Charge density studies of silicates

Silicates comprise more than 95% by weight of the earth s crust and mantle, and are widely used in glasses, ceramics, sieves, catalysts, and electronic devices. Crystals of silicates are often hard, and may show considerable extinction in their diffraction pattern, which means not only that small samples must be used, but also that ambient temperatures may be adequate for charge density studies.2 

The Resource Book of Crystal Structures by Mak and Zhou (1992) includes a description of the rules governing silicate structure, as well as a number of detailed examples, to which the reader is referred for necessary detail. 

2 As noted below, the temperature parameters for zeolites are often considerably larger than those for simple silicates. Disorder may contribute to this difference. 

The study of the nature of the Si—O bond through analysis of its charge density is based on net ionic charges, heights of peaks in atom deformation density maps, and, more recently, topological analysis of the total charge density. 

The deformation density peaks in the Si—O bonds of coesite are located away from the midpoint of the bond towards the oxygen atom, and also tend to be displaced towards the interior of the SiOSi angle. Downs reports a bending of the 

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