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# Twin Crystals

List of Contents

### Twinning:

The symmetrical intergrowth of two or more individual crystals are known as the ”twin crystals”. In a twinned crystal the lattice directions of one unit exhibit a definite crystallographic relation to the lattice directions of the other.

### Twin Plane: This plane exists between two twinned crystals. The twin plane divides the two twinned crystals into two halves, which are mirror images of each other. The twin plane is always perpendicular to the twin axis.

### Twin Axis:

The components of twin crystals are geometrically related to each other in such a way if one part is derived from the other by an angular rotation of 180 degrees about some crystal line common to both. The twin axis, therefore, may be defined as the axis about which a 180-degree rotation of one half of the twinned crystal would convert it into a single crystal.

### Composition Plane:

Two halves of a twin meet at a common plane, which is known as the ”composition plane”. The composition plane commonly but not always coincides with the twin plane. In the twin crystal of rutile, the twin plane and the composition plane is (011). If the composition plane is irregular, it is called the ”composition surface”.

### Twin Law:

The relation between the parts of the twin crystals is described by the ”twin law”. It is the ”twin law” that states, whether there is an axis or a twin plane, describes the crystallographic orientation for this axis or plane.

### Reentrant Angles: The reentrant angles exist in twinned crystals, which are a ”v-shaped” depression found at the junctions between crystal pairs. Twinning is commonly detected by the presence of these angles.

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