Relaxed interfaces of the 100, ¯101, 001, 20¯1 and 101 contact twins of the gypsum crystal were theoretically examined and their perturbed structure was described. The obtained twin energies were 13.6, 145, 255, 826 and 848mJ/m2 for the 100, 001, ¯101, 20¯1 and 101 laws, respectively. The five twin laws could be divided into three classes, reflecting the deep differences due to the character of the face upon which the contact twin was supposed to form. The original composition planes of the 100, 001 and ¯101 laws corresponded to stepped faces, while those of the 20¯1 and 101 laws corresponded to kinked faces. The highest probability of occurrence of the 100 contact twins was strictly related not only to the lowest twin-energy value but also to the fact that, among the three stepped faces, (100) was the only one belonging to the athermal equilibrium shape of the crystal.

The Five Twin Laws of Gypsum (CaSO4•2H2O): a Theoretical Comparison of the Interfaces of the Contact Twins. M.Rubbo, M.Bruno, F.R.Massaro, D.Aquilano: Crystal Growth and Design, 2012, 12[1], 264-70