Glass composition and glass substrate

Abstract

A glass substrate for use as the substrate of an information recording medium such as a magnetic disk, magneto-optical disk, DVD, or MD or of an optical communication device, and a glass composition for making such a glass substrate, contains the following glass ingredients: 45 to 75% by weight of SiO2; 1 to 20% by weight of Al2O3; 0 to 15% by weight, zero inclusive, of B2O3; SiO2+Al2O3+B2O3 accounting for 65 to 90% by weight; a total of 7 to 20% by weight of R2O compounds, where R=Li, Na, and K; and a total of 0 to 12% by weight, zero inclusive, of R′O compounds, where R′=Mg, Ca, Sr, Ba, and Zn. Moreover, the following conditions are fulfilled: B2O3=0% by weight, or Al2O3/B2O3≧1.0; and (SiO2+Al2O3+B2O3)/(the total of R2O compounds+the total of R2O compounds)≧3.

Dependencies are inferred from the claim text.