Structure of Silicates

Silicates are also oxides, but they contain two or more oxides, out of which one is essentially silica. Earth s crust contains 48% by weight O2, 26% Si, 8% Al, 5% Fe, and 11% Ca, Na, K, and Mg combined. Thus, the earth s crust and also mantle consist mainly of silicates. 

Formation of nonbridging oxygen by the addition of an alkali oxide to silica. 

The number of NBOs are proportional to the number of moles of alkali or alkaline earth metal oxides added. 

Increasing the number of NBOs results in the progressive breakdown of the silicate structure into smaller units. 

A specially good example showing how a large group of important compounds can develop all the above described types—primary valence lattices, primary valence networks and primary valence chains—is afforded by the silicates whose structure elucidation is due in the first place to the experiments of W. L. Bragg and his co-workers over many years. For these very compounds the old method borrowed from organic chemistry of description by the aid of constitutional formulas was shown to be inappropriate. 

The work of W. L. Bragg and also the experiments of L. Pauling and F. Machatschky have led to the establishment of astonishingly simple principles which render the structure and properties of these widely distributed compounds intelligible. 

The main forces, which prevail in these lattices are those between the silicon and oxygen atoms which are best viewed as homopolar primary valences, although they probably have in part a certain heteropolar aspect. 

We can now base the structure of the silicate lattices on the following principles. 

We may derive from these simple rules two possibilities as extreme cases. 

The earth s crust is about 48 percent by weight oxygen, 26 percent silicon. 8 percent aluminum, 5 percent iron, and 11 percent calcium, sodium, potassium, and magnesium combined. Thus it is not surprising that the earth s crust and mantle consist mainly of silicate minerals. The chemistry and structure of silicates can be quite complex indeed and cannot possibly be covered in detail here. Instead, a few guidelines to understanding their structure are given below. 

The addition of alkali or alkali-earth metal oxides to silica must increase the overall OjSi ratio of the silicate. 

It thus follows that a critical parameter that determines the structure of a silicate is the number of NBOs per tetrahedron, which in turn is determined by the 01 Si ratio. How this ratio determines structure is discussed below but before addressing this point, it is important to appreciate that in general the following principles also apply  

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Murata, K. J. (1943). Internal structure of silicate minerals that gelatinize with acid. American Mineralogist, 28, 545-62. 

Santamaria-Perez D, Vegas A, Liebau F (2005) The Zintl-Klemm Concept Applied to Cations in Oxides II. The Structures of Silicates 118 79-135... 

In summary such marked structural differences can be assumed to affect the structure of the initial nanoparticles in the solidification process. Improved peak resolution, deconvolution, Q1 ratio-possible ring structure matching, model calculations, and comparison with NMR and other structural data can make Raman a quite useful, inexpensive, and mobile accessory for studying the structure of silicates both in aqueous solutions and in solid phase. 

The electron crystallography method (21) has been used to characterize three-dimensional structures of siliceous mesoporous catalyst materials, and the three-dimensional structural solutions of MCM-48 (mentioned above) and of SBA-1, -6, and -16. The method gives a unique structural solution through the Fourier sum of the three-dimensional structure factors, both amplitude and phases, obtained from Fourier analysis of a set of HRTEM images. The topological nature of the siliceous walls that define the pore structure of MCM-48 is shown in Fig. 28. 

Comparing the Raman spectra of silicate melts with various NBO/Si and NBO/T ratios with the Raman spectra of crystals of the same chemical composition, the above authors subdivided the anion structure of silicate melts into three compositional ranges ... 

Hess P. C. (1977). Structure of silicate melts. Canadian Mineral, 15 162-178. 

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. 

Lazarev, A. N. Vibrational spectra and structure of silicates (english translation). New York Consultants Bureau 1972. 

Unlike Si(IV), Cd(II) often takes an octahedral configuration as well as a tetrahedral one it is rather difficult for Si(IV) to take an octahedral configuration under ambient conditions. The isopolycyanocadmate(II) systems built of the CN-linkages among tetrahedral and octahedral Cd atoms can mimic the structures of silicate minerals composed of the tetrahedral Si and other octahedral cations. 

Bragg, W. L. (1930). The structure of silicates. Zeit. Kristallogr. 74, 237-305. Brammer, L., Zhao, D., Lapido, F. T., and Bradock-Wilking, J. (1995). Hydrogen bonds involving transition metal centres—a brief review. Acta Cryst. B51, 632-40. 

McNicol et al. (49) used luminescence and Raman spectroscopy to study structural and chemical aspects of gel growth of A and faujasite-type crystals. Their results are consistent with a solid-phase transformation of the solid amorphous network into zeolite crystals. Beard (50) used infrared spectroscopy to determine the size and structure of silicate species in solution in relationship to zeolite crystallization. 

Figure 3. Schematic drawings for pore-wall structures of siliceous MCM-41, organic-grafted MCM-41 and hybrid mesoporous materials.

Ceramicists and physicists have studied the structures of silicate glasses and have interpreted the results in terms of characteristics such as the size, polarizability, and coordination number of the ions, whereas metallurgists have employed a less quantitative description in terms of the acidity or basicity of the components constituting the slag.424... 

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. 

In the silicates, the tetrahedral SiCU units are either isolated or share corners with other tetrahedra, giving rise to an enormous variety of structures. In many silicates, silicon may be replaced to a certain extent by other elements, such as aluminium, so the structures of silicates are further extended to cover cases where such partial substitution occurs. 

Ionic radius. The wide variation of metal-oxygen distances within individual coordination sites and between different sites in crystal structures of silicate minerals warns against too literal use of the radius of a cation, derived from interatomic distances in simple structures. Relationships between cation radius and phenocryst/glass distribution coefficients for trace elements are often anomalous for transition metal ions (Cr3+, V3+, Ni2+), which may be attributed to the influence of crystal field stabilization energies. 

Synthesis of silica-based materials with controlled skeleton structures, such as zeolites, requires controlling the structure of oligomeric silicate species at the first reaction step. Organic quaternary ammonium ions, which are known as organic templates in zeolite synthesis (1 ), have a role in making up the specific structures of silicate anions, whereas silicate anions randomly polymerize in aqueous solutions containing alkali metal ions, resulting in the presence of silicate anions with different structures. 

Analytical Procedure. The structures of silicate anions in the solutions and solids have been examined with the trimethylsilylation technique combined with gas-liquid chromatography and "Si NMR. The molecular weight distribution was measured by applying gel permeation chromatography to the trimethylsilylated derivatives. 

Figure 5. Structures of Silicate Ions in K-silicate Solution. (Reproduced from ref. 19. Copyright 1981 American Chemical Society.)...

Because the structure of silicates oligomers tend to be as condensed as possible, structures with long linear chains and large cyclic rings are less plausible. On the basis of this statement, structures containing three-membered rings and threefold substituted Si sites appear the most likely candidates. Therefore, the most probable structure of the analysed species is 7e. In combination with two other samples... 

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