Effect of the Si/Al ordering on structural parameters and the energetic stabilization of vermiculites - A theoretical study

The effect of the Si/Al distribution in the tetrahedral sheets of the vermiculite mineral has been investigated employing density functional theory. The structures of six models for vermiculite with the structural formula (Mg₄)(Mg₁₂ (Si₈Al₈)O₄₀(OH ₈·24(H₂O) per unit cell were fully optimized. The models differ by the T⋯Mg²⁺⋯T' coordination of the interlayer Mg²⁺ cations by two central cations from the adjacent tetrahedral sheets of the 2:1 vermiculite layers (T,T' = Si,Al). We observed the formation of very strong hydrogen bonds between water molecules solvating the interlayer Mg²⁺ cations and the surface basal oxygen atoms of the 2:1 layers. The directionality of hydrogen bonds is the major factor determining the layer stacking in the vermiculite structure. Results showed that the most stable model is that where only silicon atoms in the tetrahedral sheets coordinate all interlayer Mg²⁺ cations.