GeoClassroom Physical Geology Historical Geology Structure Lab Mineralogy Petrology


Twin: Two or more intergrown crystals of the same mineral, which are related by an element of symmetry that was not present in the untwinned crystal

3 Types

  1. Penetration: one crystal growing into another
    • related by a rotation axis
    • Some minerals that commonly display penetration twinning: fluorite, staurolite, orthoclase
  2. Contact: mirror image of crystal across composition plane
    • related by a mirror plane
    • Some minerals that commonly display contact twinning: gypsum, quartz,
  3. Polysynthetic: repeated or multiple contact twins on a microscopic scale with successive composition surfaces parallel
    • related by mirror planes
    • Some minerals that commonly display polysynthetic twinning: plagioclase feldspars
  4. Cyclic - repeated or multiple contact twins for which successive composition planes are not parallel
    • related by rotation axis
    • Some minerals that commonly display cyclc twinning: ruitle, aragonite

Some terms and definitions:
  • "Twin plane" or "Twin axis"- symmetry elements that relate twins
  • Composition plane - spot where actual structural change occurs

Causes of twinning:
  1. Growth twinning: error in stacking of unit cells when crystal first starts to grow
  2. Transformation twin: variations in temperature and pressure stress lattice - lattice relieves stress by "warping" -
          warping causes twin to form
    • This is essentially a polymorph changeover
        => 2 different minerals (e.g., calcite structure warps to aragonite structure)
  3. Deformation twin: some kind of pressure stress on crystal structure - crystal relieves stress by twinning
    • this does not involve polymorphs - all one mineral

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