The pressure dependence of the refractive index for a glass of the composition 0.50SiO_2 centre dot 0.25Al_2O_3 centre dot 0.25CaO at pressures up to 5.0 GPa is determined using a polarizing interference microscope and an apparatus with diamond anvils.The densities and compressibilities of the glass are calculated from the measured refractive indices within the framework of the theory of photoelasticity.The results are compared with the data obtained earlier for vitreous silica.The lower compressibility of the glass under investigation as compared to the compressibility of fused silica is in agreement with the degree of polymerization that is calculated with due regard for the formation of triclusters from the relationship NBO/T = (2O +3O_t - 4T)/T,where NBO (nonbridg-ing oxygen)is the number of nonbridging oxygen atoms;O is the number of gram-ions of oxygen,except for of tricluster oxygen;O_t is the number of gram-ions of tricluster oxygen;and T is the total number of gram-ions of network formers.
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