Silicate clays structure

Figure 2.16. Common groups of layer silicate clay structures found in soils, pictured terms of their tetrahedral (iHk) and octahedral ( ) sheets. The usual locations of - /uctural charge and exchange cations are indicated by — and + signs.

Layer-silicate structure, as in other silicate minerals, is dominated by the strong Si-O bond, which accounts for the relative insolubility of these minerals. Other elements involved in the building of layer silicates are Al, Mg, or Fe coordinated with O and OH. The spatial arrangement of Si and these metals with O and OH results in the formation of tetrahedral and octahedral sheets (see Fig. 8-2). The combination of the tetrahedral and octahedral sheets in different groupings, and in conjunction with different metal oxide sheets, generates a number of different layer silicate clays (see Table 8-1). 

Fig. 8-2 Structure of a 1 1 (kaolinite) and a (montmorillonite) layer-silicate clay mineral.

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. 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

Among all layered silicate clays, the smectite family of 2 1 layer lattice structures are preeminent in their ability to adsorb organic molecules and to catalyze their chemical transformations. All metal oxides in the soil environment may exhibit some degree of surface reactivity. However, the adsorptivity and reactivity of typical smectites are facilitated by their relatively high internal surface areas 700 m2/g) and external surface areas (10-50 m2/g). 

Provided in this chapter is an overview on the fundamentals of polymer nanocomposites, including structure, properties, and surface treatment of the nanoadditives, design of the modifiers, modification of the nanoadditives and structure of modified nanoadditives, synthesis and struc-ture/morphology of the polymer nanocomposites, and the effect of nanoadditives on thermal and fire performance of the matrix polymers and mechanism. Trends for the study of polymer nanocomposites are also provided. This covers all kinds of inorganic nanoadditives, but the primary focus is on clays (particularly on the silicate clays and the layered double hydroxides) and carbon nanotubes. The reader who needs to have more detailed information and/or a better picture about nanoadditives and their influence on the matrix polymers, particularly on the thermal and fire performance, may peruse some key reviews, books, and papers in this area, which are listed at the end of the chapter. 

In this chapter, an overview of the fundamentals of PNs is described, according to the author s understanding and experience as well as support from numerous references and review articles. The content of this chapter covers all kinds of inorganic nanoadditives, but, because the most widely investigated and thus understood nanoadditives used to enhance the thermal and fire resistance of the polymers are clays (natural or synthetic) followed by the CNTs and colloidal particles, the focus of the chapter is primarily on clays, particularly on the silicate clays and LDHs, as well as the CNTs. This includes structure, properties, and surface treatment of the nanoadditives, design of the modifiers, synthesis, characterization of the structure/morphology, and thermal and fire... 

Layered silicate clays intercalated by pillaring poly-oxocations are precursors to an important class of mi-croporous catalysts. Smectite clay was the only host structure known to be pillarable by purely inorganic oxo ions. Recently, layered double hydroxides (LDH) pillaring oxo ions were reported by Pinnavaia and coworkers [79, 80]. 

Clay minerals are hydrous layer silicates of colloidal dimensions, with most if not all of the individual platy particles in the colloidal range of c. 1 nm-1 pm (van Olphen, 1976 Van Damme et al., 1985). The term phyllosilicate (phyllo = leaf like) is applied to the broad group of hydrous silicates with layer structures. The essentia] components of the phyllosilicate structure are two-dimensional tetrahedra and octahedra of oxygen atoms (or ions). The coordinating atoms (or cations) in the centre of the tetrahedra are for the most part Si, but Al3 or Fe3+ may also be present. The coordinating cations in the octahedra are usually Al3, Mg2+, Fe3 or Fe2. Some clay structures (e.g. hectorite) can be synthesized in a reproducible and relatively homogeneous form. 

Phyllosillcates (clay) are the most commonly used diluents because they are relatively inert, available in large quantities at a low cost, and are easily handled during manufacturing and application. Van Olphen (2) defines clays or clay materials as fine-grained, crystalline, hydrous silicates. The structures of clays impart a certain degree of reactivity upon the microbial formulated with them. Several... 

All the monolith composites were prepared at a 1 1 ratio between the magnesium silicate clay binder and the AC or alumina. After premixing of the dry powders by careful addition of water a dough was formed. This dough was extruded as honeycomb monolithic structures with parallel channels of square section at a cell density of 8 cells cm and a wall thickness of 0.9 mm using a Bonnot single screw extruder. 

On a time series of Quaternary marine terraces in northern California, Brimhall et al. (1992) conducted the first mass balance analysis of soil formation over geologic time spans. This analysis provided quantitative data on well-known qualitative observations of soil formation (i) the earliest stages of soil formation (on timescales of 10 -10 yr) are visually characterized by loss of sedimentary/rock structure, the accumulation of roots and organic matter, and the reduction of bulk density and (ii) the later stages of soil development (>10 yr) are characterized by the accumulation of weathering products (iron oxides, silicate clays, and carbonates) and the loss of many products of weathering. 

The geoscience community has been working on nanoparticulate materials for many decades and has made remarkable progress towards understanding the bulk structures of finely crystalline silicate clays and oxyhydroxide, oxide, and hydroxide minerals. More recently, the application of spectroscopic methods has revealed a great deal about... 

Layer of maximum accumulation of silicate clay minerals, or maximum development of blocky structure, or both... 

Micas. If the double-chain amphibole structure diagrammed in Figure 2.8a is extended in two dimensions by the bonding of all three basal 0 atoms of each tetrahedron with Si atoms of other tetrahedra, a sheet silicate (phyllosilicate) is formed with the structure shown in Figure 2.9a. This polymer, extended infinitely in two dimensions, has the formula (Si40to) and is the basis of the mica structure (as well as the layer silicate clays, as will be discussed later in this chapter). 

Layer Silicates. Although the common primary minerals include island, chain, sheet, and framework silicates, the most stable and persistent silicates, which occur as weathering products (secondary minerals) in the clay fraction of soils, are sheet silicates. Figure 2.9a depicts the structure of the tetrahedral sheet in these minerals, which is comparable to the tetrahedral structure of mica. For the layer silicate clays, however, numerous structural combinations of the tetrahedral sheet with octahe-drally coordinated metal cations are possible. 

Many of the layer silicate clays common in soils are based on the mica structure (shown in Figure 2.9b) in which two tetrahedral sheets sandwich a single sheet of octahedrally coordinated cations. Consequently, they are termed 2 1 layer sihcates. Conceptually, it is useful to start with the neutral framework of the talc and pyro-phyllite structures, representing the trioctahedral (Mg in the octahedral sheet) and dioctahedral (AF in the octahedral sheet) members of the 2 1 group. These have the ideal formulae given below ... 

Figure 2,14, Dependence of 2 1 layer silicate clay expansion in water on structural layer charge.

Unlike layer silicate clays, the oxides of Fe and A1 are not inclined to develop structural charge as a result of isomorphous substitution. Consequently they have very low cation exchange capacities despite sometimes possessing impressively large surface areas. The surfaces do, however, develop limited charge (negative or positive) in response to the pH of the surrounding solution, and this process will be discussed... 

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