Structure of the Clay Minerals

We have already learnt that the building unit of the silicates is the Si04 pyramid. By linking these pyramids in various ways the different kinds of silicate anions are formed. The classification of the silicates is based on these kinds of ions. The silicate anions of clay minerals have a layered structure, the top view of which is again represented in figure 8.5. 

The silicate construction sheet in figure 8.5 consists of tetrahedrons and is called T-sheet. Clay minerals also have a second building 

Unit of physical properties, 1st part of table %(m/m) [unless specified differently] Unit of analysis of main elements, 2nd part of table %(m/m) 

Unit of analysis of trace elements, 3rd part of table mg/kg 

In figure 8.6 every octahedron shares the four OH -ions in the sheet with four neighbouring octahedrons, i.e. 1/4 of each of those four ions belongs to the octahedron. The OH-ions above and below the Al3+-ion fully belong to the octahedron. The formula of an octahedral unit in the sheet is thus Al3+ + 4x1/4 OH + 2 x OH = Al(OH)3. Now that we have been introduced to the O and T sheets, the next step is to link these sheets and in this way form clay minerals. This process in represented in figure 8.7. 

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This section reviews several of the methods that are used to categorize clays. First, the structure of clay minerals will be discussed. Next, the mechanism of formation for kaolinite will be reviewed followed by a description of the types of deposits in which clays are found. The section will end with a description of the types of clays used in the ceramics industry. 

Fixation of difficultly-exchangeable ammonium by clay minerals is limited to the 2 1 type minerals. For information on the structure of clay minerals reference should be made to Hendricks (1945) Ross and Hendricks (1945) Gieseking (1949) Grim (1953) ... 

R. Prost, Interactions between adsorbed water molecules and the structure of clay minerals Hydration mechanism of smectites, Proc. Int. Clay Conf. 1975, p. 351 (1976). 

While the structure bears a resemblance to bioorganic bilayer membranes, the overall similarity to the structures of clay minerals is striking. First, consider the repeat distance in the sodium salt of the calixarene, 13.7 A, compared to... 

Figure 3. Diagrammatic projections of the structures of clay minerals, (a) Kaolinite (b) mont-morillonite (c) mica (d) chlorite (e) sepiolite. (After Mackenzie and Mitchell [1966].)...

G. Brown, Ed., The A-ray Identification and Crystal Structures of Clay Minerals, Mineralogical Society, London, 1961. 

A solid phase, as discussed in detail in Chap. 2, is composed of varying amounts of mineral and organic matter which influence the crumb structure and the binding capacity, by the association of clay minerals with organic matter of the solid. The ability of a solid phase to sorb organic pollutants is also influenced by variable system conditions and differing environmental conditions. 

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). 

Brown, G. (1953) The occurrence of lepidocro-dte in British soils. J. Soil Sd. 4 220—228 Brown, G. (1980) Associated minerals. In Brindley, G.W. Brown, G. (eds.) Crystal structures of clay minerals and their X-ray identification. Min. Soc., London, 361-410 Brown, G.E.Jr. (1990) Spectroscopic studies of chemisorption reaction mechanisms at oxide/water interfaces. In Hochella, M.F.Jr. 

Mineral Surfaces. Organic matter is chemically adsorbed (deriva-tized) at the surfaces of clay minerals, zeolites, and related minerals (105) and is at times protected, concentrated, and degraded by contact with the solid surfaces. For example, porphyrins are protected (106), as are optically active amino acids by montmorillonite (107). This may result in part from the position of the organic matter in lattice spaces, as shown by Stevenson and Cheng (108) for proteinaceous substances keyed into hexagonal holes on interlamellar surfaces of expanding lattice clays, or from the fact that there are ordered structures at solid-water interfaces (109). 

Brindley, G. W., and Brown, G. (1980). Crystal Structures of Clay Minerals and Their X-Ray Identification. Mineralogical Society Monograph No. 5.The Mineralogical Society, London. 

Brindley, G.W., 1961b. Kaolin, serpentine and kindred minerals. In G. Brown (Editor), The X-Ray Identification and Crystal Structures of Clay Minerals. London Mineral. Soc., London, pp. 51-131. 

Structures of Clay Minerals. The term "clay minerals" generally refers to fine-grained (< lpm) sheet silicates. Many detailed discussions of the structures and compositions of clay minerals exist (e.g., 14-16). and their interesting chemical properties have been reviewed previously (H). However, a short introduction to their structures is necessary to understand their HRTEM images. 

The experimental studies of water interactions with clay minerals are very extensive. The structure, dynamics and interactions of interlayer water with the surface of clay minerals were reviewed in several papers [32, 33] and described in a number of books [15, 34, 35]. Therefore, we will review only the most important studies concerning experimental investigations of the structure and interactions of water molecules on clays. 

The most powerful methods for the study of adsorption mechanism of nitroaromatic compounds on clay minerals have become in situ spectroscopic investigations. Handerlein et al. [152, 153] and Weissmahr et al. [154-156] have investigated the adsorption of NACs particularly on illites, montmorillonites and homoionic kaolinites. The substituted nitrobenzenes on the surface of smectites were investigated by Boyd et al. [157, 158], The main focus in the experimental study of adsorption of NACs on the surface of clay minerals is the influence of the type of clay mineral, the effect of exchangeable cation of the mineral, the effect of the structure and the kind of substituents of NAC compound on the position and orientation of NACs to the surface of mineral, the character of interaction between NACs and the surface of mineral, the adsorption energy. 

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