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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
- Penetration: one crystal growing into another
- related by a rotation axis
- Some minerals that commonly display penetration twinning: fluorite, staurolite, orthoclase
- Contact: mirror image of crystal across composition plane
- related by a mirror plane
- Some minerals that commonly display contact twinning: gypsum, quartz,
- 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
- 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:
- Growth twinning: error in stacking of unit cells when crystal first starts to grow
- 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)
- 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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