Mineralogy
Twinning


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

  2. Contact: mirror image of crystal across composition plane

  3. Polysynthetic: repeated or multiple contact twins on a microscopic scale with successive composition surfaces parallel

  4. Cyclic - repeated or multiple contact twins for which successive composition planes are not parallel


Some terms and definitions:
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" -

  3. Deformation twin: some kind of pressure stress on crystal structure - crystal relieves stress by twinning


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Mineralogy
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