Computational simulations of the structure of Japan twin boundaries in quartz

The structures of Japan twin boundaries in quartz are studied through molecular dynamics simulations and energy minimization calculations. Four types of twinning are grouped under the Japan twin law, comprising 10 subtypes of geometrical configurations based on the structural handedness and composition plane. For each subtype, the twin displacement vector is determined and the structures of the twin boundaries with {112̄2} or {1̄1̄ 22} composition plane are simulated. It is shown that six subtypes composed of two crystals with same-handed structure have the same type of SiO₄ tetrahedral linkage at twin boundaries. The twin boundary structures of these six subtypes can be further divided into two enantiomorphic structures composed of right- or left-handed quartz. The other four subtypes are composed of combinations of right- and left-handed quartz, and are structurally distinct from the same-handed group. The twin boundary energies of the same-handed group are lower than for the opposite-handed group. The Si-O bond lengths in the region of the twin boundary differ by no more than 0.05 Å from those in the bulk quartz for all subtypes. The differences between the Si-O-Si angles at the twin boundaries and in the bulk exceed 20°, whereas the differences in O-Si-O angles are less than 10°. The present calculations demonstrate that SiO₄ tetrahedra at Japan twin boundaries are very stiff in comparison with inter-tetrahedral forces, consistent with previous reports for silica polymorphs, and that structural matching of twin individuals is primarily achieved through the rotation of SiO₄ tetrahedra.