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Twins are a pair of crystals that are intimately connected and united, and twinning is the process by which twins form.
Terms commonly used in the explanation of twin Laws are: Twin plane, Twin axis, and composition 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.
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:
In this type the component parts of a twin crystal are held together along a well defined composition plane.
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 are those twin crystals that have very clearly separated halves that are held together by relationships that are intuitively clear.
Common twin laws
Here is a brief summary of the twin laws that are most frequently found in various crystallographic systems.
Given that it is found in minerals belonging to the spinal group, it has this name. The twin place, as it is in most cases, is the octahedral face in this 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.
In this law the prism of IInd order is a twin plane. Quartz shows development of twins according to this law.
In this law c-axis is the twinning axis. Twins are generally intergrown. Some quartz twin also based on this law.
Contact twins result on this law in which pyramid is a twinning plane.
In this system, crystals show twinning in a variety of ways of which following are more common.
a) when the twinning is repeated and the prism angle is around 60.
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.
The following laws are the most common and in no case exclusive.
The c-axis serves as the twin axis. The twinning of the minerals frequently takes the form of contact or interpenetration.
The mineral shows twining with clino dome as the twinning plane.
Man Bach law:
Here, the basal pinacoid is a twinning plane.
The twinning plane is in this plane, running parallel to the b-pinacoid.
Per cline law
The axis that is parallel to the b-axis is commonly referred to as the twin axis. The twins might be polysynthetic repeats.