The Genesis of Twin Crystals

Abstract

Interest has recently been indicated in a possible cause for the occurrence of twins. Apparently the only popular approach to twinning has been through the empirical rules of twinning enunciated by the French School. The writer's views, now some twenty years old, are herewith presented as a rational, structural approach to the cause of twinning. Since a stable crystal represents the least energy state of a collection of atoms, it is evident that the twin boundary must correspond with a higher energy state. It is shown that this higher energy is largely associated with the higher coordination spheres of the atoms. That a twin can form at all requires that the first coordination conditions of the atoms at the boundary are satisfied. This view is purely structural and has been substantiated in twins which have been considered with regard to structure. On the basis of genesis, twins may be divided into the categories of growth twins, transformation twins, and gliding twins. Growth twinning is caused by an accidental departure from equilibrium during growth, and is favored in the nuclear stage, especially under conditions of supersaturation. Transformation twinning is a consequence of high-low inversion. Glide twinning is caused by a specific type of structural shear in plastic deformation. Parting is, in part, a consequence of the high energy in the twin boundary. The act of twinning has analogies with the act of transforming from one polymorph to another: growth twinning corresponds with reconstructive transformation; glide twinning corresponds with displacive transformation. Transformation twinning is a special case of unmixing. The oft-repeated saying that twinning represents the result of an effort on the part of the crystal to assume a greater symmetry is the exact opposite of the true state of affairs.