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

• "Twin plane" or "Twin axis"- symmetry elements that relate twins
  
• Composition plane - spot where actual structural change occurs

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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