Two-layer clays

Fig. 77. The arrangement of layers of water molecules (dashed lines) in hydrated montmor-illonites and the corresponding interlayer spacings in (a) a one-layer clay (b) a two-layer clay (c) a three-layer clay. The spacing adopted by a particular montmorillonite depends on the nature of the silicate layer and on the exchangeable cation as well as on certain other conditions of intercalation (e.g., relative humidity and temperature) (470).

Two-layer clays (1 1 clay, TO) Consist of a tetrahedral silica and an octahedral alumina layer. The layers are fixed in a well-defined distance by the hydrogen bonds (H—OH) between the silica and alumina layers. 

FIGURE 9.4 Representation of the structure of kaolinite, a two-layer clay, in which layers of hydrated Si(lV) oxide alternate with layers of Al(III) oxide. 

Figure 17.4 Representation of the structure of kaohnite, a two-layer clay.

Structurally, clays can be classified as either two-layer clays in which oxygen atoms are shared between a tetrahedral sheet and an adjacent octahedral sheet, and three-layer clays in which an octahedral sheet shares ojg gen atoms with tetrahedral sheets on either side. These layers composed of either two or three sheets are called unit layers. A unit layer of a two-layer clay typically is around 0.7 nanometers (nm) thick, whereas that of a three-layer clay exceeds 0.9 nm in thickness. The structure of the two-layer clay kaohnite is represented in Figure 17.4. Some clays, particularly... 

Finer-grained materials such as silts and clayey silts are typically used for monolithic ET cover systems and the top layer of a capillary barrier ET cover system because they contain finer particles and provide a greater storage capacity than sandy soils. Sandy soils are typically used for the bottom layer of the capillary barrier cover system to provide a contrast in unsaturated hydraulic properties between the two layers. Many ET covers are constructed of soils that include clay loam, silty loam, silty sand, clays, and sandy loam. 

Multilayer Adsorption of Water. As the amount of water in the clay increases over that needed for a one- or two-layer hydrate, the study of the properties of the water becomes experimentally more difficult. This is important because it is only at water contents in excess of the two-layer hydrate that a conflict arises between the short-range and long-range interaction models. In support of the short-range model, two studies are noteworthy. A small angle... 

In terms of composition, the simplest of the marine clay minerals is kaolinite in which tetrahedral and octahedral layers alternate (Figure 14.5a) creating a two-layer repeating imit. In three-layer clays, the repeating unit is composed of an octahedral... 

The problem with limited selectivity includes some of the minerals which are problems for XRD illite, muscovite, smectites and mixed-layer clays. Poor crystallinity creates problems with both XRD and FTIR. The IR spectrum of an amorphous material lacks sharp distinguishing features but retains spectral intensity in the regions typical of its composition. The X-ray diffraction pattern shows low intensity relative to well-defined crystalline structures. The major problem for IR is selectivity for XRD it is sensitivity. In an interlaboratory FTIR comparison (7), two laboratories gave similar results for kaolinite, calcite, and illite, but substantially different results for montmorillonite and quartz. 

The aged pillaring reagents were heated to 65 C and 325 bentonite, American Colloid Company, whose major constituent is the layered clay mineral, montmorilIonite, was added. There was always a 5-fold excess of aluminum in solution and the volume of solution per gram of clay was always 45 cc/g or more. The reaction was carried out for 2 hours. The slurry was filtered and the solids washed two times with water, dried, sized, and calcined at 500 C for 2 hours in air. 

Examples are the mica group (e.g., KMg3(OF[)2Si3A10io) of which biotite, (K(Mg,Fe)3(OH)2Sl3A10io) (Figure 1.55) and talc (Mg3(OFl)2Si40io) are members, and contains a sandwich of two layers with octahedrally coordinated cations between the layers and clay minerals such as kaolin, Al4(OH)8Sl4C3io. 

The peculiar layer structure of these clays gives them cation exchange and intercalation properties that can be very useful. Molecules, such as water, and polar organic molecules, such as glycol, can easily intercalate between the layers and cause the clay to swell. Water enters the interlayer region as integral numbers of complete layers. Calcium montmorillonite usually has two layers of water molecules but the sodium form can have one, two, or three water layers this causes the interlayer spacing to increase stepwise from about 960 pm in the dehydrated clay to 1250, 1550, and 1900 pm as each successive layer of water forms. 

MgOn(OH)j -] units (Fig. 7.5), and the illite type in which the octahedral sheet is sandwiched between two layers of tetrahedra (cf. micas such as muscovite, Fig. 7.4). Many important clay minerals such as vermiculite, biotite, and smectites (notably montmorillonite and beidellite, the princi-... 

Mixed layered clays, most often ordered, are present up to temperatures near 200°C at depths of 500 to 1500 meters. The minerals form in two distinct zones. At shallow depths (between 100 and 200°C) mixed layering is between 90 and 0% montmorillonite. Above 200°C or so no expandable minerals are present. In the second zone (1.5Km. depth) one finds ordered interlayering showing the superstructure reflection... 

In this type of system, the polymer chains are constrained by a surface. They can lie between two hard surfaces such as in the galleries within two parallel clay platelets (as is illustrated in Figure 7), have one layer absorbed on to a hard surface as a coating, with the other free (as in Figure 8), they can be absorbed by the surfaces of exfoliated clay platelets (Figure 9), or by the surface of a solid reinforcing particle completely surrounded by an elastomeric phase (Figure 10). 

Figure 10. Schematic illustration of the structure of a montmorillonite like clay. Two layers of silica-oxygen tetrahedra are joined together by an octahedrally coordinated aluminium-oxygen/hydroxide layer to give an individual clay sheet. (Reproduced with permission from W. Jones in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 387.)...

The meaning of one-, two-, and three-layer clays is best illustrated diagrammatically (see Fig. 77). Layers in this context refer to the interlamellar water, though the precise chemical nature of this entrained water is not easily established and is, in any case, a function of the parent silicate. In some sheet silicates the water is believed to take up an ice-like monolayer. Recent studies reveal that the interlamellar ion and associated water are rather mobile above room temperature. Such water is readily, but not always... 

When pillared smectites without tetrahedral substitution are calcined, there is no reaction between the pillars and the smectite layers. By contrast, a considerable structural transformation occurs when pillared beidellite is calcined, which has been interpreted as the growth of a three-dimensional quasi-zeolitic framework between the two-dimensional clay layers. The acidic properties of the product are comparable with those of zeolite Y and much more pronounced than those of calcined pillared smectites without tetrahedral substitution. 

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