  Structure and properties of magnesium silicate and borate melts and glasses
were investigated by using newly parameterized interaction potentials in
molecular dynamics simulations and compared with those of calcium silicate and
borate. The competition between the depolymerization of the silica network and
the formation of new bonds with modifier ions leads to the enhancement of the
elastic moduli with increasing modifier content in alkaline earth silicate
glasses. Compared with calcium silicate, the higher elastic moduli of magnesium
silicate result from a higher connectivity of the overall glass network due to
the incorporation of fourfold coordinated magnesium and a more rigid connection
between SiO4 tetrahedra and modifier ions. In contrast to the silicates, the
effect of modifier content on the elastic moduli of alkaline earth borates is
dominated by the fraction of fourfold coordinated boron (N4). Calcium borate
with higher N4 shows a more rigid network structure and higher elastic moduli.
