TWINNING

• TWINNING : Two or more crystals of same or relative mineral in which individual parts are reversed to each other

• TWIN PLANE: Imaginary plane which divide the twin crystal into two equal halves

• TWIN AXIS : Axis along which rotation takes place to bring twin into untwinned state

TYPES OF TWINNING

• Contact : Consist of two halves united by compositional plane

  eg: Orthoclase

• Penetration : Twin appears as individuals were crossing each other

  eg: Staurolite, Pyrite, Fluorite, etc

• Repeated : Consist of three or more individual crystals twinned according to same law

• Polysynthetic : Twin planes are parallel to each other

  eg: Plagioclase, calcite

• Symmetric or Cyclic : Twin planes are not parallel to each other (mimectic twin)

  eg: Aragonite, Rutile, Gypsum, etc

IMPORTANT MINERALS AND THEIR TWIN

• ORTHOCLASE

  • Braveno : (021)

  • Manebach : (001)

  • Carlsbad : (001)

• QUARTZ

  • Brazil : (11 -2 0)

  • Japan : (11 -2 2)

  • Douphine : (0001)

• GYPSUM

  • Swallow tail : (100)

  • Butterfly

• STAUROLITE

  • Skew twin

  • Cruciform

  • Malatese cross

• RUTILE, CASSITERITE : Geniculate

• PYRITE : Iron cross

• PLAGIOCLASE : Polysynthetic

• ARAGONITE : Cyclic

• MICROCLINE : Cross-hatched, Tartan , Girdiron