A TENSOR CLASSIFICATION OF TWINNING IN CRYSTALS

A classification scheme for twinning in crystals is proposed. It is ba sed on a tensor distinction of properties across the twin interface. T he classification employs concepts from the theory of transformation t winning. The adequacy of such a scheme for other main types of twins, namely growth twins and nonferroelastic mechanical twins, is examined, and found to be satisfactory. All twins can be divided into four fund amentally distinct categories: ferroelastic or S-twins, nonferroelasti c-ferroic or N-twins, Bollmann or B-twins and translation or T-twins. A comparison is made of the attributes of these four types of twins. A compact and informative twin symbol is introduced, which carries info rmation about the tensor properties in which the twins differ. Informa tion about any underlying total or partial coincidence sublattice is i ncorporated by attaching a subscript t or p to the main twin symbol. S everal examples are discussed. This paper also introduces a rigorous d efinition of prototype symmetry in terms of the 'nondisruption conditi on'.