TWIINING 2 (Types of Twinning)

Types of twinning

Simple twinning can result to contact twins or penetration twins.

  i.            Contact twins share the same composition surface and often appear as mirror images across the boundary.  Plagioclase, quartz, gypsum, calcite and spinel often exhibit contact twinning.

(a) 

(b)

Figure 40: Types of contact twins: (a) Maneback twin in orthoclase,  (b) Contact twin in calcite

ii  Penetration twins, the individual crystals appear as though they were passing through each other in a symmetrical manner.  Orthoclase, staurolite, pyrite and fluorite often display such a twin.

(a)

(b)

 Figure 41: Types of penetration twins: (a) orthoclase,  (b) staurolite

If several parts of a twin crystal are aligned by the same twin law, they are referred to as multiple or repeated twins. Multiple twins can either be polysynthetic or cyclic.

(a) Polysynthetic twinningarises if the multiple twins are aligned in a parallel manner.  Albite, calcite and pyrite often show this type of twinning.

(a)

(b)

Figure 42: Types of polysynthetic twin, for example; (a) pyrite, (b) Calcite

  (b)    Cyclic twinning: Here, the multiple twins are not parallel but have polygonal shapes.  Rutile, aragonite and chrysoberyl often show such twinning.

(a)

(b)

Figure 43: Types of cyclic twin, for example; (a) aragonite, (b) rutile

Other types of twinning:

 

 

 Figure 44: other types of twinning: Brazil, Dauphine, Japanese twins in quartz and Elbow twin in rutile (respectively)

 (For Sources, view the post titled Bibliography)