range up to 6.0 GPa has been measured using a polarizing interference microscope and an apparatus with diamond
anvils. The changes in the relative density, which characterize the compressibility of the K2O
Al2O3 6SiO2 glass, have been estimated in the pressure range under investigation from the measured refractive indices within the frame-
work of the theory of photoelasticity. The results have been compared with the data previously obtained for the Na2O
Al2O3 6SiO2 glass. Although the molar contents of Al2O3 and M2O (where M = K or Na) are identical in these glasses, the KAlSi3O8 glass exhibits a higher compressibility, which agrees with the lower degree of depolymeriza-
tion of this glass as compared to that observed in the NaAlSi3O8 glass. The pressure derivative of the bulk modulus
, which is calculated from the Birch–Murnaghan equation for the KAlSi3O8 glass (
= 7–9), is higher than that for the NaAlSi3O8 glass (
= 5.5–6.0). An increase in the pressure derivative of the bulk modulus 
upon replace-ment of the Na+ cations by the K+ cations is explained by the inhibition of compression of the large K+ cations,
which are located in cavities and have a considerably larger orbital radius than the Na+ cations. This manifests
itself in the fact that the curves describing the dependences of the change in the relative density (d – d0)/d (com-
pressibility) on the pressure P for the KAlSi3O8 and NaAlSi3O8 glasses converge at pressures above 4.0 GPa.