Assuming the flat {010} form to be the original composition plane, the interfaces of the 100, ¯101, ¯201, 001 and 101 penetration twins of gypsum crystals were theoretically examined. The related twin energies were 357, 64, 2171, 2124 and 10844mJ/m2, respectively. The observed experimental growth shape was interpreted, for each twin law, with the aid of the theoretical one. For each twin law, a comparison was made, between the energies obtained from contact and penetration interfaces, and a global evaluation was made of their probability of occurrence. For the penetration twins, a correlation was proposed between 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 occurred.

The Five Twin Laws of Gypsum (CaSO4•2H2O): a Theoretical Comparison of the Interfaces of the Penetration Twins. M.Rubbo, M.Bruno, F.R.Massaro, D.Aquilano: Crystal Growth and Design, 2012, 12[6], 3018-24