Layered Silicates

When these minerals are heated they dehydrate in a remarkable way by extruding little wormlike structures as indicated by their name (Latin vermiculus, little worm) the resulting porous light-weight mass is much used for packing and insulation. The relationship between the various layer silicates is summarized with idealized formulae in Table 9.10 (on page 357). 

Table 9.10 Summary of layer silicate structures (idealized formulae) ...

Vermiculite (All,7Mgo.3)Si3.6Alo,40lo(OH)2 2 1 layer-silicate... 

A specific example of this would be the weathering of K-feldspar and the formation of kao-linite (see Table 8-1 for mineral definitions), a layer-silicate clay ... 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

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). 

Once a layer-silicate clay forms, it does not necessarily remain in the soil forever. As conditions change it too may weather and a new mineral may form that is more in equilibrium with the new conditions. For example, it is common in young soils for the concentrations of cations such as K, Ca, or Mg in the soil solution to be high, but as primary minerals are weathered and disappear, cation concentrations will decrease. With a decrease in solution cations, a layer-silicate such as vermiculite will no longer be stable and can weather. In its place. 

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

Polymer layered-silicate day nano-composites (PLCN) attracted lately major interests into the industry and academic fields, since they usually show improved properties with comparison by virgin polymers or their conventional micro and macro-composites. Improvements induded increase in strength, heat resistance (Giannelis, 1998), flammability (Gilman, 2000) and a decrease in gas permeability (Xu et ah, 2001) as well as an increase in biodegradability (Sinha et al., 2002). 

However, the field of polymer day silicate has only started to speed up recently, mixing the appropriate modified layered silicate with synthetic layered silicates has long been known (Theng, 1979). The interest in these materials came from two important findings, first has been reported by Toyota research group of a Nylon-6 (N6)/Na-MMT nano-composites (Okada et ah, 1990) where very small amounts of layered silicate loadings resulted in the improvements of thermal and mechanical properties and second the findings of Vaia et ah (1993) about the... 

Three main types of composites can be formed when the layered clay is incorporated with a polymer, as shown in Figure 2 (Alexandre and Dubois, 2000). Types of composites formed mostly depend on the nature of the components used (layered silicate, organic cation and polymer matrix) and the method of preparation. 

The nano-scale structures in polymer layered-silicate nano-composites can be thoroughly characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD is used to identify intercalated structures. XRD allows quantification of changes in layer spacing and the most commonly used to probe the nano-composite structure and... 

Alexandre, M. and Dubois, P. 2000. Polymer-layered silicate nano-composites preparation, properties and uses of a new class of materials. Materials Science and Engineering 28 1- 63. 

Giannelis, E.P. 1996. Polymer layered silicate nanocomposites. Advanced Materials 8 29-35. 

Giannelis, E.P. 1998. Polymer- layered silicate nanocomposites synthesis, properties and applications. Applied Organometallic Chemistry 12 675-680. 

Homminga, D., Goderis, B., Hoffman, S., Reynaers, H. and Groeninckx, G. 2005. Influence of shear flow on the preparation of polymer layered silicate nanocomposites. Polymer 46 9941-9954. 

Ray, S.S. and Bousmina, M. 2005. Biodegradable polymers and their layered silicate nanocomposites In greening the 21 century materials world. Progress in Materials Science 50 962-1079. 

Ray, S.S. and Okamoto, M. 2003. Polymer/layered silicate nanocomposite a review from preparation to processing. Progress in Polymer Science 28 1539-1641. 

Sinha, R.S., Yamada, K., Okamoto, M. and Ueda, K. 2002. New polylactide/layered silicate nanocomposite A novel biodegradable material. Nano Betters 2 1093-1096. 

Vaia, R.A., Ishii, H. and Giannelis, E.P. 1993. Synthesis and properties of two dimensional nanostructures by direct intercalation of polymer melts in layered silicates. Chemistry of Material 5 1694-1696. 

Xu, R., Manias, E., Snyder, A.J. and Runt, J. 2001. New biomedical poly(urethane uera)-layered silicate nanocomposites. Macromolecules 34 337-339. 

Zanetti, M., Camino, G., Thomann, R. and Mulhaupt, R. 2001. Synthesis and thermal behaviour of layered silicate-EVA nanocomposites. Polymer 42 4501- 4507. 

This mbber is very tacky in nature and contains acrylic group, which makes it polar in nature. Nanocomposites have been prepared based on this elastomer with a wide range of nanohllers. Layered silicates [53-55] have been used for this preparation. Sol-gel method [56,57], in situ polymerization [58], and nanocomposites based on different clays like bentonite [59] and mica [60] have been described. The mechanical, rheological, and morphological behaviors have been investigated thoroughly. 

McDowell, S.D. and Elders, W.A. (1980) Authigenic layer silicate minerals in borehole Elmore 1. Salton Sea geothermal field, California, U.S.A. Contr. Mineral. Petrol., 74, 293-310. 

Metallic cobalt exhibits this phenomenon, and so do layered silicates and layered halides like Cdl2 or Bil3. In X-ray diffraction, stacking faults cause the appearance of diffuse streaks (continuous lines in the diffraction pattern). 

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