A composite of two or more crystal individuals having a definite
crystallographic relationship to each other. The orientation of one individual
may be the mirror image of the other across a twin plane, or an orientation
that can be derived by rotating the twin portions about a twin axis, or some
other rational twin law. Twinned individuals in a twin crystal commonly show
reentrant angles between crystal faces or on cleavage planes. A twin crystal
may exhibit symmetry higher than that of its crystal individuals.
Dictionary of Mining, Mineral, and Related Terms