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The structural behavior of Al³⁺ in peralkaline melts and glasses in the system Na₂O-Al₂O₃-SiO₂

¹ Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C., U.S.A.
² University of California at Davis, Davis California, U.S.A.

Correspondence: ^* E-mail: mysen{at}gl.ciw.edu

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, H, passes through a maximum with increasing bulk Al/(Al + Si). This H 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 H 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.

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