Alumina‐silica Relationship in Glass*

A bstract Assuming from a consideration of silicate minerals that aluminum should replace silicon in silicates, the authors investigated the system Na 2 O‐CaO‐SiO 2 ‐Al 2 O 3 . Densities and refractive indices have been determined and molecular refractions have been calculated. Partial molecular refractivities of Al 2 O 3 indicate a coordination number of 4 for aluminum and deviate appreciably from values for compounds in which aluminum has a coordination number of 6. Interionic distances have been calculated for Si—O and Al—O. As a result of these studies, it is concluded that the aluminum atom isomorphously replaces the silicon atom in the random fetrahedral network. With this replacement, Ca ++ can increasingly replace Na + in the interstices of the open structure.