Sheet silicate

Sheet silicates can be classified into two main groups, two-layered silicates and three-layered silicates. Two-layered silicates such as kaolinite have idealized formulas Al2Si20s(OH)4 and may be considered to be the condensation products of Al(OH)6 octohedral sheets with tetrahedral sheets of Si203(0H)2. 

In three-layered silicates, an octahedral layer is sandwiched between two tetrahedral layers. They are further divided into those having a dioctahedral structure and those having a trihedral structure. The former have the idealized formula Al2(Si40io)(OH)2 in electrically neutral structure, where only two-thirds of all possible octahedral sites are occupied by Al. The latter have the idealized formula Mg3(SUOio)(OH)2, and the Mg ions occupy all three such sites in a unit cell. The diversity of clays arises from deviations with respect to the ideal formulas. Aluminum 

Acidity of clays stems from different sources. The water molecules belonging to the hydration shell of exchangeable cations are subjected to a strong electrical polarizing field therefore they have a degree of dissociation several orders of magnitude larger than liquid water.  

Brensted acid sites are also generated by the dehydroxylation of pillars C AlisOtC OH )2 ( HjO), 2] + — 7H++6.5AI2OS+2O.5H2O 

The acidity of clays and pillared clays has been studied by infrared spectroscopy using pyridine as a probe molecule. Fig. 3.57 shows the infrared spectra of pyridine adsorbed on beidelite pillared with aluminum hydroxide oligomers after thermal treatment under vacuum at increasing temperatures. The intercalated beidellite contains Lewis acid sites and Bransted acid sites characterized by 1454 cm and 1540 cm , respectively. The intense band at 1454 cm is associated to Lewis acid sites on the pillars, since the band does not show up in the spectra of proton exchange clays. 

Structure of kaolinite (a) shows the different layers in the structure (b) shows the composition of the two layers. 

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Fig. 16.4. Silicate structures, (a) The Si04 monomer, (b) The Si207 dimer with a bridging oxygen. ( ) A chain silicate. (d) A sheet silicate. Each triangle is the projection of on Si04 monomer.

With its oxygen functionality, graphite oxide has chemical properties more akin to those of layered disulfides or sheet silicates than to those of graphite (Gi, T1,A2). Many studies have been of an extremely applied nature the possibility of fluorination (LI, N1), redox potentials in the presence of hydrogen peroxide (V2), the apparent density (L2), the adsorption isotherms with nitrogen (L3), and the diffusion of Cs in graphite oxide (R2). 

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. 

Table III. 29. Si chezniC LL shifts for some sheet silicates...

Sheet silicates (Q ) with significant isomorphic replacement of Si by AF+ or Fe +. These were decomposed by poly(acrylic add) to silica gel. The chlorite, thuringite, formed a strong cement but was much affected by water. 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

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 ... 

Tetrahedra linked via three vertices correspond to a composition MX1 1X3 2 or MX2 5 = M2X5. Small units consisting of four tetrahedra are known in P4O10, but most important are the layer structures in the numerous sheet silicates and aluminosilicates with anions of the compositions and [AlSiOj-]. Because the terminal vertices of the single... 

Some arrangements of tetrahedra in sheet silicates. The lower image in each case represents a side view... 

Connection of the layers in (d) cation-poor sheet silicates, (e) cation-rich sheet silicates. Octahedron vertices that do not act as common vertices with tetrahedra are occupied by OH- ions... 

The sheets consisting of tetrahedron-octahedron-tetrahedron layers in cation-poor sheet silicates are completely planar due to the symmetrical environment of the cation layer. If the sheets are electrically neutral as in talc, the attractive forces between them are weak as a consequence, the crystals are soft and easy to cleave. The use of talc as powder, lubricating agent, polishing material and filling material for paper is due to these properties. 

Clay materials show a different behavior. They are either cation-poor or cation-rich sheet silicates. They can swell by taking up varying amounts of water between the sheets. If the intercalated cations are hydrated as in montmorillonite, they act as cation exchangers. Montmorillonite, especially when it has intercalated Ca2+ ions, has thixotropic properties and is used to seal up drill holes. The effect is due to the charge distribution on... 

In 1963, Armin Weiss (then at the University of Heidelberg, Germany) reported the intercalation of amino acids and proteins in mica sheet silicates (Weiss, 1963). Some years later, U. Hoffmann, also from Heidelberg, published an article titled Die Chemie der Tonmineralien (The Chemistry of Clay Minerals), in which he mentioned possible catalytic activity of clays in processes which could have led to the emergence of life (Hoffmann, 1968). 

Nagy, K. L., 1995, Dissolution and precipitation kinetics of sheet silicates. Reviews in Mineralogy 31, 173-233. 

Figure 4.4 Infinite chain silicates (single, double, and sheet) (a) infinite single chain silicate with two corners shared per tetrahedron (pyroxene structure) (b) infinite double chain, with alternate two and three corners shared (am-phibole structure) (c) infinite sheet structure, with each tetrahedron sharing three corners (sheet silicates). (From Putnis, 1992 Figure 6.3, by permission of Cambridge University Press.)...

The clay minerals can now be discussed in terms of their relationship with the phyllosilicates (sheet silicates). It is important to keep clearly in mind here the difference between clay - the material which is dug out of the ground, and which may be a mixture of different clay minerals, together with various nonclay minerals (such as quartz, pyrite, etc), as well as unaltered rock fragments and incorporated organic material (Grim, 1968) - and the clay minerals themselves, which are crystalline compounds of specified stoichiometry and structure. At this stage, we are only considering the structure of the clay minerals. 

NMR properties, 33 213, 274 in NMR studies of zeolites, 33 254-264 in sheet silicate studies, 33 342-345 -magnesium oxide catalyst, lattice parameter, 35 75... 

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. 

Kieflfer S. W. (1980). Thermodynamics and lattice vibrations of minerals, 4 Application to chain and sheet silicates and orthosilicates. Rev. Geophys. Space Phys., 18 862-886. 

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