The state of magnesium in silicate glasses and melts

Abstract

The properties of silicate glasses and melts containing magnesium are analyzed in comparison with the properties of glasses and melts in which magnesium is replaced by aluminum. In particular, the properties of the glass and the melt of the diopside composition CaMgSi2O6 are analyzed in comparison with the properties of the glass and the melt of the anorthite composition CaAl2Si2O8. It is demonstrated that the properties of aluminosilicate and magnesium silicate glasses and melts differ not so strongly as should be expected upon replacement of modifier ions by network-former ions. By using the parameters γn characterizing the cation field strength, it is shown that Mg2+ cations can fulfill both the function of network formers like Al3+ cations and the function of modifiers like Ca2+ cations. The degree of depolymerization of the glass and the melt of the composition CaMgSi2O6 is estimated to be 0.4–0.5 from the dependences of the change in the relative density (d − d0)/d at different pressures on the degree of depolymerization NBO/T (the ratio of the number of nonbridging oxygen atoms to the number of network-former cations) for silicate glasses and the dependence of the isothermal bulk modulus Kt on the quantity NBO/T for silicate melts.