Linking silicon-oxygen tetrahedra

If the structure is composed entirely, or nearly entirely, of linked silicon-oxygen tetrahedra, one can obtain the three crystalline silica structures, showing a- and yff-modifications. These are quartz, cristobalite and tridymite. Also the irregular netting of the tetrahedra leads to fused sihca glass. Of the crystalline forms, quartz is the most dense and yff-cristobalite, next to fused silica, is the most open. In yff-cristobalite, zigzag... 

Teciosilicates involve the sharing of all four oxygens in each tetrahedral unit with adjacent tetrahedrons to form a three-dimensional framework of SiOj units linked together. The product is a strongly bonded structure with a silicon-oxygen ratio of 1 2. The greater portion of the earth s crust is composed of minerals found within this classification. 

The process of cross linking of silicate tetrahedron through oxygen atoms can be extended to three oxygen atoms per silicon atom when two - dimensional sheets are formed. 

Silicon atoms bond strongly with four oxygen atoms to give a tetrahedral unit (Fig. 16.4a). This stable tetrahedron is the basic unit in all silicates, including that of pure silica (Fig. 16.3c) note that it is just the diamond cubic structure with every C atom replaced by an Si04 unit. But there are a number of other, quite different, ways in which the tetrahedra can be linked together. 

The selectivity of glass for alkah metal ions is connected with the presence of oxides of trivalent metals in the glass structure. Zachariasen [450] states that silicate glass has a random cross-linking, where each silicon atom lies in the centre of a tetrahedron formed of oxygen atoms (see planar scheme (6.5.5)). 

In Figure 22.33(c), each silicate tetrahedron is linked to three others, forming an infinite sheet structure. Each silicon in this structure has one unshared oxygen and three shared oxygens. The stoichiometry is then 1(1) -I- 3(1/2) = 2Vi oxygens per silicon. The simplest formula of this sheet is Si205 . The mineral talc, also known as talcum powder, has the formula Mg3(Si205)2(0H)2 and is based on this sheet... 

Atoms of silicon are small and have four valence electrons, so that fonr strong bonds can be formed with adjacent atoms. In the crystal stractures of rock-forming minerals, the adjacent atoms are always oxygen, so that each Si atom is snrronnded by four O atoms at the comers of a tetrahedron. Each oxygen in the SiO group has an unsatisfied valence that can link the group to other atoms. 

Fig. 19. The structures of some fibrous zeolites. The structure of one tetrahedron chain is shown in (d ), in which the large circles represent silicon or aluminium atoms, the small circles oxygen atoms, and the heights of the atoms are given m A The same chain is represented diagrammatically in (d), where the numbers show the heights of silicon and aluminium atoms as multiples of c/8 (the c axis is 6 6 A., so that 3c/8 = 2 5 A.. 5c/8 = 4 1 A.). The linked chains are shown in (a), (6), (c) as arranged in edingtonite, thomsonite and the natrolite group, respectively, and m each the imit cell is indicated by dotted lines.

Fig. 20. A lattice of two-dimensional laminae. Large circles represent oxygen atoms, small circles silicon or aluminium atoms. The upper diagram represents a portion of an infinite sheet of tetrahedra in which all vertices are supposed to point upward. If these vertices lie on a reflection plane, a second similar tetrahedron sheet, in which all vertices point down, is linked to the first. The lower diagram represents the appearance of the linked sheets, which form a tetrahedron framework of finite thickness, when viewed in the direction indicated by the arrow in the upper diagram. 

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