A Study on edge surface structures and reactivity for clay minerals by molecular orbital method

Yotsuji, Kenji*; Takeda, Seiji ; Kimura, Hideo

To clarify the dissolution mechanism of clay minerals, the basal and edge surface structures of dioctahedral 2:1 phyllosilicate was optimized using semi-empirical molecular orbital method. The stability of the surface structure was estimated by the bond length, bond strength and the bond energy on bonds of interest along the basal and edge surfaces. The optimized basal surface structure of montmorillonite resides at a local minimum on the potential energy surface, because each normal mode for the optimized structure has real vibrational frequency. The ideal (010)-type edge surface of pyrophyllite was initialized using crystal chemical methods. The edge surface structure that was protonated allowing for pH-dependency was optimized. On optimized edge surfaces structural relaxation occurred in all pH-conditions, then it was found that Si-O bonds in the silanol groups were stronger, and Al-O bonds in the aluminol groups were weaker, than corresponding bonds in the bulk structures, respectively. The rate-determining step is thought to be governed by hydrolysis of outer Si-O bonds on edge surfaces in alkaline dissolution of pyrophyllite and by hydrolysis of Al-O bonds in bridging Si-O-Al bonds on edge surfaces in acid dissolution.