Actions

Twin operation

From Online Dictionary of Crystallography

Revision as of 14:21, 20 November 2017 by BrianMcMahon (talk | contribs) (Added German and Spanish translations (U. Mueller))
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

Opération de maclage (Fr). Zwillingsoperation (Ge). Operazione di geminazione (It). 双晶操作 (Ja). Operación de macla (Sp).


The operation (action) of an element of symmetry that generates a twin.

Let Hi be the oriented point group of the i-th individual of a twin. The intersection group of the oriented vector point groups Hi of the individuals is indicated as H* = ∩iHi. The symmetry of a twin is identified in vector space by a point group K which is a supergroup of H*. The coset decomposition of K with respect to H* gives the possible twin laws, each coset representing a twin law, and each operation in a coset representing a twin operation; the operations in a coset are equivalent under the operations of H*.

Operations in H describe the vector point symmetry of the individuals, whereas those in the cosets obtained by decomposing K in terms of H* connect the different individuals. To underline their different nature, the twin operations are often associated with a 'colour' and K is a thus a chromatic vector point group, known as a twin point group.

See also

  • Chapter 3.3 of International Tables for Crystallography, Volume D