Tectosilicate framework

Tectosilicates (framework) Continuous framework of tetrahedra, each sharing all four oxygens 1 2 Silica (e.g., quartz, Si02), feldspars [e.g., orthodase iqSiaAlOg)], feldspathoids... 

The final class of silicates is the tectosilicates (framework silicates). The tectosil-icates consist of an infinite network of tetrahedrons, where all four oxygen atoms are shared, giving them an empirical formula of SiOj. An example of a tectosilicate is shown in Figure 12.23. 

Subclass Tectosilicates (framework silicates) The tectosilicates or framework silicates have a structme where all of the four oxygens of an Si04" tetrahedron are shared with four adjacent tetrahedra. The ratios of Si to 0 is thus 1 3. The basic structural group then becomes SiOj. The minerals - quartz, cristobalite, tridymite etc. - are based on this structure. If some of the Si ions are replaced by AF, a charge imbalance is produced which is neutralized by incorporation of other cations in different coordination sites within the framework structure. Thus, the feldspar and feldspathoid minerals are also based on the tectosilicate framework. 

Cristobalite (alpha) [14464-46-1] [Named after Cerro San Cristdbal near Pachuca, Mexico] (ICSD 47219 and PDF 39-1425) SiO, M = 60.0843 46.74 wt.% Si 53.26 wt.%0 (Tectosilicates, framework) Coordinence Si (4) Tetragonal as 497.1 pm cs 691.8pm (Zs4) P.G. 422 S.G. P4,2,2 Uniaxial (-) e= 1.482 0)= 1.489 S= 0.007 6-7 2330 Habit coarse aggregate. Color colorless. Streak white. Diaphaneity transparent to translucent. Luster vitreous( Le., glassy). Fracture conchoidaL Twinning (III). Oiemical resistant to Strong mineral acids, attacked by HF and molten alkali-metal hydroxides. Mdting point 1713 C... 

Figure 1. Illustration of the linkage of Si04 tetrahedra in different classes of silicates, (a) Nesosilicates (separate tetrahedra) (b) sorosilicates (linked tetrahedra) (c) cyclosilicates (closed rings) (d) and (e) inosilicates (single and double chains) (f) phyllosilicates (sheets) (g) tectosilicates (framework).

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