Compositional transformations as well as thermodynamics and mechanism of dissolution for clay minerals

A systematic density functional theory study was conducted to understand the compositional transformation and dissolution reactions of clay minerals that are associated with a wide spectrum of geochemical processes. Compositional transformations of clay minerals are preferred for homo- rather than lower-valent ions, and the local abundance of elements largely determines the compositional transformation processes and explains a number of experimental observations. The salt form, extent of compositional transformations, temperature and pressure were explored as four factors and showed disparate results; e.g., temperature rather than pressure has a more pronounced effect, although it provides inconsistent trends for different compositional transformations. Owing to compositional transformations, the dissolution thermodynamics of clay minerals is altered significantly, and priorities of dissolution rank are B3+ < Ga3+ < Fe3+ < Al3+ < Mg2+ at octahedral sheets and Al3+ ≤ Ge4+ < Ti4+ ≤ Si4+ < Zr4+ at tetrahedral sheets. In montmorillonite, dissolution of octahedral Al3+ sites requires nearly twice the energy as dissolution of tetrahedral Si4+ sites and becomes more difficult with an increase of Al3+ to Mg2+ transformations. A two-step mechanism for montmorillonite dissolution has been proposed, as exfoliation of tetrahedral Si4+ sheets that converts montmorillonite to kaolinite and then dissolution of octahedral Al3+ sites.