Twinning

1
Twinning

• Crystals that are related to one another by some
  geometric relation.

2
Simple or Contact twins

• e.g. the fish tail of gypsum

3
Penetrative

• e.g. Carlsbad in Orthoclase

4
Polysynthetic (multiple)

• e.g. Plagioclase

5
Cyclic

• e.g. aragonite

6
Parting

• Occurs when a mineral breaks along a twin plane