The structural behavior of Al3+ in peralkaline glasses and melts along the Na2Si3O7-Na2(NaAl)3O7 join has been examined to 1200 °C at ambient pressure with 29Si MAS NMR and Raman spectroscopy. The distribution of Al3+ among coexisting Q4, Q3, and Q2 structural units in the glasses and melts was determined as a function of bulk Al/(Al + Si) and temperature. The Al3+ resides principally in Q4 structural units, which contain more than 70% of the total amount of Al3+. The Q2 units contain the smallest amount of Al3+ among the Q4, Q3, and Q2 structural units. There is no evidence for temperature-dependent distribution of Al3+ among the coexisting structural units at least to 1100–1200 °C.
The equilibrium constant for the speciation reaction, 2Q3 ↔ Q4 + Q2, passes through a maximum with increasing bulk melt Al/(Al + Si) at constant temperature. Its temperature-dependence also depends on bulk melt Al/(Al + Si) so that the enthalpy of the speciation reaction, ΔH, passes through a maximum with increasing bulk Al/(Al + Si). This ΔH ranges between 13 and 23 kJ/mol for bulk melt Al/(Al + Si) between 0 and 0.33. These composition-dependent maxima in the equilibrium constant (at constant temperature) and ΔH of the speciation reaction reflect the changes in Al/(Al + Si) of Q2, Q3, and Q4 structural units with Al/(Al + Si) of the melt.