Twinning (effects of)
From Online Dictionary of Crystallography
Maclage (effets de) (Fr). Geminazione (effetti della) (It).
Morphological and optical effects
At the morphological level, twinning introduces re-entrant angles that, in principle, are specific marks.
Except for twins by merohedry, the orientation of the optical indicatrix changes through the inteface between adjacent crystals (individuals). Thus, apart from particular orientations, if observed by a polarizing microscope between crossed polars, a twin never can be brought by rotation to a position of complete extinctions (dark field).
Overlap of lattices
By effect of a twin operation, both the direct and reciprocal lattice of the individuals forming a twin are overlapped. Overlapping (restoration) of nodes belonging to different individuals can be: (i) exact and total (twinning by merohedry); (ii) exact but partial (i.e. only a fraction of the nodes of an individual lattice is restored; twinning by reticular merohedry); (iii) total but approximate (twinning by pseudomerohedry), approximate and partial (twinning by reticular pseudomerohedry). Twin lattice, twin index and twin obliquity are entries related to the overlap of lattices.
For the effects of twinning on a diffraction pattern see: Twin (diffraction pattern of).