Structures of some silicates and glasses

In this chapter are described the phenomena of gas flow in glasses and in crystals. While the structure of alkali halides and similar more simple crystals is well known, it is not so with the large families of derivatives of silica. Nevertheless, considerable advance has been made towards an understanding of structural relationships in some at least of these families of compounds. Since many silicates are of importance to the present chapter, a few of the known structural relationships will be given for substances such as silica, silicate glasses, zeolites, mica, clays, feldspars and ultramarines, the first four of which are of great importance in discussing the permeability data. 

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 

As already observed, aluminium tetrahedra may replace a part of the silica tetrahedra in the three-dimensional network, which becomes anionic, since Si+ ++ is replaced by A1+++. Cations now enter the lattice to restore electrostatic balance, with a corresponding distortion of the parent structure. In this way nepheUne is built up from tridymite. Sometimes more cations are incorporated than are needed for electrostatic balance, and the nett positive charge is then in its turn balanced by the incorporation of anions SO4 , Cl, S . This type of structure is typical of the ultramarines, of which lazurite, whose simplest empirical formula is NagAleSi6024 (S , SO4 ), is an example. 

Taylor (1) has classified the zeolites, according to their structural properties, into 

In the first class are chabasite and analcite, in which the water-containing interstices do not collapse markedly upon 

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