Types of silicate

Together with the structural principles established by the Bragg school concerning the many types of silicates, Goldschmidt s ideas were taken further by Linus Pauling in California to establish the modern science of crystal chemistry. A good early overview of the whole field can be found in a book by Evans (1939, 1964). 

In studies of amphiboles (44), isolated strips of triplechain silicates were discovered embedded in the double-chain parent structure. It was later realized that new types of silicate structures, composed of recurrent triple chains, existed in nature. The part that HREM played in the identification of this new family of triple-chain silicates, which constitute a further step in the progression pyroxene, amphibole,. .. mica, was crucial. 

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

EPR Labile, rhombic type spectrum corresponding to Ti-superoxo species spectral features sensitive to the type of silicate structure, temperature, solvent and pH... 

When all four oxygens of the tetrahedra are shared (Si 0 = 1 2), a fully polymerized ion results, and a three-dimensional framework is formed. Quartz, an example of this type of silicate array, is discusssed in the section on silica minerals. 

Since silicates and aluminosilicates are by far the predominant rock-forming minerals, the crystal structures of most species have been determined. Liebau (1980) presents an overview of the structures of silicate and aluminosilicate minerals, and one can consult Berry, Mason, and Dietrich (1983, especially pp. 382-389) or other mineralogy texts for an introduction to the subject. The multivolume work of Eitel (1965) provides a general treatment of the crystal chemistry of all types of silicate materials. 

Glass fibers are the most common reinforcing fiber due to their excellent combination of mechanical properties, dielectric properties, thermal stability and relatively low cost. As a result, there are many different types of silicate glass fibers, all with varying properties designed for various applications (see Table 1.32). The majority... 

Fig. 4.4.10. Differentiation of three types of silicate glasses according to hardness.

Limestone is composed of calcium carbonate (CaC03) in the form of the mineral calcite (Figure 13.2). Chalk and marble are also made of calcite which is the second most abundant mineral in the Earth s crust after the different types of silicates (which include clay, granite and sandstone). 

Two molecular types of silicates are referred to as asbestos. Chrysotile is a magnesium silicate built upon a layered structure of silicate rings and Mg(OH)2. The layered structure causes the sheets to roll into cylinders approximately 200A in diameter. Amphibole asbestos may contain a variety of cations but is built upon a double chain silicate structure. The chrysotile asbestos is always found as an asbestiform crystal while the amphiboles may be either acicular or asbestiform. 

By using the ammonium ions with four alkyl groups larger than the methyl group, the other types of silicate anions with cage-like structures are formed. For example, in tetraethylammonium (N+(C2H3) ) silicate aqueous solutions, the prismatic hexameric silicate anion (SigO g, prismatic hexamer) is formed (13,14,17,18). 

Effect of Addition of Sodium Ions to Tetramethylammonium Silicate Aqueous Solution. In zeolite synthesis, alkali metal cations are combined with organic quaternary ammonium ions to produce zeolites with different structures from the one produced with only the organic quaternary ammonium ion (2) It is then expected that other types of silicate species are formed in the silicate solutions when organic quaternary ammonium ions and alkali metal cations coexist. In such silicate aqueous solutions, however, alkali metal cations only act to suppress the ability of the organic quaternary ammonium ions to form selectively silicate species with cage-like structures (13,14,28,29). 

To demonstrate that alternative types of silicate framework can be used for this reaction, experiments were carried out with copper-exchanged MCM-41. Yields of up to 87% of the aziridine with e.e. of 37% were obtained using PhI=NTs as the nitrene donor. Using... 

Telescope have proven the existence of a significant amount of frozen material and various types of silicates and PAHs in disks (van den Ancker et al. 2000 van Dishoeck 2004 Bouwman et al. 2008). The PAH features at 3-12 pm probe the incident radiation field and density distribution of the upper disk (e.g. Habart et al. 2004). 

Range of Composition (mol% MjO) Type of Silicate Entities Present... 

Rectystallization of 7 from dichloromethane affords single crystals which are suitable for X-ray diffraction analysis. Unfortunately, Because of a twinning problem, the results of the crystal measurements were not satisfactory for the determination of precise geometric parameters. However, the molecular array of 7 has been elucidated and the results are shown in Fig. 1. The analysis revealed that 7 is a new type of silicate dimeric species. 

The above considerations of the chemical durability and corrosion of glass were concerned with current types of silicate and borosilicate glasses. Special types of glass may exhibit quite different behaviour and also require special test methods. 

In 1 of these plants, silica is taken up through the roots as dissolved Si(OH)4 and is precipitated within and around the cells as hydrated opaline deposits which often replicate the shapes of the associated cellular structures. Upon death and decomposition of a plant, these siliceous deposits are released to the soil as discrete and generally microscopic structures known as phytoliths or plant opal . Most phytoliths in soils are derived from the aerial parts of plants. However, silicification can be extensive also in the roots and rhizomes of certain grasses, so that soils developed under this kind of vegetation receive significant quantities of biogenic silica from the underground portions of the plants. A rarer type of siliceous deposit in some vascular plants is tabashir (Jones et al., 1966). This material apparently is restricted to the bamboos, where it occurs within the hollow stems as solid, transluscent, opaline masses up to several cm thick. 

Other types of silicates have sheets containing Si20s units, with cations lying between the sheets and linking them together. One example is talc, Mg3(Si205)2(OH)2. Additional structural types of silicon oxides exist. 

This concludes our account of the silicates. While we have tried to discuss the more important structural and chemical features of these minerals we would again emphasize that our treatment has been in no sense exhaustive from a mineralogical point of view (Si, A1)04 tetra-hedra can be linked together in many different ways and numerous types of silicate exist in addition to those which we have discussed. The reader who wishes for a more detailed account of these structures is referred to the work by Bragg already mentioned. 

Silicate minerals provide a wide range of site symmetries, some of which are not usually found in conventional oxide systems, and frequently the phases have wide ranges of composition enabling studies of cation substitution effects. The principal types of silicate are listed in Table 10.3. 

Nanoparticles of ternary I-lIl-VE semiconductor compounds were fabricated within three types of silicate glass matrices distinguished by alkali components. Their fundamental absorption edge depends on the chemical nature of semiconductor. CuInTe2 and CuGaTc2 particles are formed only in the multi-alkali matrix, while CuInS2 is less sensitive to the matrix type. 

It was first clearly shown by Bartenev [21] that when the cooling rate is decreased, the solidification temperature of silicate glass decreases proportionally. This behaviour was confirmed for many types of silicate glass and organic polymers in subsequent papers by various authors [22] and [23]. 

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