OD character and twinning - Selective merohedry in class II merohedric twins of OD polytypes

In twinning by merohedry, the twin operations belong to the crystal lattice point group. When twinning concerns an OD structure, ie. a polytype in whi ch layer pairs are geometrically equivalent, the definition of merohedry sh ould take into account not only the point group of the crystal and that of the lattice, but also the point group of the family structure. If the latte r is a subgroup of the point group of the twin, some of the twin laws resto re only a part of the reciprocal lattice nodes corresponding to the family reflections (the reflections common to ail the OD structures of a family). For this kind of twinning the definition of selective merohedry is introduc ed: it can occur when the Laue point group of the crystal is lower than the twin point group, i.e. in the so-called "class II twins". Detection of sel ective merohedry from the diffraction pattern is discussed.