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

Having assumed the flat {010} form as the original composition plane (OCP), the interfaces of the 100, 1̅01, 2̅01, 001, and 101 penetration twins of gypsum crystal are theoretically examined and their relaxed structures are illustrated. The related twin energies (γPT) are 357, 64, 2171, 2124, and 10 844 erg cm–2, respectively. The observed experimental growth shape is interpreted, for each twin law, with the aid of the theoretical one. For each twin law, a comparison is made, between the γPT values obtained from contact and penetration interfaces, and a global evaluation is done on their occurrence probability. Finally, for the penetration twins, a correlation is proposed among the area of the two-dimensional common cell at the twin interface, the obliquity of the cell, the twin energy, and the supersaturation domain of the mother phase in which the twin can occur.