The Dissolution of Silica and Alumina in Silicate Melts: In situ High Temperature Calorimetric Studies

High temperature calorimetry using molten silicate solvents has been used to evaluate, in situ, the mixing properties of two network-forming oxides Al₂O₃ and SiO₂ in several silicate melts at 1760 K. In K₂O-SiO₂ solvents, the partial molar enthalpy of mixing of supercooled liquid SiO₂ ranges from -11.4 to +4.6kJ/mol as SiO₂ concentration is increased from 66 to 75 mole %. For mixed cation solvents (0.2CaO, 0.8K₂O)-SiO₂, the partial molar enthalpy of mixing ranges from -10.6 to -3.6kJ/mol for the same composition range. The energetics reflect the depolymerization of the silicate melt framework and the formation of melt species with Si-O-Mⁿ⁺ bonds. Decreased field strength of the network modifying cation, Mⁿ⁺, favors Si-O-Mⁿ⁺ formation. However, as predicted by Le Chatelier's Principle, since Si-O-Mⁿ⁺ formation is exothermic, this species becomes less dominant as temperature is increased. This temperature-dependence of the depolymerizing reactions, which define silicate melt structure, may relate to the configurational changes of alkali silicate melts which result in non-Arrhenian structural relaxation. The enthalpy of solution of Al₂O₃ depends on the composition of the silicate solvent and the concentration of Al₂O₃. The enthalpy of solution ranges from 33.3 to 151.7kJ/mol for the concentration range 0 to 10mole % Al₂O₃ in potassium silicate solvents. In mixed cation solvents, the enthalpy of solution ranges from 33.6 to 64.4kJ/mol in the concentration range 0 to 10mole % Al₂O₃. The dissolution of Al₂O₃ in these silicate solvents reflects the interaction between supercool ed Al₂O₃ liquid and the existing melt species and is also influenced by the identity of the charge-balancing cation. In potassium end-member melts, the low degree of interaction between K⁺ and the polymerized framework means that Al³⁺ cation repulsions dominate the melt energetics and results in an ordered melt with predominance of K-O-Al species and a strong concentration dependence of the enthalpy of solution. In mixed cation melts the ability of Ca²⁺ to perturb the melt network results in a melt that has a variety of structural environments for AlO₄ tetrahedra. Accordingly the concentration dependence of the heat of solution is less pronounced.