earlier to the variety of structural modifications which may he observed in sheet silicates. Clearly it is a matter of considerable in jortance to he able to determine if, for example, the aluminium content within a clay arises p a ely from octahedral substitution (as in montmorillonite) or whether there is some tetrahedral component (as in heidellite). a1 MASNMR readily provides the necessary answers. Figvire 1 illustrates the a1 spectrum for a synthetic heidellite material with Na as charge balancing cation. Aluminium in two distinct chemical environments is observed, with chemical shifts corresponding to octahedrally and tetrahedrally co-ordinated aluminium. 

Kaolinite, Al2Si205(0H)4 or Al203-2Si02-2H20 , is a sheet silicate with A1 atoms in octahedral and Si atoms in tetrahedral coordination the corresponding electrostatic bond strengths are ... 

Clays composed of layers are called layered silicates. The most common sheets are tetrahedral silicon and octahedral aluminum (see Figure 3.2, Figure 3.3, and Figure 3.4). Three common representative clays in soil are 1 1 kalo-inite, 2 1 fine-grained micas, and 2 1 smectites that is, kaolinites have one sheet of silicon tetrahedra and one sheet of aluminum octahedra. The finegrained mica and smectites have two sheets of silicon tetrahedra and one sheet... 

Fundamental structural units of detrltal silicates, (a) octahedron, (b) octahedral layer found in sheet silicates, (c) tetrahedron, and (d) tetrahedral layer found in sheet silicates. Source From Grim, R. E. (1968). Clay Mineralogy, 2nd ed., McGraw-Hill Publishing Company, p. 52. 

Rocks that contain talc, or the chemically and structurally similar minerals mentioned previously, are usually the result of alteration and recrystallization of rock formations that contained magnesian silicate minerals. Steati-zation or serpentinization are the terms given the processes that create layered (sheet) silicates from chain or other tetrahedral arrangements adopted by silicate minerals (see Fig. 2.1). The recrystallization process is expedited by temperature and pressure, and especially through the action of hydrothermal solutions. 

Clays are layer silicates (phyllosilicates) of particle size less than about 4 pm, produced by the weathering of aluminosilicate rocks. Clay minerals fall roughly into two structural classes the kaolinite type, based on paired sheets of tetrahedral (SiC>44-) and octahedral [A10n(0H) g " or... 

It is a matter of some significance catalytically to be able to ascertain whether the aluminum present in a natural clay or its synthetic analogue is in a state of octahedral substitution (as in montmorillonite) or whether there is some tetrahedral substitution (as in beidellite). 27A1 MAS NMR readily provides the necessary answers. For example, Diddams et at. (462) in a study of the synthesis, characterization, and catalytic performance of synthetic beidellites and their pillared analogues, monitored the fate of AI from the gel precursor to the sheet silicate and to its pillared state by 27A1 MAS NMR (see... 

Figure 10.10. Three combinations of tetrahedral (T) and octahedral (O) layers in sheet silicates 0) TO, (li) TOT and (Hi) TOT O TOT.

In a T-O-T layer (also termed a 2 1 layer), tetrahedral sheets attach to both sides of an octahedral sheet, thereby giving a structural formula of M2T40io(°h)4 to M3T4O10 (OH) 4. The T-O-T layer forms the basis of the sheet silicates pyrophyllite (M Al) and talc (M - Mg). 

Now the stoicheiometry of all mineral phases is complex and beset by problems of isomorphous substitution. In the sheet silicates such as talc, the incorporation of material between the layers, as occurs in intercalation compounds, is also common. Beneath this complexity, though, lies the fact that the tetrahedral-octahedral framework of these materials is of a fixed and inflexible metal to oxygen stoicheiometry. 

Show how the fundamental silicate tetrahedral unit (Si04 ) links to other silicate tetrahedra to form rings, chains, double chains, sheets, and space-filling crystalline networks (Section 22.1, Problems 1-4). 

The reactions to illustrate weathering of complex silicates during acid hydrolysis (eqns. 4.13 4.14) predict that clay minerals will be an important solid product and this is confirmed by looking at soils. Clay minerals are important constituents in most soils. These sheet silicates that are less than 2 pm (Section 4.2.3) are constructed of layers of atoms in tetrahedral and octahedral coordination, known as tetrahedral and octahedral sheets. 

Layer Silicates. Although the common primary minerals include island, chain, sheet, and framework silicates, the most stable and persistent silicates, which occur as weathering products (secondary minerals) in the clay fraction of soils, are sheet silicates. Figure 2.9a depicts the structure of the tetrahedral sheet in these minerals, which is comparable to the tetrahedral structure of mica. For the layer silicate clays, however, numerous structural combinations of the tetrahedral sheet with octahe-drally coordinated metal cations are possible. 

Illite A phyllosilicate (layered silicate) with a structure of parallel silicate tetrahedral sheets. Each sheet has the general formula Si20s. 

Other related cell (rat tumor and lung cells)/silicate (Chrysotile-sheet silicate asbestos, silicon, silica, hornblende) interaction studies performed using ESCA have been reviewed in detail elsewhere by Seal and his coworkers [177-180]. The general feature inherent to all of these systems is that there seems to be a particular type of chemical interaction between the cells and the silicate involving direct coupling between the cell and the tetrahedral silicate subunit with a corresponding release of these components to the octahedral subunit (i.e., the Al, Mg, and particularly the Fe). The results also suggest that some of the Fe is inserted into the cell. 

Layered Silicates The structure of Layered silicates (LS)/clays consists of a 2-D layer of two fused silicate tetrahedral sheet with an edge-shared octahedral sheet of metal atoms, such as A1 or Mg. This model was proposed by Hofifinann et al. [19]. 

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