Twinning (endemic conditions of)

 Geminazione (condizioni endemiche di) (It)

When a lattice necessarily contains at least one sublattice that supports either twinning by reticular merohedry or twinning by reticular pseudomerohedry, it is said that an endemic condition of twinning does exist. The following cases are known.

Rhombohedral lattice (hR)

A hR lattice always (symmetry [math] \bar 3[/math]m) always contains a hp lattice (symmetry 6/mmm). Consequently, the crystal structures based on an R lattice are endemic candidates to twinning by reticular merohedry via the symmetry elements that occur in the 6/mmm point group of the sublattice, but not to the [math] \bar 3[/math]m point group of the lattice.

Cubic lattices