Silicates tectosilicate framework

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. 

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).

Framework silicates are also termed tectosilicates. Their common feature is the three-dimensional connection of tetrahedra sharing all four vertices. They are subdivided into ... 

One may summarize the various observations as follows. In strongly acid solutions, A1(H20)63+ is dominant. From pH = 6 upwards, Al3+ becomes wholly insignificant, polymeric ions are no longer important, and Al(OH)4 or A102 are dominant. The important Al(OH)4 is tetrahedral in structure and so should favor tectosilicate formation with silicates condensed into a 3-D framework. 

The formal systematic replacement of Si02 units in quartz or other polymorphs of silicon dioxide by AIO(OH) or M A102 (M = monovalent metal) is well known to lead either to sheet silicates or to tectosilicates, the zeolites being the most remarkable representatives.All these solid-state compounds have as a common feature interstitial holes, which are occupied by easily extractable cations, or have three-dimensional frameworks which can be modified and used for many applications (ion exchange, catalysts, specific coordination sites,... 

M stands for cations such as protons, alkali, or alkaline earths, n stands for then-valency, X indicates numbers between 2 and 40, and y indicates numbers between 1 and 20. The metal ions are exchangeable xSi02 comprises the tetfahedral [SiOJ units of the framework, and nH20 comprises all structural hydroxyl groups and the interlayer water molecules. This formulation refers to layered silicate hydrates of different structures, cationic forms, and degrees of hydration. It does not differentiate the M-SHs from other silicate types, such as nesosilicates, inosilicates, and phyllosilicates. Liebau [16] proposed a classification criterion (Table 2) based on the 0/Si ratio in the framework. The layered metal silicate hydrates with values between 2.25 and 2.1 are positioned between the traditional phyllosilicates (0/Si ratio 2.5) and the tectosilicates (0/Si ratio 2.0). 

Framework silicates, also referred to as tectosilicates, are characterized by a tetrahedral ion-to-oxygen ratio of 1 2. The typical tetrahedral ions are silicon and aluminum, but, in some cases, germanium, titanium, boron, gallium, beryllium, magnesium, and zinc may substitute in these tetrahedral sites. All tetrahedral ions are typically bonded through oxygen to another tetrahedral ion. Silicon normally composes from 50% to 100% of the tetrahedral ions. 

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