Coordination numbers silicates

The physical and chemical properties of silicate glasses depend on the composition of the material, ion size, and cation coordination number (9). A melt or glass having a Si02/Na20 ratio of 1, ie, sodium metasiUcate [1344-09-8] is expected to possess a high proportion of (SiO ) chains. At a ratio of 2, sheets might predominate. However, litde direct evidence has been shown for a clear predominance of any of these stmctures. The potential stmctures of sihcate melts of different ratios are discussed in detail elsewhere (10—12). 

The electrostatic valence rule usually is met rather well by polar compounds, even when considerable covalent bonding is present. For instance, in calcite (CaC03) the Ca2+ ion has coordination number 6 and thus an electrostatic bond strength of s(Ca2+) =. For the C atom, taken as C4+ ion, it is s(C4+) =. We obtain the correct value of z for the oxygen atoms, considering them as O2- ions, if every one of them is surrounded by one C and two Ca particles, z = -[2s(Ca2+) + s(C4+)] = -[2 j + ] = -2. This corresponds to the actual structure. NaN03 and YBOs have the same structure in these cases the rule also is fulfilled when the ions are taken to be Na+, N5+, Y3+, B3+ and 02. For the numerous silicates no or only marginal deviations result when the calculation is performed with metal ions, Si4+ and 02 ions. 

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). 

Boon J. A. and Fyfe W. S. (1972). The coordination number of ferrous ions in silicate systems. Chem. Geol, 10 287-298. 

Since the vek computation was only rough, as Fersman himself admitted, the hardness values calculated from the formulae (3.3) and (3.4) were not sufficiently accurate and failed to consider all crystallochemical factors. Next, Sobolev in 1946 established a relationship between hardness and coordination number, attempting to extend the applications of Goldschmidt s formula to complex minerals, including silicate minerals. It was discovered by that time that crystal hardness with both ionic and covalent bonds, depends on ... 

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... 

More than 60 years ago, the important proposal3 was made that the strength, s, of an X—O bond in an XO -coordinated polyhedron in a crystal like a silicate should depend on the valence, z, and the coordination number, n, of the X+z metal cation such that s = z/n. With this simple definition, it was found for instance that the sum of the strengths of each of the t bonds in a crystal reaching a given oxide ion,... 

FIGURE 2. A plot of the average bond lengths, , of the XOn coordination polyhedra observed for silicate and oxide crystals vs the Pauling bond strength, s, of the X—O bond. The Roman numeral superscript denotes the coordination number of the X-cation... 

Experimental trends in Si shielding observed experimentally arise from variations in the coordination number (i.e. the number of atoms in the 1st coordination sphere), the extent of polymerization of the silicate tetrahedra, the degree of replacement of one net-work forming cation by another (e.g. coupled Na+, Al+3 for Si+4 substitution), the size of the rings of tetrahedra present and the Si-O-Si angles (1,2). Similar trends are seen in gas-phase molecules, species in aqueous solution and in both crystalline and amorphous solids. Polarized double-zeta basis set Hartree-Fock level calculations using small molecular cluster models reproduce these trends semiquantitatively, as we will show. 

A few silicates are known in which there are simple, discrete orthosilicate, Si044 anions. In such components, the associated cations are coordinated by the oxygen atoms, and various structures are found depending on the coordination number of the cations. There are a number of compounds of the type M2Si04, where M2+ is Mg2+, Fe2+, Mn2+ or some other cation with a preferred coordination number of 6, in which the Si044 anions are arranged to provide an interaction with six oxygen atoms in an octahedron symmetry. 

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