SIMILARITIES IN THE BEHAVIOR OF MAGNESIUM AND CALCIUM IN SILICATE-GLASSES

The dependence of the infrared reflection spectra of mixed sodium-alka line earth-silicate glasses on magnesium or calcium content shows simi larities. This is surprising in view of previous observations indicati ng a possible network-forming role for magnesium. In these glasses, a well localized band for calcium or magnesium is observed at frequencie s similar to those found in CaO or MgO. However calculation of the eff ective charge carried by each type of cation gives a degree of ionicit y, Z/Z0, of 0.9 for calcium but only 0.7 for magnesium. This differenc e explains why magnesium might have some network-forming character in alumino-silicate glasses.