Silicate melts structural units

Lead silicate glasses having different mole ratios of Si02 to PbO and containing different amounts of alkali/alkaline earth metal oxides were prepared by conventional melt-quench method. The different structural units of Si present in these glasses were identified by Si MAS NMR. ... 

It has been shown above that silicate melts contain distributions of silicate polyanions of different sizes. These polymeric anions can all be constructed from five structural units ... 

When silicate minerals crystallize from these melts, only particular structural units can be accommodated by the growing mineral. For example pyroxene chains contain only middle groups ( Si) and olivines contain only orthosilicate ( Osi) anions. 

Masson (I965) assumed that the discrete silicate polyanions of different sizes mix ideally. However the activity coefficients (y) in equation (9) will also disappear if they lie in a geometrical series and cancel out (Masson, 1972). This is equivalent to allowing the ideal mixing of the structural units of the polymers. It should be of interest, therefore, to treat silicate melts as ideal solutions of structural units or structons (Huggins, 195 ) and cations. 

Thus, by combining what is already known of the nearly ideal mixing properties of silicate structural units in the melt (Masson, 1972) with the requirement that crystallizing mineral phases can only accommodate certain of those present in the polymers, it may be possible to construct tables of approximate solubility products for silicate minerals in melts similar to those for salts in aqueous solution. 

Zirconia. Zircon (zirconium silicate), the most widely occurring zirconium-bearing mineral, is dispersed in various igneous rocks and in zircon sands. The main deposits are in New South Wales, Australia Travancore, India and Florida in the United States. Zircon can be used as such in zircon refractories or as a raw material to produce zirconia. The zircon structure becomes unstable after about 1650°C, depending on its purity, and decomposes into Zr02 and Si02 rather than melting (see Zirconiumand zirconium compounds). 

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