Network silicates

In a similar way, the sheets of the two-dimensional network silicates, shown in Figure... 

Fig. 17-10. Three-dimensional network silicates granitic minerals.

Quartz is an important network silicate (Section 12.10). A number of additional tetrahedral silicate-like materials possess some AIO4 tetrahedra substituted for the Si04 tetrahedra. Such structures offer a little larger hole in comparison to the entirely Si04 structures allowing alkali and alkaline-earth cations to be introduced. Feldspar (orthoclase) is one such mineral. The alumino-silicate networks are almost as hard as quartz. For feldspar and other tetrahedral networks the number of oxygen atoms is twice the summation of silicon and other MO4 cations. 

The physical properties of the silicates correlate closely with their structures. Talc, Mg3(Si40io)(OH)2, is an example of an infinite layered structure (see Fig. 22.If). In talc, all of the bonding interactions among the atoms occur in a single layer. Layers of talc sheets are attracted to one another only by van der Waals interactions, which (being weak) permit one layer to slip easily across another. This accounts for the slippery feel of talc (called talcum powder). When all four vertices of each tetrahedron are linked to other tetrahedra, three-dimensional network structures such as cristobalite (see Fig. 22.Ig) or quartz (Fig. 22.2) result. Note that the quartz network carries no charge consequently, there are no cations in its structure. Three-dimensional network silicates such as quartz are much stiffer and harder than the linear and layered silicates, and they resist deformation well. 

The simplest network silicates are the pyroxenes, which contain extended chains of linked SiO tetrahedra (Figure 18.11). If two such chains are laid... 

If the linking of silicate chains continues in two dimensions, sheets of SiO tetrahedral units result (Table 18.4). Various clays and mica have this sheetUke structure. Clays, which are essential components of soils, are aluminosilicates— some Si + ions are replaced by Al ions plus other cations that take up the additional positive charge. Feldspar, a component of many rocks and a network silicate, is weathered in the following reaction to form clay. 

In compounds with metals, isolated boron atoms behave as metal atoms and, with an excess of transition metals, boron can form intermetallic compounds and alloys if n > 2 in the stoichiometry M B. Although boron atoms can exist as metallic atoms in metallic alloys they have the tendency to bind to each other if given the opportunity, even in metals. If the boron content increases to values greater than in M2B covalently bound boron pairs, chains and nets are formed in the metal matrix (Figure 4.9). As in the case of network silicates the stoichiometry of the metal boride is related to the type of boron network in the lattice. A few representative metal borides are listed in Table 4.4. 

TectosiHcates Network silicates (silica and feldspars) 1 2 ratio of Si 0 Quartz SiOg Orthoclase K(AISi30g) Anorthite Ca(Al2Si20g) 100% sharing ... 

Multicomponent Silicate Systems. Most commercial glasses fall into the category of sihcates containing modifiers and intermediates. Addition of a modifier such as sodium oxide, Na20, to the siUca network alters the stmcture by cleaving the Si—O—Si bonds to form Si—0-Na linkages (see Fig. 3c). 

Aluminosilicates. These silicates consist of frameworks of silica and alumina tetrahedra linked at all corners to form three-dimensional networks familiar examples are the common rock-forming minerals quartz and feldspar. Framework silicates generally form blocky crystals, more isotropic... 

The viscosity of liquid silicates such as drose containing barium oxide and silica show a rapid fall between pure silica and 20 mole per cent of metal oxide of nearly an order of magnitude at 2000 K, followed by a slower decrease as more metal oxide is added. The viscosity then decreases by a factor of two between 20 and 40 mole per cent. The activation energy for viscous flow decreases from 560 kJ in pure silica to 160-180kJmol as the network is broken up by metal oxide addition. The introduction of CaFa into a silicate melt reduces the viscosity markedly, typically by about a factor of drree. There is a rapid increase in the thermal expansivity coefficient as the network is dispersed, from practically zero in solid silica to around 40 cm moP in a typical soda-lime glass. 

A higher content of AI2O3 and SiOj is critical for the composition of the protective films in the tubing, assuming the water contains silicates or silicic acid. The protective films have a maximum thickness of 1.5 mm and cannot grow further. The corrosion process can be stopped even in copper pipe networks with type I pitting [21] by providing a reaction tank with impressed current aluminum anodes. 

The uses in the glass and ceramics industries reflect the diagonal relation between boron and silicon and the similarity of vitreous borate and silicate networks (pp. 203, 206 and 347). In the UK and continental Europe (but not in the USA or Japan) sodium perborate (p. 206) is a major constituent of washing powders since it hydrolyses to H2O2 and acts as a bleaching agent in very hot water ( 90°C) in the USA domestic washing machines rarely operate above 70°, at which temperature perborates are ineffective as bleaches. 

In general, greatly reduced rates of attack are observed for impure or dilute nickel alloys compared with pure nickel when exposed to SO2 + O2 atmospheres. Haflan et al. have attributed this to the segregation of impurities at the sulphide/oxide interface causing breakup of the sulphide network. For example in the case of silicon additions, it has been shown that silicates form and it has been proposed that these alter the wetting characteristics of the sulphide and prevent the establishment of an interconnected sulphide network. 

More than 90% of the rocks and minerals found in the earth s crust are silicates, which are essentially ionic Typically the anion has a network covalent structure in which Si044-tetrahedra are bonded to one another in one, two, or three dimensions. The structure shown at the left of Figure 9.15 (p. 243), where the anion is a one-dimensional infinite chain, is typical of fibrous minerals such as diopside, CaSi03 - MgSi03. Asbestos has a related structure in which two chains are linked together to form a double strand. 

Many properties of silicates can be understood in terms of the type of network lattice formed. In the one-dimensional networks, shown in Figure 17-8, the atoms within a given chain are strongly linked by covalent bonds but the chains interact with each other through much weaker forces. This is consistent with the thread-like properties of many of these silicates. The asbestos minerals are of this type. 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

The concept of silicates as inorganic polymers was implicit in the ideas developed by W. H. Zacheriasen in the early 1930s. He conceived of silicates as consisting of macromolecular structures held together by covalent bonds but including network-dwelling cations. These cations were not assumed to have a structural role but merely to be present in order to balance the charges on the anionic polymer network. 

Carbon black is reinforced in polymer and mbber engineering as filler since many decades. Automotive and tmck tires are the best examples of exploitation of carbon black in mbber components. Wu and Wang [28] studied that the interaction between carbon black and mbber macromolecules is better than that of nanoclay and mbber macromolecules, the bound mbber content of SBR-clay nanocompound with 30 phr is still of high interest. This could be ascribed to the huge surface area of clay dispersed at nanometer level and the largest aspect ratio of silicate layers, which result in the increased silicate layer networking [29-32]. 

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