2. Twinning
A twin is a symmetrical intergrowth of two or more
crystals of the same substance
Twinning can be considered a type of planar defect
The surface on which two individuals are united is
known as the composition surface. If this surface is
a plane, it is called the composition plane and can
be irregular or regular
3. Twinning
The operations that relate a crystal to its twinned
counterpart are symmetry operations:
‣ twin plane - reflection by a mirror plane
‣ twin axis - rotation about a crystal direction common
to both, with the angular rotation normally 180°
‣ twin center - inversion about a point
5. Contact & Penetration
Contact twins have a regular composition surface
separating the two individuals; the twin is defined by
a twin plane
Penetration twins are made up of interpenetrating
individuals having an irregular composition surface,
and the twin law is usually defined by a twin axis
direction
6. Polysynthetic & Cyclic
Multiple twins that have three or more individual
crystals twinned according to the same law and if all
the composition surfaces are parallel, the resulting
group is a polysynthetic twin
If successive composition planes are not parallel, a
cyclic twin results
11. Causes of Twinning
Growth twins - error in stacking of unit cells when
crystal first start to grow
Transformation twin - variations in temperature and
pressure stresses the lattice - lattice relieves the
stress by “warping”
Deformation twin - some kind of stress is put on the
crystal structure - crystal relieves stress by twinning
12. Structural Defects
As a crystal grows, structural defects are
incorporated into the mineral
Such imperfections affect growth rate, crystal
morphology, and basic properties of crystalline
materials, such as strength, conductivity,
mechanical deformation, and color
