A Comparison Of The Densities Of Lithium And Sodium Borate And Silicate Glasses With Very High Alkali Contents

The densities of alkali borate and alkali silicate glasses as a function of composition show similar trends. The origins of the respective density trends are distinct in each family. In the alkali borates the density changes are initially controlled by the fraction of fourfold coordinated borons. This structural unit is extremely dense in comparison with the trigonal boron groups. In the alkali silicates the density is a strong function of the numbers of non-bridging oxygens of the silica tetrahedra. A useful way to plot the borate data is to compare the volume per mole B2O3 with the ratio of alkali to boron, as denoted as R. In both lithium and sodium borates this volume initially decreases due to the coordination change of boron. At higher alkali concentrations, non-bridging oxygens are formed on trigonal borons at the expense of tetrahedral borons and the modified molar volume increases linearly. In silicate glasses, the volume per mole SiO2 increases with increasing ratio of alkali oxide to silica, J. As J is increased, in either the lithium or sodium case, the modified molar volume increases linearly. This is indicative of the conversion of bridging oxygens into non-bridging oxygens through the addition of alkali oxide.