Electric-Field-Dependent Covalent Interactions between H⁺ and Surface O Atoms Promote the Structural Disintegration of Montmorillonite

The structural stability of clay minerals is an important geochemical process. However, a long-standing challenge is to understand the surface reaction mechanisms for mineral structure stability. Quantum mechanical analysis indicates that a strong surface reaction, electric-field-dependent covalent interactions between H⁺ and basal oxygen (O) atoms of montmorillonite (MMT) surface, occurs in hydrothermal experiments. The covalent interactions strongly depend on the orbital hybridization of siloxane atoms at the MMT surface and increase with increasing acid concentration and temperature. The electric-field-dependent covalent interactions between H⁺ and surface O atoms weaken the Si-O bonding energy in MMT crystals, consequently contributing to the structural disintegration. A critical adsorption pressure of H⁺ at the MMT surface for complete disintegration was estimated to be -241.6 MPa. For example, the transformation rate of MMT is 34.7% at an adsorption pressure of -203.5 MPa, while it reaches up to 97.4% when adsorption pressures are below -241.6 MPa. Our findings will enhance the understanding and awareness of mineral weathering and soil acidification, which depend on the clay mineral structure.