Effect of the Na/Mg mixing on the structure and properties of aluminosilicate melts.

• Na/Mg mixing in aluminosilicates causes non-linear variations in their properties. • Entropy is not modelled by an ideal mixing law but by a sub-regular solution model. • Na and Mg are preferably arranged in sub-networks, reducing structural disorder. • Preferential role-sharing as network modifier and charge compensator between Na/Mg. • Extending sub-regular model and role-sharing to all alkaline/alkaline-earth mixing. This study documents the mixed modifier effect occurring between Na and Mg in aluminosilicate glasses. Rheological, thermodynamic, and structural data were acquired on compositions with 37.5 mol% Na 2 O+MgO, 12.5 mol% Al 2 O 3 and 50 mol% SiO 2. When Na substitutes Mg, melt viscosity decreases non-linearly, as does glass configurational entropy, showing large deviations from the ideal mixing law. This suggests ordering in the glass atomic structure upon Na/Mg mixing, as supported by structural data. Indeed, as Na 2 O/(MgO+Na 2 O) increases, Raman spectra reveal a shift of the 2Q3= Q2+Q4 equilibrium to the left-hand side, while 27Al MAS NMR spectra reveal that the Al coordination tends to 100 % Al[IV]. 23Na MAS NMR data further indicate that the network modifier/charge compensator role of metal cations also changes upon Na/Mg mixing. Those results therefore suggest important changes in melt/glass structure as alkali cations substitute calc-alkaline ones, with important rheological and thermodynamical impacts. [ABSTRACT FROM AUTHOR]