1.89: Pseudo symmetry
A crystal space can in general be divided in N components S 1 to S N . When a coincidence operation φ(S i )→S j brings the i -th component S i to coincide with the j -th component S j , for any i and j , φ is a symmetry operation of the space group.
Sometimes, φ brings S i close to, but not exactly on, the position and orientation of S j : in this case the operation mapping S i onto S j is not crystallographic but the linear and/or rotational deviation from a space group operation is limited. For this reason, it is preferable to describe the crystallographic operation φ as a pseudo symmetry operation .
Pseudo symmetry operations for the lattice play an important role in twinning, namely in the case of twinning by pseudomerohedry and twinning by reticular pseudomerohedry.