Reactivity and point defects of double oxides with emphasis on simple silicates

Heterogeneous solid-state reactions in quasibinary oxide systems are analyzed. As long as local thermodynamic equilibrium prevails, diffusion processes through the reaction product are rate-controlling. The diffusion coefficients are governed by point-defect concentrations. Point-defect thermodynamics allow calculation of relative point-defect concentrations as a function of the relevant thermodynamic variables, if the disorder type of the crystalline product is known. Disorder types in ternary ionic crystals are introduced. On this basis, several reactions leading to simple silicates of the form A ₂ BO ₄ are discussed in terms of ion mobilities and Gibbs energies of formation, and the possible reaction mechanisms are analyzed. Finally some remarks are included on the influence of the gas atmosphere on the reaction rate, on powder reactions and on phase boundary-controlled reaction rates.