Twinning and types of twin and common twin laws

A group of two crystals mutually united and intimately related are called TWINS and the phenomenon of their formation is called twinning.

Terminology

Terms commonly used in the explanation of twin Laws are: Twin plane, Twin axis, and composition plane.

Twin plane:

It is such a plane in a twin crystal, which is common to both the halves of the crystal and across which one half may appear to be the reflection of the other.

Twin axis:

It is a crystallographic direction along which a rotation of some degrees seems to have produced the resultant twins. The twin axis is other than the axis of two fold, four fold, and symmetry.

Types of twins:

The following types are:

Contact Twins:

In this type the component parts of a twin crystal are held together along a well defined composition plane.

Penetration twins:

In these twins the contact plane is not well defined. In fact the two parts of twin crystal may appear to be inter penetrating to each other.

Simple Twins:

When a twin crystals has very well defined two halves held together according to easily understanding relationships these may be said as simple twins.

Common twin laws

Following is a brief outline of the most commonly observed twin laws in different crystallographic system.

Isometric system

Spinal law:

It is so named because of its presence in minerals of spinal group. In this law, octahedral face is the twin palce, which is also in most cases.

Tetragonal system

Rutile law

The face of pyramid of IInd order is the twinning plane. This is the most common law for the crystals of the tetragonal system.

Hexagonal system

Brazilian Law:

In this law the prism of IInd order is a twin plane. Quartz shows development of twins according to this law.

Dauphine law:

In this law c-axis is the twinning axis. Twins are generally intergrown. Some quartz twin also based on this law.

Japanese law:

Contact twins result on this law in which pyramid is a twinning plane.

Orthorhombic system

In this system, crystals show twinning in a variety of ways of which following are more common.

a)         When  the  prism  angle  is  about  600  and  the  twinning  is

repeated. b)  When the prism angle is 700

Staurolite Twinning. This mineral shows cruciform twins of two types.

Right angled cross: These result when the face is a twinning plane

Sea horse twin in which the face is a twin plane.

In both cases the twins are of penetration type.

Monoclinic system

The following laws are the most common and in no case exclusive.

Carlsbad law:

The c-axis is the twinning axis. The minerals commonly show interpenetration or contact type of twinning.

Baveno law:

The mineral shows twining with clino dome as the twinning plane.

Man Bach law:

Here, the basal pinacoid is a twinning plane.

Triclinic system

Albite law

In this plane parallel to b-pinacoid is the twinning plane.

Per cline law

The twinning axis is easily defined as the one parallel to b-axis. The twins may be repeated polysynthetic type.