Structure of Aluminosilicate Melts

Abstract

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.