Structural behavior of Al 3+ in silicate melts: In situ, high-temperature measurements as a function of bulk chemical composition

Abstract

The anionic structure of aluminosilicate melts has been determined along the join Li 2Si 2O 5Li 2(LiAl) 2O 5 (LS2-LA2) with microRaman spectroscopy in the temperature range 25°-1460°C. Those data are compared with the structural behavior of melts along the join Na 2Si 2O 5Na 2(NaAl) 2O 5 (NS2-NA2) in the same temperature interval. In these systems, Li' and Na + serve both to charge-balance Al 3+ in tetrahedral coordination and as network-modifiers. The NBO/ T ( T = Si + Al) equals unity in the Al/(Al + Si) range examined (0–0.3). In the Al-free endmember glass and melt systems, the three species, Q 4, Q 3, and Q 2 coexist and the expression, (1) 2 Q 3 ⇔ Q 2 + Q 4, describes the equilibrium. Substitution of Na- or Li-charge-balanced Al 3+ for Si 4+ results in stabilization of an additional, more depolymerized structural unit, Q 1. An additional equilibrium, (2) 2 Q 2 ⇔ Q 1 + Q 3, is needed, therefore, for a complete description of the equilibria. In the LS2-LA2 system, the ΔH for this latter reaction ( ΔH 2) ranges between ∼0 and −87 kJ/mol and is negatively correlated with Al/(Al + Si). In the NS2-NA2 system, the ΔH 2 is positive with values between 16 and 37 kJ/mol and is positively correlated with Al/(Al + Si). Equilibrium (1) is affected by equilibrium (2) in the Al-bearing melts, so that in the NS2-NA2 melt system, equilibrium (1) shifts to the left with temperature ( ΔH 1 = −10–−15 kJ/mol), whereas in the LS2-LA2 system, equilibrium (1) shifts more strongly to the right with temperature than in the absence of Al ( ΔH 1 is positively correlated with Al/(Al + Si) with values in the range 6–48 kJ/mol). Activity coefficients for Q 2 units in the melts calculated from liquidus phase relations in combination with the determined mol fractions of structural units in the melts show simple relations between activity coefficient of the unit and its molar abundance in the melts.