Structure of Clays

The octahedral sheet is further divided into two types the dioctahedral type 

The silicate layer is an electrically neutral structure. If part of tetrahedral Si4+ or octahedral Al3+ is isomorphously substituted by lower valency cations the sheet becomes negative. In this case the clay will have exchangeable cations between every layer to compensate for the negative charge of the layers. The cation exchange capacity (CEC) of a clay is equivalent to the layer charge and is dependent on the degree of isomorphous substitution. 

Montmorillonite, which is the most commonly used clay for catalysing organic reactions, is a smectite clay in which the unit layer is composed of one octahedral sheet containing aluminium and magnesium ions sandwiched between two sheets of silica oxygen tetrahedra. In between the layers are hydrated metal ions, usually Na+ or Ca2+ in the natural materials. 

The layers extend in the a and b axis directions and are stacked one above the other in the c direction. The thickness of one layer is about 0.96 nm but the 

Smectite (0.25 x 0.6) Montmorillonite MJAh.JtffoKSLOOj OHk Beidellite MJAlzKSU-xAUOirfOHk Hectorite Mjc[Mg3 xLiJ (Si4)0 io(OH)2 Saponite MJMg3](Si4 xAU)O10(OH)2 

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Fig. XI-12. End-on view of the layer structures of clays, pyrophillite, and mica. (From Ref. 163.)...

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,... 

Table I. Chemical Structures of Clay and Mineral Fines Stabilizers...

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. 

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. 

The surfaces of colloidal particles are often charged and these changes can arise from a number of sources. Chemically bound ionogenic species may be found on the surface of particles such as rubber or paint latex particles. Charged species may be physically adsorbed if ionic surface active materials, for example, have been added. A charged surface may occur on a crystal lattice. An example is the isomorphous substitution of lower valency cations such as aluminium for silicon in the lattice structure of clays. A further example is the adsorption of lattice ions... 

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

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... 

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. 

A purified Na -exchanged montmori1lonite ("Kunipia F", 1.19 meq/g) was ion exchanged with aqueous Fe(N03)3. The Fe -exchanged montmori1lonite (abbreviated as Fe-Mont) was washed and dehydrated under relatively mild conditions of 120°C/0.5 Torr for 24 h to avoid collapse of the layer structure of clay. The BET surface area of Fe-Mont thus obtained was 37 m /g. 

Section 15.13 discusses the structure of clays in more detail. 

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. 

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