Structure of Silicates

1.4.2 Structure of Silicates
Silicates are also oxides, but they contain two or more oxides, out of which one is essentially silica. Earth’s crust contains 48% by weight O2, 26% Si, 8% Al, 5% Fe, and 11% Ca, Na, K, and Mg combined. Thus, the earth’s crust and also mantle consist mainly of silicates. In silicates, two types of oxygens can be seen. They are bridging and nonbridging oxygens (NBOs). Oxygen atoms are bonded to two Si atoms in the case of bridging oxygens, whereas they are bonded to only one in the case of nonbridging oxygens. NBOs are formed by the addition of either alkali or alkaline earth metal oxides to silica, as shown in Figure 1.18. In this, O? denotes an NBO. NBOs are negatively charged. Local neutrality is maintained by having cations end up adjacent to the NBOs.
1- Silica
As already stated, the basic building block of silicates are (SiO4)4? tetrahedra. This building block is possessed by the silica structure. When this block is joined to the neighboring tetrahedra, the four negative charges on each block is balanced, and the structure becomes neutral.
2- Sheet Silicates
When three out of four oxygens are shared, with an O/Si ratio of 2.5, the sheet structure results. Examples for this structure are clays, talc [Mg3(OH)2 (Si2O5)2], and mica [KAl2(OH)2(AlSi3O10)].