Twinning

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Revision as of 08:21, 7 December 2006 by Doos (talk | contribs) (Transformation)
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Basic

Minerals do not always grow under ideal conditions to form perfect crystals, instead during growth (or after) some crystals may form twins.

There are basically two types of twins:

  • Contact twins
  • Penetration twins

And there are typically 3 causes of twinning:

  • During crystal growth
  • Transformation
  • Deformation

Types of twins

Contact twins

Contact twins share a single plane (or face).

Geneculate (knee-type) twinning in zircon


Penetration twins

Penetration twins share a single axes (usually a rotation axis).

Fluorite penetration twin (two cubes intergrown)
Staurolite penetration twin (two hexagonal prisms intergrown)


Causes of twinning

During growth

During the growth of a crystal there may be changes inside the magma (like temperature, pressure and flow) which causes elements of the magma not to keep building a crystal in the same orientation as it did.
Most twinning occurs under these circumstances.

Transformation

When a fully grown crystal experiences sudden change in tempearture and pressure the internal arrangement of atoms can shift. A good example of this is quartz.
At temperatures above 573°C quartz is hexagonal and named beta-quartz. This beta-quartz will transform to alpha-quartz (which is trigonal) below that temperature and twinning will occur during the transformation.

Deformation

When a fully grown crystal undergoes mechanical stress (such as pressure) the crystal lattice can be distorted. One can imagine this by laying the shell of a matchbox on a table and apply slight pressure to it. The matchbox will shear and its form is thus distorted. When two matchbox shells are placed on top of eachother, the pressure will cause them to shear into an arrow head.
An example of this is calcite.

Deformation twin (contact)