Bond distances silicates

Higgins and Woessner (106) correlated 29Si chemical shifts in the framework silicates cristobalite, quartz, albite, and natrolite with the mean Si—O bond distances and arrived at the following relationship ... 

In all silicate minerals formed under crustal conditions silicon is coordinated to four oxygen atoms. In high-pressure transformations, silicon commonly increases its coordination number. The longer- the Si—O distances in tetrahedral silicates the higher the pressure transformations to phases with octahedral silicon. The average Si—O bond distance for the pressure transformation is 159 pm. This distance is achieved at room temperature at pressures in all measured silicates and may be a minimum for tetrahedryl Si—O bonds 300 kbar is an upper pressure limit for the silicon tetrahedron and SOkbar is a lower pressure limit for octahedral silicon. Temperature has little effect on Si—O bond distances in either tetrahedra or octahedra... 

It has been suggested that virtually all samples of MgO (and probably silicates as well) contain small amounts of HjO and CO2 (Freund, 1981). These impurities lead to formation of O species. The species CO4 " has also been postulated to occur on the surface of MgO exposed to COj. This species has recently been studied by ab initio 8CF Hartree-Fock-Roothaan MO calculations, both in its anion form and as the protonated cluster C(0H)4 (Gupta et al., 1981). Its calculated equilibrium bond distance is intermediate between those observed for B emd N in tetrahedral coordination with oxygen, and there seems to be no intrinsic source of instability. Thus, such a species seems stable. However, the calculations indicate a charge on C in 04" " very similar to that in COj, arguing for the formulation O 04 as first approximation to the electronic structure, rather than the C°(04 ) formulation suggested by Freund (1981). Perhaps such a species is better formulated as a chemisorption complex of a anion (a bent, 18-valence-electron system) and an O. . . 0 ... 

SiO bond lengths are both now lengthened to 1.84 A (from the minimum geometry). The OH bond distances are 1.20 A. The most interesting result is the drop in the activation barrier, A , (which should be close to the activation energy barrier, AE ) to the value 21.87 kcal mol -1 (see Table 3). This value is remarkably close to the activation energy for silicate dissolution (Lasaga, 1984). 

In framework silicates, the apparent thermal contraction of the T-0 bonds has been related to a transverse vibration of the oxygen atoms normal to the T-O-T bonding plane [2.34,35]. Increased amplitudes of this mode, on time average, would increase the bond distance, which is compensated for by a movement of the Si atoms towards the oxygen. Consequently, if bond lengths... 

Elementary silicon is also of principal importance to the covalent chemistry of silicon, because all pathways to such silicon compounds involve the element itself. Of course, the silicate systems are distinctly separate. Their properties and uses arise from the high strength of the silicon—oxygen bond. To illustrate the differences between related compounds of carbon and silicon, some bond energies and bond distances are shown in Table 1. More recent compilations give considerably higher Si—X bond energies [66]. 

The charges on the oxygen atoms due to partial ionic character of the bonds to the metal atoms in the silicates and other salts should be taken into consideration in making this calculation. These charges lead to further decreases in the Si-O, P-O, S-O, and Cl-0 distances, of amount depending on the nature of the metal and the structure of the crystals. Because of uncertainties in the system of equations used in this paper, this refinement in the calculation has not been carried out. 

---