Solutions of Polysilicates

On the other hand, it is likely that most reactions of silica and silicates in aqueous solutions do involve a temporary expansion of the coordination number from 4 to 5 or 6. Also in certain crystalline silicates, the silicon atom is coordinated with six oxygen atoms, for example, in thaumasite (82). This was also confirmed by Edge and Taylor (83). who worked out the structure of this calcium silicate sulfate carbonate crystal and demonstrated the presence of Si(OH), ions. 

From infrared spectra, Borisov and Ryzhenko (87) conclude that in solutions of sodium metasilicate (Si02 Na20 ratio of 1 1) there are mainly ions of (H0)2Si02 with some cyclic tetramer (H0)4(Si404)04 , which increases in concentration with increasing ratio, and also that silicon is coordinated with four oxygen atoms. 

except in very rare instances, the silicate ions in solution or in crystals appear to contain silicon with a coordination number of four. 

As discussed at the beginning of this chapter, silicates with Si02 Na20 ratios of 4 1 to about 25 1 are usually referred to as polysilicates. They do not have the high content of sodium and silicate ions as do the conventional soluble silicates, which have ratios less than 4 1. However, they contain much more alkali than necessary to stabilize sols of smallest particle size manufactured and sold as colloids. 

The borderline between a polysilicate solution and an alkali-stabilized sol of colloidal silica can logically be drawn at the particle size of the silica below which the increased solubility of the silica as Si(OH), at ordinary temperature causes the 

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