The (100) Contact Twin of Gypsum

We investigated the structure of the interface between two gypsum crystals forming a (100) contact twin. The athermal twinning and adhesion free energies were calculated using empirical potential functions. By minimizing the twin energy, the optimal interface configuration was obtained and carefully described. The layer expansion and in plane deformation occurring at the twin interface is analyzed in terms of the atomic relaxation displacements. A comparison with previous studies on this twin has been made. A discussion on the formation of the (100) contact twin follows where we show that this twin can be formed by two-dimensional nucleation.