Silicates model structures

Some other models for the structure of C-S-H gel that have been proposed are incompatible with the evidence. A proposal identifying it with natural tobermorite, based on IR and extraction results (S37), appears to ignore both composition and degree of crystallinity. Another, assuming it to be closely related to the CH structure, with incorporation of monomeric silicate ions (G38), is inconsistent with the observed silicate anion structure. As noted earlier, one assuming three-dimensional anionic clusters (C23) is inconsistent with the Si NMR evidence, and with the overwhelming proportion of the other evidence on silicate anion structure. 

Bish, D. L., and C. W. Burnham (1984). Structure energy calculations on optimum distance model structures application to the silicate olivines. Amer. Mineral. 69, 1102-9. 

CSH samples were synthesized with a Calcium/Silicium ratio 0.9 following a soft Chemistry hydrothermal procedure which allows to produce model compounds of cement a calcio-silicate layered structure with a pristine interlamellar spacing of 14 A. 

Tanabe has reviewed the earlier work with silica-magnesia, silica-zirconia, and other amorphous siliceous materials. In a model for binary siliceous oxide catalysts, only the non-siliceous component was considered in terms of proton affinity and co-ordination number. Tanabe and co-workers " proposed a general model for mixed oxide catalysts in which acidity is caused by an excess of negative or positive charge in a model structure of the binary oxide. The hypothesis is shown to fit 28 of the 31 binary oxides tested. One of these oxides,... 

Fig. 2 Model structure of layered silicates (montmorillonite) where usually silicon sits in the tetrahedral locations of the oxygen network. The octahedral positions may variously be iron, aluminium, magnesium or lithium, and the exchangeable cation in the gallery is given by M " [4]...

Layered Silicates The structure of Layered silicates (LS)/clays consists of a 2-D layer of two fused silicate tetrahedral sheet with an edge-shared octahedral sheet of metal atoms, such as A1 or Mg. This model was proposed by Hofifinann et al. [19]. 

Bish DL, Burnham CW (1984) Stracture energy calcnlations on optimnm distance model structures Application to the silicate ohvines. Am Miners 69 1102-1109... 

The study of silicate glass structure is another area where molecular modeling can be a... 

Table 2.3. Comparison of measured axial coefficients of thermal expansion and values calculated from computergenerated model structures for h-quartz-type and keatite-type alumino-silicates...

Birchall ID, Espie AW (1986) Biological implications of the interaction (via silanol groups) of silicon with metal ions. In Evered D, O Connor M (eds) Silicon biochemistry. Ciba Foundation Symposium 121. Wiley, New York, pp 140-159 Bish D, Burnham CW (1984) Structure energy calculations on optimum distance model structures application to the silicate olivines. Am Mineral 69 1102-1109 Blum A, Lasaga AC (1988) Role of surface speciation in the low-temperature dissolution of minerals. Nature 331 431-433... 

The picture of cement microstructure that now emerges is of particles of partially degraded glass embedded in a matrix of calcium and aluminium polyalkenoates and sheathed in a layer of siliceous gel probably formed just outside the particle boundary. This structure (shown in Figure 5.17) was first proposed by Wilson Prosser (1982, 1984) and has since been confirmed by recent electron microscopic studies by Swift Dogan (1990) and Hatton Brook (1992). The latter used transmission electron microscopy with high resolution to confirm this model without ambiguity. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

Before data from the Galileo mission became available, the interior structure of the moon was still basically unknown. Hie data obtained during two encounters of the probe with Europa (E4 and E6) on December 19, 1996, and February 20, 1997 (Anderson et al., 1997), indicated the presence of an inner core with a density of 4,000 kg/m3. This could be a metal core with a radius of about 40% of that of the moon, surrounded by a rock mantle with a density of 3,000-3,500 kg/m3. Two further approaches of the probe to Europa made refinement of the model possible (Anderson et al., 1998), and they concluded that the moon s interior may consist of a mixture of silicates and metals. If the moon does in fact have a metallic core, estimation of its diameter is not possible because of its unknown composition. 

Callisto orbits Jupiter at a distance of 1.9 million kilometres its surface probably consists of silicate materials and water ice. There are only a few small craters (diameter less than a kilometre), but large so-called multi-ring basins are also present. In contrast to previous models, new determinations of the moon s magnetic field suggest the presence of an ocean under the moon s surface. It is unclear where the necessary energy comes from neither the sun s radiation nor tidal friction could explain this phenomenon. Ruiz (2001) suggests that the ice layers are much more closely packed and resistant to heat release than has previously been assumed. He considers it possible that the ice viscosities present can minimize heat radiation to outer space. This example shows the complex physical properties of water up to now, twelve different crystallographic structures and two non-crystalline amorphous forms are known Under the extreme conditions present in outer space, frozen water may well exist in modifications with as yet completely unknown properties. 

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. 

Predictably, alkyltrisilanols react with dialkylzincs to yield larger aggregates, such as 133, which are much more inorganic in nature than those formed by mono- and disilanols (Scheme 84). Often these zinc siloxanes are without direct zinc-carbon bonds and resemble the silicates for which they serve as model compounds.192 The specific structures of these products depend heavily on the substituents of the silanetriols and diorganozincs, as well as the reaction stoichiometries.193... 

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