9.17: Twinning by metric merohedry
Twinning by metric merohedry is a special case of twinning by merohedry which occurs when:
- the lattice of the individual has accidentally a specialized metric which corresponds to a higher holohedry
- the twin operation belongs to this higher holohedry only
If H is the individual point group, D(H) the corresponding holohedry and D(L) the point group of the lattice, twinning by metric merohedry corresponds to D(L) ⊃ D(H) ⊇ H
Twinning by metric merohedry can be seen as the degeneration of twinning by reticular merohedry to twin index 1, or of twinning by pseudomerohedry to twin obliquity zero.