Smectite group, silicates

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. 

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

The montmorillonite (Chapter 1, Table 1.2) clay mineral can be used as a model substance in the study of the interfacial processes of rocks and soils. It is a layer silicate, a member of the smectite group. Its structure is appropriate for modeling the most important interfacial processes in geological formations. Besides, it is a fairly widespread mineral in rocks and soils, and plays an important role in the nutrient cycle of soils. In addition, it has many agricultural, industrial, and environmental applications. 

Smectite group (montmorillonite, beidellite, nontronite, etc.) Silicatic rocks of either origin a) Incomplete leaching of silicates (feldspars, micas) due to restricted water drculation b) After deposition either by removal of potassium from micas or neoformation from solutions To kaolinite by subsequent leaching, to illite or glauconite by addition of potassium and iron, to chlorite Either residual or neoformation... 

If, in three-layer sheet silicates, part of the Si in the tetrahedral layer is substituted by Al, then negative surface charges will occur that are compensated for by alkali cations, rather weakly bound between the three-layer stacks. In that way the large group of mica is formed. On the other hand, the formal partial substitution of Al by Mg in the pyrophyllite lattice produces a charge deficiency that will be compensated for by monovalent or divalent atoms such as Na or Ca to create the minerals of the smectite group. 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

The g-values and A values of Table IV reveal that the particular layer silicate has more effect on ESR parameters of adsorbed Cu " - than saturation of exchange sites with different cations such as Na+ and Ca +. Also, the smectites as a group have lower g and higher A values than vermiculite. From the perspective of molecular orbital theory, low g and high A values correspond to more covalent bonds between Cu + and the ligand (19). Thus,... 

As a function of their structural properties, clays interact differently with organic and inorganic contaminants. Two major groups of clay minerals are selected for discussion here (a) kaolinite, with a 1 1 layered structured aluminosilicate and a surface area ranging from 6 to 39 m g" (Schofield and Samson 1954) and (b) smectites with a 2 1 silicate layer and a total surface area of about 800m g" (Borchardt 1989). 

Saponite is a naturally occurring phyllosilicate clay of the smectite (montmorillonite) group. It is a magnesium-rich hydrated aluminum silicate and is present as a component of some commercial magnesium aluminum silicate clays. Saponite is a mineral with an approximate empirical formula owing to the variability in cation substitution see Table I. 

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