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Twinning

GLY 4200

Twinning, 2012

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Twinning Illustration

• The twin plane cannot be a part of the normal symmetry of a crystal

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Twinning Operations

• Reflection across a mirror plane. The added mirror plane would then be called a twin plane

• Rotation about an axis or line in the crystal. The added rotation axis would then be called a twin axis

• Inversion through a point. The added center of symmetry would then be called a twin center

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Twin Law

• Must define two things: The type of twin operation The orientation of the twin element

associated with the operation

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Types of Twinning

• Twins may also be classified on the basis of their physical properties

• There are two basic types of twin Contact twins Penetration twins

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Contact Twinning

• Have a planar composition surface separating two individual crystals

• These are usually defined by a twin law that expresses a twin plane

• Orthoclase twinned on the Braveno Law

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Polysynthetic Twinning• A type of multiple contact

twinning is called polysynthetic

• The compositions surfaces are parallel to one another, they are called polysynthetic

• Plagioclase commonly shows this type of twinning, called the Albite Twin Law, with {010} as the twin plane

• Such twinning is one of the most diagnostic features of plagioclase

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Cyclic Twinning

• Another type of contact twinning

• If the composition surfaces are not parallel to one another, they are called cyclical twins

• Shown here is the cyclical twin that occurs in chrysoberyl

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Penetration Twins• Have an irregular composition surface

separating 2 individual crystals• Are defined by a twin center or twin axis• Twinned crystal of orthoclase twinned on

the Carlsbad Law with [001] as the twin axis

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Origin of Twinning

• Twinning can originate in 3 different ways Growth twins Transformation twins Glide or deformation twins

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Growth Twins• When accidents occur during crystal growth and a

new crystal is added to the face of an already existing crystal, twinning can occur if the new crystal shares lattice points on the face of the existing crystal, but has an orientation different from the original crystal

• Such growth twins can be contact twins or penetration twins

• Both Carlsbad and Albite twins are growth twins

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Transformation Twins• Occurs when a preexisting crystal undergoes a

transformation due to a change in pressure or temperature

• This commonly occurs in minerals that have different crystal structures and different symmetry at different temperatures or pressures

• When the temperature or pressure is changed to that where a new crystal structure and symmetry is stable, different parts of the crystal become arranged in different symmetrical orientations, and thus form an intergrowth of one or more crystals

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Brazil Twin

• Dauphiné and Brazil twinning in quartz commonly forms this way during a decrease in temperature   

• Photomicrograph shows a Brazil twin in amythyst in crossed polars

• Click to play video

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Tartan Twinning

• Combination of albite twinning and pericline twinning in alkali feldspar results when high temperature sanidine (monoclinic)  transforms to low temperature microcline (triclinic) is known as "tartan“, “gridiron, or “cross-hatch” twinning pattern

• One of the most characteristic diagnostic properties for the identification of microcline

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Deformation Twins

• During deformation atoms can be pushed out of place

• If this happens to produce a symmetrical arrangement, it produces deformation twins

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Examples of Deformation Twinning

• Plagioclase and calcite are two common minerals exhibiting this behavior

• Deformational albite twins characteristically "pinch out" rather than running throughout the grain

• They may also be bent• Calcite deformation enhances ductility in

some rocks, such as marble

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Deformation Twinning in Calcite

• The mineral calcite can be easily twinned during deformation, producing polysynthetic twins on {01bar12}

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Manebach Twinning

• A type of contact twinning seen in orthoclase on the {001} plane

• Diagnostic of orthoclase when it occurs

• Monoclinic system 

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Gypsum • Swallow tail twins {100}

are commonly observed in the mineral gypsum CaSO4●2H2O)

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Orthorhombic Cyclic Twinning

• Cyclical Twins - The minerals aragonite (CaCO3) , chrysoberyl (BeAl2O4), and cerrusite (PbCO3) commonly develop twinning on {110}, which results in a cyclical twin that gives these minerals a pseudo-hexagonal appearance

• Diagram illustrates {110} in aragonite

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Twinning in Staurolite

• Two types of interpenetration twins occur in staurolite

• {031} twins from a right-angled cross

• {231} twins form a cross at about 60°

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“Fairy Cross” Twins

• The staurolite crystal shown has twinning on {031}

• The small crystals on the surface are garnets

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Staurolite Twin

• The common 60° twin on {231}

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Tetragonal System

• Twinning usually occurs on {011} forming cyclical contact twins

• Rutile (TiO2 - left) and cassiterite (SnO2 – below) often show this type of twinning

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Calcite Twinning• Most common twin laws

that are observed in calcite crystals are {0001} and the rhombohedron {01bar12}

• Both are contact twins, but the {01bar12} twins can also occur as polysynthetic twins that result from deformation

• Photo shows {01bar12} twin

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Quartz Twinning

• Brazil Law - {11bar20} - is a penetration twin that results from transformation

• Dauphiné Law - [0001] - is also a penetration twin that results from transformation

• Japanese Law - {11bar22} - is a contact twin that results from accidents during growth

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Quartz Twin Pictures

• Japanese Law

• Dauphiné twin

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Isometric System

• Spinel law – Twin reflection on (bar1 bar1 1) plane

• Twin rotation on [111] and [001]

• On [001], known as the Iron Cross, the twin axis gives the mineral apparent 4-fold symmetry about 3 perpendicular axes

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Spinel Law Photo

• Twin plane clearly visible

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Iron Cross Twin

• Iron Cross in Pyrite