Silicates ring structures

The molecular structure of Li-, Na-, and K-silicates in 0.2 to 3 mole SiOj/L aqueous solutions has been investigated by FTIR and Raman spectroscopy to help exploring their solidification process. These silicates were found to be only partially dissociated and their average molecular weight (AMW) varies with the type of the alkaline ion, the alkaline/silicon ratio, and the concentration. It is demonstrated that these differences are associated with differences in the Qn connectivity ratios of [Si04] tetrahedra and in the dominating siloxane ring structures which can be identified by their vibrational spectra. 

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. 

Fig. 3. Mineralization linked to the ring structures I - Urkuveem (Mo with Ag and Bi, greisen type), II -Keyukveem (polymetallic), III - Kitivelgin (Au, arsenic-antimony association), IV - Belaya Sopka (Sn, cassiterite-sulfide association), V - Shestakovka (Sn with Ag and Bi, cassiterite-silicate association). Total productivities Pj, m %) for more promising catchment areas (outlined) and/or linear and ring structures are numbered.

Figure 7.2 Silicate anion structures (o) orthosilicate, (6) pyrosilicate, (c) three-silicate ring, (d) six-silicate ring, (e) pyroxene, (/) amphibole, and (g) phyllosilicate.

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. 

Approximately 85% of the of the peak area shown in Figure 1 can be assigned to 19 specific silicate structures 14.51. A schematic of these structures is presented in Table I, together with the chemical shift for each distinct silicon atom. It is evident from this table that silicate oligomers tend to occur as single and multiple ring structures, rather than as linear chains. 

It is possible to obtain such silicates as solids by evaporation to dryness of the organic quaternary ammonium silicate aqueous solutions. In this case, however, the process is very slow and accordingly a problem of impurity incorporation arises. It was reported that silicate solids consisting of the double four-membered ring structure were abruptly separated out after an exothermal reaction on stirring the mixture of (2-hydroxyethyl)trimethylammonium hydroxide aqueous solution and tetraethoxysilane (26). 

The silicate anion with a double five-membered ring structure is mainly formed as a crystalline solid from the tetra-n-butylammonium (N+(n-C Ho) ) silicate solutions whose N/Si ratios range from 0.78 to 1.0 (20,21). Pyridinium ions are also effective in forming silicate anions with cage-like structures (27). 

Neutron diffraction has been applied to the chloroaluminate melts to determine the shape and structure of a number of anions there. They turn out to have chain and ring structures in the higher members similar to those in liquid silicates and borates (Fig. 5.62). 

Although these anions are simpler than the structures deduced on the basis of transport measurements, they agree with the structure (see Section 5.Q. Chains are present, as are O and SiOj. However, up to 1997, no peaks have yet been registered that are characteristic of the ring structures known to exist in the corresponding soUds, the suggested presence of which in the liquid silicates fits stoichiometry, bond angles, and the behavior of the heat of activation for viscous flow as a function of composition (Fig. 5.71). 

SisOoQCNCCHs s, which has the double four-membered ring structure in the silicate skeleton (-97.7 or -99.3 ppm) (38,39). One large peak assigned to SigOogCSiCCHg Jg appears on the gas chromatogram of the solid trimethylsilylated by the method of Gotz and Masson. These indicate that the solid consists only of the cubic octameric silicate structure. 

A normal-co-ordinate analysis of the vibrations of Mg2Si04 has been carried out to investigate the group behaviour of the SiO ion. An i.r. and Raman study of a number of silicates with SiaO ring structures has been undertaken to show that the essential spectroscopic features of these rings are modified under the influence of structure change with or without modification of the local symmetry of the ring. 

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