Energetics of Silica‐Poor Glasses in the Systems MgO – SiO ₂ and Mg _0.5 Ca _0.5 O – SiO ₂

Abstract

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 CO 2 laser melting. They have been studied by high‐temperature oxide melt solution calorimetry with 2 PbO · B 2 O 3 as solvent. The enthalpies of formation from the oxides at room temperature ( ) have been calculated from the solution enthalpies. Glasses in the Ca 0.5 Mg 0.5 O – SiO 2 system show greater energetic stability than those in the MgO – SiO 2 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 0.5 Mg 0.5 O – SiO 2 system but not in the MgO – SiO 2 system. The thermodynamic observations support earlier structural studies in these systems.