Co‐Ordination state of aluminium ions in CaO–Al2O3–SiO2 glasses

The state of co‐ordination of aluminium ions in synthetic glasses in the system CaO–Al2O3–SiO2 was studied by determining the effects of varying the alumina content (from 1 to 7 moles) on the density and refractive index of glasses of composition 10 CaO, 8 SiO2, n‐Al2O3 (CaO/SiO2 ratio 1.17:1). The partial molecular refractivity of alumina indicated a fourfold co‐ordination of aluminium ions. The increase in density and refractive index due to the replacement of varying amounts of silica by alumina at a fixed level of CaO (36—44%) has been shown to be due to lower partial specific volume of alumina compared with that of silica and is not due to sixfold co‐ordination of aluminium ions as reported in the literature.