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 2Al(3+) tetrahedrally coordinated cations. This structure is dominated by SiO4, (Si,Al)O-4, and AlO4 entities.