STABILITY OF 1-LAYER POLYTYPE OF KAOLIN BY MEANS OF MOLECULAR DYNAMICS SIMULATION

Abstract

An energy map for 1-layer kaolin polytypes was developed, plotting internal energies calculated from results of Molecular Dynamics (MD) simulations. The energy map indicates stabilities of various kaolin 1-layer polytypes by the internal energies as a function of the interlayer shift. The interlayer shifts were described on the basis of a pseudo-hexagonal lattice which contains an asymmetric unit of kaolinite structure. The MD simulations were carried out at grid points on a (001) plane of pseudo-hexagonal lattice with increments of 1/12 along each axis. In order to remain interlayer configurations in each structure, we adopted a constraint that the positions of non-hydrogen atoms had been fixed. The present energy map shows two energetic minima, which correspond to structures of kaolinite and its enantiomorph, and has no minimum at (1/3, 1/3) in fractional coordinates of a projection onto (001) plane of pseudo-hexagonal lattice, which had been clearly shown in the electrostatic potential map by previous workers. We also tried to follow trajectories of interlayer shifts during all-atom relaxations by MD simulations (NPT ensemble, 293 K, 0.1 MPa). The trajectories of interlayer shifts from different initial structures converged to five spots on the energy map, which include the two energetic minima. Average internal energies have lower values at the two minima on the energy map than at other three spots which trapped some trajectories. More long simulation time or higher temperature may complete relaxations of the trapped structures. Both of the two approaches, the energy map and following trajectories, support that the kaolinite and its enantiomorph are the most stable structures as 1-layer kaolin polytype.