Structure of Aluminosilicate Melts
In peralkaline and meta-aluminous melts, essentially all Al3+ (>95%) occupy tetrahedral coordination, whereas for peraluminous melts, complex mixtures of aluminum triclusters with 4-fold coordinated Al3+ and Al3+ in 5- and 6-fold coordination with oxygen describe the structure. Aluminum in tetrahedral coordination requires electrical charge-balance. With alkali metals (M+) in this role, the proportions are M+=Al3+. The overall structure is dominated by three-dimensionally interconnected tetrahedra to form 6-membered rings of tetrahedra. The Al/(Al+Si) of these tetrahedra are simple positive functions of the bulk melt Al/(Al+Si). When tetrahedrally-coordinated Al3+ is charge-balanced by divalent cations, the M2+ cation charge-balances 2Al3+ tetrahedrally coordinated cations. This structure is dominated by SiO4, (Si,Al)O4, and AlO4 entities.In peraluminous melts, where there is insufficient proportion of M2+ and M2+ cations for charge-balance, aluminum exists in triclusters with Al3+ in tetrahedral coordination. In peralkaline aluminosilicate melts, there coexist discrete structural units with different degree of silicate polymerization. These units are termed Qn-species where the superscript, n, is the number of bridging oxygen in individual units. Equilibria among these units are of the type, 2Qn = Qn+1 + Qn−1. In these melts, Al3+ is distributed among these units. The Al3+ in peralkaline aluminosilicate melts strong preference Q4 units. This preference is, however, temperature-dependent as reflected in changes in the ΔH of the Qn-species reaction.