CLASSIFICATION OF TWINS

I. BISECTING TWIN AXIS AND PRINCIPAL TWIN AXIS

Abstract

The authors proposed new elements of symmetry, the bisecting twin axis and the principal twin axis, to describe systematically the morphology of twin crystals. The bisecting twin axis is an axis that occurs within the twin plane and bisects a reentrant corner formed by the two individual crystals. The principal twin axis is an axis of symmetry perpendicular to the bisecting twin axis and to the twin plane; it comprises 6, 4, 3, ^-6, ^-4 and ^-3.
In an ideal form, with which two individual crystals involved in twinning have developed to an equal size, several bisecting twin axes exist on both sides of the reentrant corners which bound the two individuals; the arrangement of these axes corresponds to the symmetry of the principal twin axis perpendicular to the twin plane.
When a twin crystal is described by means of the bisecting twin axis, the principal twin axis and the twin plane, what has been heretofore defined as a composition plane would mostly turn out to be a twin plane. These newly proposed elements of symmetry would become an important basis for grouping the twinned point groups.