In the natural world, crystals often seem to overlap, as their mutual orientation is unorganized. Such forms are ramdom and should not be confused with “twinned crystals”.
A twinned crystal is the result of the combination of two or more individual crystals which have the same properties and which follow well-defined geometrical rules. The reason for the association lies in the internal structure of the crystal. A twinned crystal can then be single (two associated crystals), or multiple, if more than two crystals are involved.
Twinned crystals can be divided in two types :
Twinned crystals formed by welding, i.e. they are joined on either side of a twin plane. Example : cassiterite (twinned cassiterite), gypsum (swallow-tail gypsum).
Twinned crystals formed by penetration which are characterized by crystalline interpenetration. Example : staurotide (Bretton cross, Greek cross or Saint André cross), feldspar (Carlsbad twin).
From a genetic point of view, twins can be classified into two categories :
Long run twins which develop at the level of the crystal nucleus and which grow at different rates.
Mechanic twins which are formed by stress on the crystal after its formation.