4. (Crystallog.) A compound crystal composed of two or more crystals,
or parts of crystals, in reversed position with reference to each
other.
The relative position of the parts of a twin may be explained by
supposing one part to be revolved 180° about a certain axis (called the
twinning axis), this axis being normal to a plane (called the twinning
plane) which is usually one of the fundamental planes of the crystal.
This revolution brings the two parts into parallel position, or vice
versa. A contact twin is one in which the parts are united by a plane
surface, called the composition face, which is usually the same as the
twinning plane. A penetration twin is one in which the parts
interpenetrate each other, often very irregularly. Twins are also
called, according to form, cruciform, geniculated, etc.
Twin (?), v. i. [imp. & p. p. Twinned (?); p. pr. & vb. n. Twinning.]
1. To bring forth twins. Tusser.