Modelling of density structure relations in silicate glasses containing Al₂O₃

A model is presented to correlate density with structure of Li₂O–Al₂O₃–SiO₂, Na₂O–Al₂O₃–SiO₂, K₂O–Al₂O₃– SiO₂ and CaO–Al₂O₃–SiO₂ glasses. Any of these systems shows a unique dependence of density on the (R₂O or RO/SiO₂) ratio. There is mostly no effect of Al₂O₃ concentration on density. The volumes of the Q₃ units in these glasses are the same as found in the corresponding binary silicate glasses. The volume of the Q₄ unit is about 2·5% smaller than in vitreous SiO₂. The AlO₄ volume increases linearly with the reciprocal of the field strength of the modifier ion. It is deduced that for a specific (R₂O or RO/SiO₂) ratio the change in density due to AlO₄ units compensates the change due to the silicate units. Additive relation is given to correlate the molar volume with glass structure. The results suggest that for Al₂O₃/(R₂O or RO)≤1 there are AlO₄ units in the structure without significant existence of AlO₆ units.