Phyllosilicates layer types

The phyllosilicates in soil clays can be classified into three layer types, distinguished by the numbers of tetrahedral and octahedral sheets combined, and five groups, differentiated by the kinds of isomorphic cation substitutions that occur. The layer types are shown in Fig. 1.3, and the groups are described in Table 1.3. The 1 1 layer type consists of one tetrahedral and one octahedral sheet. In soil clays, it is represented by the kaolinite group, with the unit cell chemical formula [Si4](Al4)Oio(OH)8, where the cation enclosed in brackets is in tetrahedral coordination and that enclosed in parentheses is in octahedral coordination. Normally there is no significant isomorphic substitution for Si(IV) or Al(III) in this group, and, as is common with soil clay minerals, the octahedral sheet has two thirds of its cation sites occupied (dioctahedral phyllosilicate). 

Figure 1.3. The three layer types for phyllosilicate structures in soil clays. All shown here are dioctahedral, with hydroxyl groups shown as shaded circles.

Table 1.8 lists specific surface area values for illitic micas as determined by nitrogen gas adsorption and by negative chloride adsorption.The specific surface areas calculated from N2 gas adsorption with the help of Eq. 1.7 show no particular trend with type of exchangeable cation. The mean value of 5, 11.2 0.5 x 10 m kg", suggests that the mineral forms particles containing seven phyllosilicate layers, as indicated previously. The external surfaces of these particles are expected to repel anions, and therefore the specific surface area determined by negative chloride adsorption should also be around lO m kg" . As shown in Table 1.8, however, the values of 5k, obtained with Eq. 1.18, are always less than 5 and decrease sharply with increasing radius of the... 

In the next section, we demonstrate how exfoliation and ordered restacking of aminopropyl-derivatized magnesium phyllosilicates in the presence of proteins, enzymes or DNA can be used to prepare new types of bio-inorganic layered nanocomposites. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

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... 

This possibility is due to the non-equivalence of Mg and Fe which segregate into corrensite and chlorite respectively. This effect is discussed in the chlorite chapter. Thus four major phyllosilicate phases could be present in an equilibrium situation. It should be noted that the expanding trioctahedral phase is or can be more aluminous than chlorite. This might lead one to think that some of the layers might in fact be dioctahedral such as those in sudoite. The importance of the differentiation of the two types of mixed layered minerals lies in the segregation of alumina and potassium in one (the dioctahedral mixed layered mineral)... 

The number and exact composition of the sheets is used to classify the phyllosilicates. The most important classification for our purposes is the distinction between 1 1 and 2 1-type minerals (Figure 2.1). In 1 1 minerals such as kaolinite, the basal oxygens of the tetrahedral sheet are free to interact with octahedral OH groups forming hydrogen bonds. In contrast, 2 1 minerals such as pyrophyllite or talc contain two tetrahedral sheets sandwiched around an octahedral sheet. These minerals have only basal oxygens exposed on the faces of the tetrahedral sheets and are linked by weak van der Waals forces. The weaker interaction of one 2 1 layer with a second 2 1 layer results in interlayer spaces which, depending on the particular mineral, may be available for contaminant intercalation. 

Clay minerals are hydrous aluminum phyllosilicates made of sheets or layers composed of tetrahedra and octahedra. This mineral type includes the following groups kaolinite, smectite, illite, and chlorite. In the case of smectite, each layer comprises two sublayers of tetrahedra with an inserted octahedral layer, where, between layers, an interlayer space where the exchangeable cations are located is formed [131-133], In Figure 2.24... 

From the discussion above, it can be seen how the atomic structure of phyllosilicate clays plays a key role in determining the final state of clay particles in aqueous media. The presence of structural charges, neutralizing cations, and the capacity of forming hydrogen bonds between different layers produces a system that can be completely delaminated, completely flocculated, or in an intermediate state having floes mixed with isolated layers. Whether the more stable situation corresponds to isolated layers, floes, or a mixture depends on the type of clay, its concentration, pH, concentration and type of supporting electrolyte, and so on. 

Figure 5. Summary plot of apparent number of layers removed or extracted of silica (0-6 weeks) vs. estimated solubilities with respect to dissolved silica, juM, using the 0-6 week data in Figure 4(a-z). The diagonal dashed lines suggest general trends in the data, while the narrow solid line at 100 /xM is the mean concentration of dissolved silica in seawater. Brackets and various types of shading separate minerals into various groups phyllosilicates, crosshatching tectosili-cates, open inosilicates, diagonal lines neso- and soro-silicates, solid.

Kaolinite is a 1 1 (T-O) phyllosilicate. The fundamental unit of its structure is an extended sheet of two constituents a silica-type layer of composition (Si4O10)4- and a gibbsite-type layer of composition (0H)6A14(0H)204 (see schematic representation in Fig. 10). Ideally, kaolinite crystals are not permanently charged. However, due to isomorphic substitution of Si by Al at the siloxane surface, kaolinite platelets carry a small, permanently negative charge (Van Olphen, 1977). Lim et al. (1980) and Talibudeen (1984) postulate that the permanent charge of kaolins is caused by contamination with small amounts of 2 1 phyllosilicates rather than a consequence of isomorphic substitution. 

Layer silicates, sheet-like phyllosilicates such as the familiar micas, are in primary rocks and in soils. The soil minerals are often called clay minerals. Since other components can also be in the clay fraction, layer silicates is a mom accurate term. A typical layer silicate is a combination of a layer of Al-, Mg-, or Fe(II)-0 octahedra plus one or two layers of Si-0 tetrahedra. The tetrahedral and octahedral sheets bond together by sharing oxygens at the corners of the tetrahedra and octahedra. Layer silicate minerals are differentiated by (1) the number and sequence of tetrahedral and octahedral sheets, (2) the layer charge per unit cell, (3) the type of interlayer bond... 

Micas are layer silicates (phyllosilicates) whose structure is based either on a brucite-like trioctahedral sheet [Mg(OH)2 which in micas becomes Mg304(0H)2] or a gibbsite-like dioctahedral sheet [Al(OH)3 which in micas becomes Al204(0H)2]. This module is sandwiched between a pair of oppositely oriented tetrahedral sheets. The latter sheet consists of Si(Al)-tetrahedra which share three of their four oxygen apices to form a two-dimensional hexagonal net (Fig. 1). In micas, the association of these two types of sheet produces an M layer, which is often referred as the 2 1 or TOT layer. 

By analogy with phyllosilicates, a group of titanium silicates whose structures are based on TOT-like layers have been called heterophyllosilicates (Ferraris et al. 1997). In these structures, rows of Ti(Nb)-octahedra (hereafter, Ti-octahedra) are introduced in a T sheet along the direction which is parallel to a pyroxene tetrahedral chain (Fig. 17). HOH layers are thus obtained where H stands for hetero to indicate the presence of the Ti-octahedra in a sheet corresponding to the T sheet of the layer silicates. Because the edges of the Ti-octahedra and Si-tetrahedra have close lengths dimensions, the insertion of the octahedra in a T sheet does not produce strain. As summarized by Ferraris (1997), three types of HOH layers (Fig. 18) are known so far. 

Smectites, by virtue of their large surface area, are particularly sensitive to the exchangeable cation type. In nature, the most common cations are Ca and Na. Calcium ions reduce bound water layer thickness and provide some deformation resistance by cross-linking clay platelet surfaces. Sodium smectite, on the other hand, has the highest affinity for water of any common phyllosilicate, and is therefore used in water-based muds (WBM) to build viscosity and to suspend fine-grained materials used to increase the mud weight. The most geochemically sensitive shales are almost invariably smectitic with saline pore fluids. 

The phenomenon of neoformation of a mixed compound has been extensively studied in the case of the Ni/SiOj system, which forms nickel phyllosiUcate of 1 1 type, also referred to as nickel hydrosilicate. 1 1 nickel phyllosilicate exhibits a stacked structure, each layer consisting of a brucite-type sheet containing Ni(II) in octahedral coordination and a sheet containing linked tetrahedral Si04 units (Figure 14.3a). 

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