Difference between revisions of "Twinning by merohedry"
From Online Dictionary of Crystallography
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| − | The twin operation belongs to the point group of the lattice but not to the point group of the crystal. Therefore, the point group of the crystal must be a subgroup of the point group of the lattice, ''i.e.'' the crystal shows only a part (''[[merohedry]]'') of the symmetry | + | The twin operation belongs to the point group of the lattice but not to the point group of the crystal. Therefore, the point group of the crystal must be a subgroup of the point group of the lattice, ''i.e.'' the crystal shows only a part (''[[merohedry]]'') of the symmetry operations belonging to its lattice which, instead, shows ''[[holohedry]]'' (complete symmetry). The twinning element of symmetry may (''Class I of twins by merohedry'') or may not belong to the Laue class of the crystal (''Class II of twins by merohedry''). |
== Examples == | == Examples == | ||
Latest revision as of 13:43, 15 July 2021
Maclage par mériédrie (Fr). Meroedrische Verzwillingung (Ge). Geminazione per meroedria(It). 欠面双晶 (Ja). Macla por meroedría (Sp).
The twin operation belongs to the point group of the lattice but not to the point group of the crystal. Therefore, the point group of the crystal must be a subgroup of the point group of the lattice, i.e. the crystal shows only a part (merohedry) of the symmetry operations belonging to its lattice which, instead, shows holohedry (complete symmetry). The twinning element of symmetry may (Class I of twins by merohedry) or may not belong to the Laue class of the crystal (Class II of twins by merohedry).
Examples
Class I: in crystals with point group 2 (Laue group 2/m) the mirror plane m acts as twin element.
Class II: in crystals with point group 4 (Laue group 4/m) a mirror plane m parallel to the fourfold axis 4 acts as twin element.
See also
- Chapter 3.3 of International Tables for Crystallography, Volume D