Clay minerals, layer lattice

Smectites (Montmorillonites). Smectites are the 2 1 clay minerals that carry a lattice charge and characteristically expand when solvated with water and alcohols, notably ethylene glycol and glycerol. In earUer Uterature, the term montmorillonite was used for both the group (now smectite) and the particular member of the group in which Mg is a significant substituent for Al in the octahedral layer. Typical formulas are shown in Table 2. Less common smectites include volkhonskoite [12286-87-2] hich. contains Cr " medmontite [12419-74-8], Cu " andpimeUte [12420-74-5], (12). 

Clays are composed of extremely fine particles of clay minerals which are layer-type aluminum siUcates containing stmctural hydroxyl groups. In some clays, iron or magnesium substitutes for aluminum in the lattice, and alkahes and alkaline earths may be essential constituents in others. Clays may also contain varying amounts of nonclay minerals such as quart2 [14808-60-7] calcite [13397-26-7] feldspar [68476-25-5] and pyrite [1309-36-0]. Clay particles generally give well-defined x-ray diffraction patterns from which the mineral composition can readily be deterrnined. 

The best formed plate textures are found in crystals with a layer lattice, and generally in all crystals having the form of thin plates. Diffraction pattern (Fig.7) indicates a texture of this type, and was obtained from crystals in the shape of thin hexagonal plates. The specific role of the oblique-texture type electron diffraction patterns have in the study of clay minerals having layer structures (B.B.Zviagin, 1964, 1967). 

Clay Minerals. The clay minerals in coal all contain water bound within their lattices. Kaolinite contains 13.96%, illite 4.5%, and mont-morillonite 5% bound water. In addition, the montmorillonite in the mixed-layer clays also contains interlayer or adsorbed water. All of the water is lost during the high-temperature ashing. 

In many layer structures, such as clay minerals, the extent of lattice adjustment on entry of guest molecules is intermediate between the behaviors of zeolites and of clathrates. The layers remain intact, but the distance between them changes substantially (8). For water-free smectite crystals the d(001) distance is 9.4 A. The van der Waals diameter of a water molecule is 2.8 A so that, in batavite, for example, the water layer in the Na form is about 14.8 — 9.4 = 5.4 A thick, corresponding with two monolayers. 

In conclusion thermal degradation studies on Nautilus pompilius indicate that mineralizing matrix and aragonite shell represent a true structural entity. By the sharing of oxygens in protein and mineral lattices we will generate phase boundaries of the type that are present, for instance, in the common clay mineral kaolinite. Here, aluminum octahedra and silica tetrahedra incorporate the same oxygens and hydroxyls, and layers composed of octahedra and tetrahedra arise (Fig. 13). 

The three-sheet or 2 1 layer lattice silicates consist of two silica tetrahedral sheets between which is an octahedral sheet. These three sheets form a layer approximately 10 A thick. The oxygens at the tips of the tetrahedra point towards the center octahedral sheet and substitute for two-thirds of the octahedrally coordinated hydroxyls. The 2 1 clay minerals include the mica and smectite groups which are by far the most abundant of the clay minerals. The pure end members of this type are talc, a hydrous magnesium silicate pyrophyllite, a hydrous aluminum silicate and minnesotaite, a hydrous iron silicate. 

For clay minerals the natural processes of weathering and erosion tend to produce small particle sizes so that usually only mild dispersion in simple mixers, blenders, or ultrasonic baths are required. Also for days, having inherent lattice charge means that when in contact with water an electric double layer is immediately created and no stabilizing (peptizing) electrolyte may be needed in this case. The converse may also apply. That is, a sample may contain too much electrolyte to be easily dispersed. Clay and other suspensions that contain a large, aggregating amount of electrolyte can be purified by a number of means to remove this electrolyte and create a reasonably stable dispersion. 

Clay Colloids. Three clay minerals are important components of the clay colloid fraction of soils, namely, montmorillonite, illite, and kaolinite (Adams, 1973). Mont-morillonite consists of one layer of aluminum oxide between two layers of silicon oxide (Figure 11.2). An important feature of this mineral is its multilayer arrangement, which permits smaller molecules such as pesticides to penetrate between them. This is referred to as an "expanding lattice" clay. Illite is also a three-layer clay but it does not form multilayers. Kaolinite is a two-layer mineral of aluminum oxide and silicon oxide. 

Acid-treated clay minerals were employed as cracking catalysts in the first commercial process, the Houdry process, widely used in the early petroleum industries to produce high-octane gasoline. The Houdry process catalysts had been discussed extensively by many investigators (2) but were eventually completely replaced by synthetic silica-alumina or zeolite catalysts. Recently, the need for new catalytic materials has revived special interest in the layer lattice silicates because of their ion-exchange properties and their expandable layer structures. 

Geologically and genetically, clay minerals are difficult to define simply and adequately, but broadly they are layer lattice silicates of secondary origin. In the same classification are the micas, talc, chlorites, and serpentines which are not strictly clay minerals. In this context, secondary origin means that mineral formation has arisen from the weathering of primary or igneous rock, e.g. granites and basalts. 

Whilst layer lattice clay minerals are finely divided microcrystalline minerals, which together with their variable composition limits their choice as ion exchangers in industrial processes, the naturally occurring zeolites are largely macrocrystalline. As industrial ion exchangers they are preferable and furthermore they are readily synthesized and in this form may possess even better ion exchange capacities than their naturally occurring counterparts. Artificial zeolites, or... 

The thermal decomposition reactions of several other layer-lattice clay minerals have also been studied by Al NMR in conjunction with other nuclides. These include a montmorillonite which contained sufficient iron for a complementary study to also be made by Mossbauer spectroscopy (Brown et al. 1987), Fuller s Earth (another... 

Clay minerals are frequently platy and a high degree of preferred orientation is required when preparing sample mounts for XRD analysis. This is related to their layer lattice characteristics and atomic sequence normal to the surface of the clay plate. There are numerous ways of preparing orientated mounts, and these include precipitation onto glass slides (Brown, 1953), suction onto unglazed ceramic tiles (Gibbs, 1965 Rich, 1975 Rhoton et al., 1993) and suction onto membrane filters (McAlister and... 

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

To a clay mineralogist, the term illite is synonomous with variability in both composition and crystallinity ( 8). The situation is even further complicated by the fact that much of the material in coal referred to as illite is actually an illite dominated mixed layered clay wherein the illite lattices are randomly interstratified with I4X clay lattices usually chlorite. This mixing of clay mineral lattices further adds to the inherent variability in both composition and crystallinity of the illitic material. The constitution of illite can therefore be expected to vary significantly from sample to sample. It should be quite apparent from the above discussion that no standard illite exists that could be used to represent illite in standard samples. 

Illite as a non-swelling clay mineral of layer structure plays a very important role in our studies. This special role is due to the fact that both sides of the surface of the silicate lamellae are made up of Si04-tetrahedron planar lattices, and this structure - even when hydrophobized - is identical with the surface structure of montmorillonite and vermiculite, both of which are of the swelling type. 

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