Twinned crystal

A twinned crystal is the association of crystals of the same species according to precise geometric laws related to the symmetry elements of the space in question. It may be single or multiple (polysynthetic twinning of plagioclases); it may occur by joining along a face or by interpenetration of crystals (e.g. the Carlsbad twinning of orthoclase). So-called mechanical twinning has a different meaning; it is caused by deformations (see Mechanics).