Clay minerals structures

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

The two types of clay mineral structures which are of interest in the present discussion are the expanding 2 1 structures (the smectites and vermiculites) and the 1 1 structures (the kaolins). 

Zvyagin B.B. (1967) Electron diffraction analysis of clay mineral structures. New-York, Plenum. 

Brcck, D. R. (1974). Zeolite Molecular Sieves. Wiley-Interscience, New York. Brenner, S. S. (1958). Growth and properties of whiskers. Science 128 569-575. Brindley, G. W. (1980). Order-disorder in clay mineral structures, pp. 125-195. In Brindley, G. W. and G. Brown, eds. Crystal Structures of Clay Minerals... 

B. B. Zvyagin, Electron-Diffraction Analysis of Clay Mineral Structures , Plenum Publishing, 1967. 

A. Ship-in-Bottle Synthesis of Metal Clusters in Zeolites and Clay Minerals Structures and Reactivity of Metal Clusters in Zeolites... 

Fig. 4.8 Schematic diagram to show how the octahedral and tetrahedral sheets, seen as separate entities in (a), can be merged to form 1 1 clay mineral structure shown in (b).

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

This was more marked in the case of illite and montmorillonite. It is considered that in all these cases, the clay mineral structure was destroyed and possibly formed a silicate glass, much like those found in furnace slags but having, perhaps, less oxygen. 

The reactions between clay minerals, NaOH solutions, and micro-wave irradiation showed that the clay mineral structures began to break down, and the released Al and Si (and some K) combined with Na from the solution to form new minerals. The exact reaction path could not be determined from the present experiments. Since the new minerals formed had approximately the same Al Si ratio as the clay minerals, no excess Al (as AI2OJ or Al(OH)g) or quartz was expected, nor was any found in the XRD pattern. The following equations, arranged with increasing time, seem most reasonable to describe the reactions observed for kaolinite and illite ... 

This reaction involves the loss of part of the clay phase, yielding only 0.8 mol of illite per mol of smectite, and presumably requires solution and reprecipitation. However, the Ar/K studies of Aronson and Hower (1976) indicate that inherited radiogenic argon from illite interlayers in original detrital smectite is retained, so that the original clay mineral structure must have persisted. 

Brindley, G.W. 1980. Order-disorder in clay mineral structures, p. 125-195. In G.W. Brindley and G. Brown (ed.) Crystal structures of clay minerals and their x-ray identification. Miner. Soc., London. 

ZvyaginBB (1967) Electron-diflEtaction Analysis of Clay Mineral Structures. Plenum Press, New Yoik Zvyagin BB (1988) Polytypismof crystal stmctiues. Comp Math Applic 16 569-591 Zvyagin BB (1993) Election diffraction analysis of mineris. MSA 23 66-79... 

A number of factors can influence the amount of exchangeable ammonium in soils. Soil porosity, clay mineral structure, presence of other cations, and organic matter content are a few examples. The cation concentration of freshwater wetlands is generally lower than that of coastal wetlands and would result in higher partition coefficients in freshwater wetlands relative to coastal wetlands. 

This chapter will provide only a brief description of clay mineral structures and properties, concentrating on those relevant to catalytic applications. There... 

Molecular Modeling of Clay Mineral Structure and Surface Chemistry... 

Various methods in computational chemistry have been applied to study clay minerals since the early 1970s. The molecular modeling of clay mineral structure and surface chemistry that we will discuss here is mainly about computational chemistry applied to the clay mineral alone or in contact with a specific aqueous solution. 

Clay minerals are isostructural with the mica, but with a random pattern of isomorphic substitutions within their tetrahedral or octahedral sheets, depending on the type of clay. We shall discuss the structural aspects of clay mineral hydrates in the first section of this chapter. Then classical simulations with Monte Carlo (MC) and molecular dynamics (MD) methods will be discussed, together with their application to clay mineral structure, surfacial water structure, adsorption of ions, adsorption of organic and inorganic compounds, and clay nanocomposites. 

We shall look into four types of molecular modeling method used to describe clay mineral structure and surface chemistry quantum mechanics (QM), molecular mechanics (MM), molecular dynamics (MD), and Monte Carlo (MC) methods. If you are more interested in clay mineral applications, you can read section IV before reading this section. 

FIGURE 6 Computational methods for clay mineral structure and surface chemistry. 

IV. COMPUTATIONAL STUDIES OF CLAY MINERAL STRUCTURE AND THE CLAY-WATER INTERFACE... 

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