Clay minerals crystal structure

In clay mineral crystals, atoms having different valences commonly will be positioned within the sheets of the structure to create a negative potential at the crystal surface. In that case, a cation is adsorbed on the surface. These adsorbed cations are called exchangeable cations because they may chemically trade places with other cations when the clay crystal is suspended in water. In addition, ions may also be adsorbed on the clay crystal edges and exchange with other ions in the water. 

Brown, G. (1984). Crystal structure of clay minerals and related phyllosilicates. In Clay Minerals Their Structure, Behaviour and Uses, ed. Fowden, L., Barrer, R.M. and Tinker, P.B., Royal Society, London, pp. 1 20. 

It suggests that water adsorption occurs mainly on structural defects of the clay mineral crystal rather than on the basal surfaces because, due to the presence of aluminol groups on one face of kaolinite, kaolinite would be, in principle, more hydrophylic than illite which exhibits only siloxane bridges on both faces of the crystal. 

BRI 80] BRINDLEY G.W., Quantitative x-ray mineral analysis of clay in Crystal structure of clay minerals and their x-ray identification, Mineraloeical Society, London, p 438-441,1980. 

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

Dresselhaus, M.S. Dresselhaus, G. Adv Phys. I98I, 30, 139. Brindley, G.W. In "X-ray Identification and Crystal Structure of Clay Minerals" Mineralogical Society of Great Britain,... 

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... 

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

Brindley, G. Brown, G. 1980. Crystal structures of clay minerals and their X - ray identification. London Mineralogical Society. 

Figure 3.3. The left structure represents kaolinite, a 1 1 clay mineral, and the right structure, a 2 1 clay mineral. These representations are intended to show surface groups, surface pairs of electrons, unsatisfied bonds, and associations between clay particles. Note that clay structures are three-dimensional and these representations are not intended to accurately represent the three-dimensional nature nor the actual bond lengths. Also, the brackets are not intended to represent crystal unit cells.

Brindley, G. W. Brown, G. "Crystal Structures of Clay Minerals... 

Clay Minerals as Lewis Acids. Lewis acid sites in a clay mineral are exchangeable (2) or structural ( 0) transition metal cations in the higher valence state, such as Fe + and Cu +, and octahedrally coordinated aluminum exposed at the crystal edges (38). Reduction of both exchanged and structural (octahedral) transition metal cations in the upper oxidation state is a reversible process (12,... 

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). 

Schofield RK, Samson HR (1954) Flocculation of kaolinite due to the attraction of opposite charged crystal faces. Discuss Faraday Soc 18 135-145 Schofield RK, Samson HR (1953) The defiocculation of kaolinite suspensions and the accompanying change-over from positive to negative chloride adsorption. Clay Miner BuU 2 45-51 Schulten HR (2001) Models of humic structures association of humic acids and organic matter in soils and water. In Qapp CE et al. Humic substances and chemical contaminants. Soil Science Society of America, Madison, Wl, pp 73-88... 

When three of the oxygens in the tetrahedra are shared (Si O ratio = 2 5), the complex ions that form take on a sheetlike configuration. The sheets can be stacked, and the associated cations are found between the sheets. Micas and clays are sheet-structure minerals with distinctive habits and physical properties, that reflect the planar silicate sheet structure (Fig. 2.1G). These normally platey minerals may also occur with fibrous-growth habits. The special crystal chemistry that produces such a distinctive habit is discussed later. 

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... 

Honjo, C., N. Kitamura, and K. Mihama (1954). A study of clay minerals by means of single crystal electron diffraction diagrams structure of tubular kaolin. 

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. 

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