Mineralogy: Twins and twinning

What is a twin?

A symmetrical intergrowth of two or more crystals of the same mineral

Twin crystal

A crystal containing twinned crystals

How are twins related

Through a symmetry element, called a twin element

What are the three kinds of twin elements

Twin plane, twin axis, and twin center

Twin plane

Reflection

Twin axis

Rotation about a crystal direction common to both individuals

What is the typical degree of rotation for a twin axis?

180°

Twin center

inversion

Twin law

A name given to a twinning type that is very common

Examples of twin laws

Albite twin law, brazil twins

Composition surface

The contact between twins

Types of twins:

Contact twins and penetration twins

Contact twinning

When there is a definite composition surface

Simple twin

A contact twin where you have two individuals

Polysynthetic twin

Three or more individuals in a twin where composition surfaces are parallel

Cyclic twins

three or more twins where the composition surfaces are not parallel

Penetration twins

Where the twin plane is very complex, and cannot be easily separated

Dauphiné law

Penetration twins that are intensely intergrown

Twinning importance

Can be diagnostic

What causes twins

A growth defect, or deformation later on

How does twinning help you tell orthoclase and microcline apart?

Microcline exhibits tartan twinning

What mineral commonly shows albite twins?

Plagioclase

Types of twins in quartz

Brazil, dauphiné, and japan

What group of minerals do you commonly see carlsbad twinning in?

Plagioclase feldspars