Which type of twinning appears as if two crystal halves are mirror images of each other?

The type of twinning that appears as if two crystal halves are mirror images of each other is contact twinning. In this phenomenon, the two halves of the crystal are related by symmetry and share a common boundary, giving them a mirrored appearance. This is a distinct characteristic of contact twinning, where the alignment of the crystal structure allows for such reflection, enhancing the overall aesthetic and symmetry of the crystal.

Growth twinning, on the other hand, occurs when two crystals develop simultaneously from the same parent crystal and can form in a way that doesn't necessarily reflect a mirrored symmetry. Reentrant twinning involves irregularities and can create complex geometrical shapes that do not exhibit mirror image symmetry. Cyclic twinning involves repetitive angles and rotations in the crystal lattice that do not establish a direct mirror image relationship. Therefore, contact twinning is the correct choice when identifying the mirrored appearance in crystal formations.