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=== Twinning by Merohedry === | === Twinning by Merohedry === | ||
In a merohedral twin, the twin law is a symmetry operator of the crystal system, but not of the point group of the crystal. This means that the reciprocal lattices of the different twin domains superimpose exactly and the twinning is not directly detectable from the reflection pattern. This type is possible in the trigonal, tetragonal, hexagonal and cubic crystal systems, which have more than one Laue group. The twin law corresponds to the two-fold operation that is present in the apparent Laue group, but not in the true space group | In a merohedral twin, the twin law is a symmetry operator of the crystal system, but not of the point group of the crystal. This means that the reciprocal lattices of the different twin domains superimpose exactly and the twinning is not directly detectable from the reflection pattern. This type is possible in the trigonal, tetragonal, hexagonal and cubic crystal systems, which have more than one Laue group. The twin law corresponds to the two-fold operation that is present in the apparent Laue group, but not in the true space group. | ||
Higher forms of twinning exist: tetartohedry (4 twin domains) and ogdohedry (8 twin domains). | Twinning in P3, P4, P6 and cubic crystals often involves only two domains (hemihedry). Only for trigonal crystals is there more than one possible twin law. Higher forms of merohedral twinning then exist: tetartohedry (4 twin domains) and ogdohedry (8 twin domains). | ||
=== Twinning by Pseudo-Merohedry === | === Twinning by Pseudo-Merohedry === | ||
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Giacovazzo, C. ed. (2002). Fundamentals in Crystallography, I.U.Cr. & O.U.P.: Oxford, UK. | Giacovazzo, C. ed. (2002). Fundamentals in Crystallography, I.U.Cr. & O.U.P.: Oxford, UK. | ||
=== Other Helpful Papers on Twinning === | |||
== Other Helpful Papers on Twinning == | |||
Parsons, S. Introduction to twinning. Acta Cryst (2003) D59, 1995-2003. | Parsons, S. Introduction to twinning. Acta Cryst (2003) D59, 1995-2003. | ||
Dauter, Z. Twinned crystals and anomalous phasing. Acta Cryst (2003) D59, 2004-2016 | Dauter, Z. Twinned crystals and anomalous phasing. Acta Cryst (2003) D59, 2004-2016 | ||
[http://journals.iucr.org/d/issues/1999/10/00/ba0026/ba0026.pdf Chandra, N., Ravi Acharya, K., and Moody, P.C.E. (1999) Analysis and characterization of data from twinned crystals. Acta Cryst. D55, 1750-1758] | |||
== | === Examples, with background references === | ||
Detecting and overcoming hemihedral twinning during the MIR structure determination of Rna1p. Hillig RC, Renault L. Acta Crystallogr D Biol Crystallogr. 2006 Jul;62(Pt7):750-65. | |||
=== Other Crystal Pathologies === | |||
Zwart, PH et al. Surprises and pitfalls arising from (pseudo)symmetry. Acta Cryst (2008) D64, 99-107. | Zwart, PH et al. Surprises and pitfalls arising from (pseudo)symmetry. Acta Cryst (2008) D64, 99-107. | ||