Difference between revisions of "Twin operation"

From Online Dictionary of Crystallography

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<br> Chapter 3.3 of ''International Tables of Crystallography, Volume D''<br>
<br> Chapter 3.3 of ''International Tables of Crystallography, Volume D''<br>
[[Category:Fundamental crystallography]]

Revision as of 09:42, 15 May 2006

Opération de maclage (Fr). Operazione di geminazione (It)

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 twin point group.

See also

Chapter 3.3 of International Tables of Crystallography, Volume D