# Merohedral

### From Online Dictionary of Crystallography

**Merohedral** is the adjectival form of merohedry and indicates a crystal that does not possess the full point symmetry of its lattice. Merohedral twins are the result of twinning by merohedry.

## Discussion

In the literature, the term *merohedral twinning* is often improperly used instead of twinning by merohedry. A merohedral crystal may undergo several different types of twinning and for this reason the term 'merohedral twinning' is misleading, as the following example shows.

A crystal belonging to the geometric crystal class 2 is merohedral because its lattice has at least symmetry 2/*m*. There are three minimal supergroups of order four of the point group 2 which correspond to three different twins.

- Twinning by reflection across the (010) plane or by inversion: this corresponds to twinning by merohedry, twin point group 2/
*m'*. - Twinning by reflection across the (100) or (001) plane: this corresponds to twinning by pseudomerohedry, twinning by reticular merohedry, or twinning by reticular pseudomerohedry if β ≠ 90º, or to twinning by metric merohedry if β = 90º; the twin point group is
*m*′2*m*′. - Twinning by rotation about the [100] or [001] direction: this corresponds to the same types of twinning as case 2 above but the twin point group is 2′22′.

Case 1 above would be a 'merohedral twin of a merohedral crystal' while cases 2 and 3 would be 'non-merohedral twins of a merohedral crystal'.

To avoid any terminological awkwardness, the adjective **merohedric** has been suggested with reference to twins, but the use of the category names like twinning by merohedry remains preferable.