Twin operation

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 H_i be the oriented point group of the i-th individual of a twin. The intersection group of the oriented vector point groups H_i of the individuals is indicated as H* = ∩_iH_i. 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