There are three basic types of the twinning of crystals; (i) Contact Twins, (ii) Penetration Twins, and (iii) Repeated Twins.
(1). Contact Twins:
When two halves of a twinned crystal are symmetrical with respect to the twin plane, it is called ”contact twin”. Contact twins have a definite composition plane, which separates the two halves and the twin is defined by a twin plane. Crystals of rutile and calcite exhibit contact twins.
(2). Penetration Twins:
When the two individual crystals appear to have penetrated one another, it is called ”penetration twin”. Penetration twins have an irregular composition surface and the twin law is usually defined by a twin axis. The main feature of penetration twin is that it can not be divided into two separate halves. Crystals of Staurolite and Flourite commonly show penetration twin.
(3). Repeated Twins:
When a twinned crystal is composed of three or more parts which are related to one another by the same twinning law, such type of twins is known as ”repeated” or ”multiple twins”. The repeated twins are unclassified into two types; one is polysynthetic twins, and the other is cyclic twins.
- Polysynthetic Twins: A polysynthetic twin is a type of multiple/repeated twin, in which all the successive composition surfaces are parallel. It is common in plagioclase felspars.
- Cyclic Twins: A Cyclic twin is a kind of repeated twin in which all the successive composition surfaces/planes are not parallel.
- Introduction to Crystallography
- Unit Cells of the Crystal Lattice
- Crystallographic Axes
- Interfacial Angle of Crystals
- Parameters of Crystal faces
- Crystallographic Notation
- Forms of Crystal faces
- Symmetry Elements of Crystals
- Six Types of Crystal Systems
- Cubic/ Isometric Crystal System
- Tetragonal Crystal System
- Hexagonal Crystal System
- Orthorhombic Crystal System
- Monoclinic Crystal System
- Triclinic Crystal System
- Crystal Groups
- Twinned Crystals or Twinning