Corresponding twins
From Online Dictionary of Crystallography
Macles correspondantes (Fr). Geminati corrispondenti (It).
Definition
In the case of twinning by merohedry, when the twin element is twofold and the crystal is centrosymmetric, the twin operation can be described either as a rotation or as a reflection (the two operations being equivalent under the action of the center).
In the case of twinning by pseudomerohedry, instead, the two twin operations are no longer equivalent even in centrosymmetric crystals but produce different twins, which are called reciprocal twins (Mügge, 1898) or corresponding twins (Friedel, 1904, 1926). One of the most classical examples is that of albite vs pericline twins in feldspars.
Pairs of corresponding twins normally do not have the same frequency of occurrence, as one would be tempted to assume from the identical value of their twin obliquity.
History
- Friedel, G. (1904). Étude sur les groupements cristallins. Extrait du Bulletin de la Société de l'Industrie minérale, Quatrième série, Tomes III et IV. Saint-Étienne, Société de l'imprimerie Théolier J. Thomas et C., 485 pp.
- Friedel, G. (1926). Leçons de Cristallographie. Berger-Levrault, Nancy, Paris, Strasbourg, XIX+602 pp.
- Mügge, O. (1898). Über Translationen und verwandte Erscheinungen in Krystallen. Neues Jahrb. Mineral. Geol. Paläontol. 1, 71-158.