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Geosciences LibreTexts

5.6: Twinning

  • Page ID
    3198
  • Basic

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

    There are basically two types of twins:

    • contact twins
    • penetration twins

    There are typically 3 causes of twinning:

    • during crystal growth
    • from transformation
    • by deformation

    Types of twins

    Contact twins

    Contact twins share a single plane (or face).

    Figure \(\PageIndex{1}\): Geneculate (knee-type) twinning in zircon

    File:Zircon twin.jpg

    Penetration twins

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

     

    Figure \(\PageIndex{2}\): Fluorite penetration twin (two cubes intergrown)

    File:Fluorite twin.jpg

    Figure \(\PageIndex{3}\): Staurolite penetration twin (two hexagonal prisms intergrown)

    File:Staurolite twin.jpg

    Causes of twinning

    During growth

    During the growth of a crystal, there may be changes inside the magma (like temperature, pressure, and flow) which cause elements of the magma to alter a crystal's orientation. 
    Most twinning occurs under these circumstances.

    Transformation

    When a fully grown crystal experiences sudden change in temperature 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 is termed beta-quartz. Below that temperature, beta-quartz will transform into trigonal alpha-quartz, and twinning will occur during this transformation.

    Deformation

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

     

    Figure \(\PageIndex{4}\): Deformation twin (contact)

    File:Deformation twin.jpg