The Five Twin Laws of Gypsum (CaSO4·2H2O): A Theoretical Comparison of the Interfaces of the Contact Twins

The relaxed interfaces of the 100, 1̅01, 001, 201̅, and 101 contact twins of the gypsum crystal are theoretically examined, and their perturbed structure is described. The obtained twin energies (γPT) are 13.6, 145, 255, 826, and 848 erg cm–2 for the 100, 001, 1̅01, 201̅, and 101 laws, respectively....

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Bibliographic Details
Published in:Crystal growth & design 2012-01, Vol.12 (1), p.264-270
Main Authors: Rubbo, Marco, Bruno, Marco, Massaro, Francesco Roberto, Aquilano, Dino
Format: Article
Language:English
Subjects:
Condensed matter: structure, mechanical and thermal properties
Defects and impurities in crystals
microstructure
Exact sciences and technology
Grain and twin boundaries
Physics
Structure of solids and liquids
crystallography
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Summary:The relaxed interfaces of the 100, 1̅01, 001, 201̅, and 101 contact twins of the gypsum crystal are theoretically examined, and their perturbed structure is described. The obtained twin energies (γPT) are 13.6, 145, 255, 826, and 848 erg cm–2 for the 100, 001, 1̅01, 201̅, and 101 laws, respectively. Hence, the five twin laws can be divided in three classes reflecting the deep difference due to the character of the face, in the sense of Hartman–Perdok, on which the contact twin is supposed to form. In fact, the original composition planes of the 100, 001, and 1̅01 laws correspond to stepped (S) faces, while those of the 201̅ and 101 laws correspond to kinked (K) faces. Furthermore, the highest probability of occurrence of the 100 contact twins is strictly related not only to the lowest γPTvalue but also to the fact that, among the three S faces, (100) is the only one belonging to the athermal equilibrium shape of the crystal.
ISSN:1528-7483
1528-7505
DOI:10.1021/cg201031s