Energetics of silica-poor glasses in the systems MgO - SiO ₂ and Mg _0.5 Ca _0.5 O - SiO ₂

A series of alkaline-earth silicate glasses, with compositions ranging from the metasilicate to the ortho- and suborthosilicate, have been synthesized by aerodynamic levitation and CO₂ laser melting. They have been studied by high-temperature oxide melt solution calorimetry with 2PbO·B ₂O₃ as solvent. The enthalpies of formation from the oxides at room temperature (ΔH298K,oxides0,gl) have been calculated from the solution enthalpies. Glasses in the Ca_0.5Mg_0.5O- SiO₂ system show greater energetic stability than those in the MgO-SiO₂ system, with a more pronounced negative enthalpy of mixing near the orthosilicate composition. This stabilization may explain why it is possible to prepare glasses poorer in silica (suborthosilicate) in the Ca _0.5Mg_0.5O-SiO₂ system but not in the MgO-SiO₂ system. The thermodynamic observations support earlier structural studies in these systems.