Difference between revisions of "Twinning"
From Online Dictionary of Crystallography
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* '''symmetry of a twin''' | * '''symmetry of a twin''' | ||
| − | An element of symmetry crystallographically relating differently oriented crystals cannot belong to the individual. The element of symmetry that relates the indivduals of a twin is called ''[[twin element of symmetry]]'' (or simply ''twin element'') and the connected operation is a ''[[ | + | An element of symmetry crystallographically relating differently oriented crystals cannot belong to the individual. The element of symmetry that relates the indivduals of a twin is called ''[[twin element of symmetry]]'' (or simply ''twin element'') and the connected operation is a ''[[twinning operation]]''. The ''[[Mallard law]]'' states that the ''twin element'' (i.e. the geometrical element relative to which the twining operation is defined) is restricted to a direct lattice element: lattice nodes (''twin centres''), lattice rows (''twin axes'') and lattice planes (''twin planes''). |
In most twins the symmetry of a twin (''twin point group'') is that of the individual point group augmented by the symmetry of the twinning operation; however, a symmetry element that is oblique to the twinning element of symmetry is absent in the twin (e.g., ''spinel twins'': ''m''<math> \bar 3</math>''m'' crystal point group; {111} [[twin law]]; <math> \bar 3</math>/''m'' twin point group. | In most twins the symmetry of a twin (''twin point group'') is that of the individual point group augmented by the symmetry of the twinning operation; however, a symmetry element that is oblique to the twinning element of symmetry is absent in the twin (e.g., ''spinel twins'': ''m''<math> \bar 3</math>''m'' crystal point group; {111} [[twin law]]; <math> \bar 3</math>/''m'' twin point group. | ||
Revision as of 10:01, 26 April 2006
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Contents
Oriented association and twinning
Crystals (also called individuals) belonging to the same phase form an oriented association if they can be brought to the same crystallographic orientation by translation, rotation or reflection. Individuals related by a translation form a parallel association; strictly speaking these individuals have the same orientation even without applying a translation. Individuals related by a reflection [either plane (reflection twin) or centre (inversion twin) of symmetry] or a rotation (rotation twin) form a twin.
- symmetry of a twin
An element of symmetry crystallographically relating differently oriented crystals cannot belong to the individual. The element of symmetry that relates the indivduals of a twin is called twin element of symmetry (or simply twin element) and the connected operation is a twinning operation. The Mallard law states that the twin element (i.e. the geometrical element relative to which the twining operation is defined) is restricted to a direct lattice element: lattice nodes (twin centres), lattice rows (twin axes) and lattice planes (twin planes).
In most twins the symmetry of a twin (twin point group) is that of the individual point group augmented by the symmetry of the twinning operation; however, a symmetry element that is oblique to the twinning element of symmetry is absent in the twin (e.g., spinel twins: m[math] \bar 3[/math]m crystal point group; {111} twin law; [math] \bar 3[/math]/m twin point group.
Classification of twins
Twins are classified following Friedel's reticular (i.e. lattice) theory of twinning. This theory states that the presence, either in the lattice or a sublattice of a crystal, of (pseudo)symmetry elements is a necessary, even if not sufficient, condition for the formation of twins. In presence of the reticular necessary conditions, the formation of a twing finally still depends on the matching of the crystal structures at the contact surface between the individuals. The following categories of twins are described under the listed entries.
twinning by reticular merohedry
twinning by reticular pseudomerohedry
- twinning by metric merohedry
- twin obliquity
The twin obliquity is a measure of the distorsion of a (sub)lattice in twins by (reticular) pseudomerohedry.
- corresponding twins
Other categories of twins
Endemic conditions for twinning
See also
Chapter 1.3 of International Tables of Crystallography, Volume C
Chapter 3.3 of International Tables of Crystallography, Volume D