Structural considerations on Al2O3-SiO2 and derived glasses

Available data of density, molar volume, refractive index and molar refraction of Al2O3-SiO2 glasses have been analyzed. The analysis shows that any of these properties can be expressed in terms of a factor of SiO4 tetrahedron and another one related to a structural species of AlO3/2 formula. These factors, together with previously found factors of the modified AlO4 tetrahedra, are useful to calculate the properties of Li2O-Al2O3-SiO2 and CaO-Al2O3-SiO2 glasses with [Al2O3/(Li2O or CaO)] > 1. Structural forms of AlO4, AlO5 and AlO6 groupings are proposed, in which the charge compensation is attained through various distributions of 2-, 3- and 4- coordinated O2- ions. Any of these groupings is equivalent to the AlO3/2 formula. It is concluded that the studied properties are not sensitive to coordination of Al3+ ions.