Mineral Twins

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Mineral Twins
Contact Twin
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Twin Definitions

• Twin Rational intergrowth of two or more single
  crystals of the same mineral
• Twin Law Describes a twin relationship
  including axis, center or plane and type of twin
• Twinning Formation of a twin

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Twin Definitions con.

• Twin Axis Plane of rotation such that one
  crystal may be rotated 180 to become concordant
  with another

• Twin Plane Plane around which components of a
  twin (crystals) are arranged symmetrically or
  reflected

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Two General Twin Types

• Contact
• Two parts are joined along a common plane

• Penetration
• Two or more parts of the crystals appear to
  penetrate into each other

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Common Twin Types

• Carlsbad
• Japan
• Swallowtail
• Baveno
• Manebach
• Polysynthetic
• Cyclic

Marcasite
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Carlsbad Twin

• Produced by 2-fold rotation on the c-axis

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Japan (Law) Twin

• Crystals form at right angles to each other

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Swallowtail Twin

• Two monoclinic minerals form a V-shape

Selenite
9
Baveno Twin

• Produced by reflection on 021

orthoclase
10
Manebach Twin

• Produced by reflection on 001

microcline
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Polysynthetic Twin

• Repeated twinning of three or more individuals
  according to the same twin law
• Common in plagioclase

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Cyclical Twin

• Composition surfaces are not parallel
• Rutile (rutile star), chrysoberyl, cassiterite

Rutile
Chrysoberyl
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Thin Section Twinning

• Tartan Twinning (Pericline Law)
• Orthoclase, microcline

• Albite Twinning (Polysynthetic)
• Plagioclase