Density of MgO–Al2O3–SiO2, MgO–CaO–SiO2 and MgO–CaO–Al2O3–SiO2 glasses and the structural role of MgO

Density and molar volume of MgO–Al2O3–SiO2, MgO–CaO–SiO2 and MgO–CaO–Al2O3–SiO2 glasses have been correlated with the structure by following the change in type and concentration of structural units with composition. It is assumed that for (MgO+CaO)≥Al2O3 all Al atoms form AlO4 tetrahedra. The type of modified units of SiO2 (Q3 and Q2) are determined by the ratio [(MgO+CaO)−Al2O3]/SiO2. In the case of (MgO+CaO)<Al2O3, there is a fraction of Al2O3 that forms units based on the AlO3/2 formula. The entire SiO2 content would be in the form of Q4 units. Any of the structural units has its own constant volume that is independent of the type of glass. The agreement between calculated and experimental density and molar volume strongly suggests that MgO has only a modifier role in the studied glasses. Formation of assumed MgO4, MgO5 and MgO6 polyhedra can be due to change in coordination of oxygen, rather being looked as structural units.