part 4: crystal forms twinning lecture 11 crystallography
TRANSCRIPT
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Part 4:
Crystal Forms
Twinning
Lecture 11 Crystallography
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Crystal FormsHabit: the general external shape of a crystal (e.g., prismatic, cubic, bladed...)
Form: refers to a group of faces which have the same relation to the elements of symmetry.
Crystals with higher degrees of symmetry tend to generate more form faces.
openform
closed form
Tetragonal Dipyramid
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Form Indexes defined by the Miller
index (hkl) of the face in the positive quadrant
enclosed in curly braces {hkl}
a{110}
Tetragonal 4/m 2/m 2/m
a2
c
Miller indices, in braces, may also be used as a shorthand representation of forms. For example, if the face (110) is present on a tetragonal crystal, three faces that are symmetrically equivalent to (110) by the 4-fold axis should also be present. These are (1,bar-1,0), (bar-1,1,0), and (bar-1,bar-1,0). The set of crystal faces in a crystal class that are symmetrically equivalent are called a form for that class and are designated by braces around the Miller indices of one of the faces. For example, the four faces mentioned above may be represented by the one symbol {110}.
This form is called a tetragonal prism
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Pyramids and DipyramidsPyramids and Dipyramids
PrismsPrisms
Low Symmetry FormsLow Symmetry Forms
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Trapezohedron, Scalenohedron, Rhombehedron, Disphenoid
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Isometric Forms
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Twinning
• If a crystal is subjected to stress during growth
• or temperature/pressure conditions different from those under which it originally formed
• two or more intergrown crystals are formed in a symmetrical fashion.
• These symmetrical intergrowths of crystals are called twinned crystals.
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Twinning Mechanism
•What happens is that lattice points in one crystal are shared as lattice points in another crystal adding apparent symmetry to the crystal pairs.
•Twinning, because it adds symmetry, never occurs in relation to the existing symmetry of the crystal.
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Twinning defined by symmetry added
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.
Carlsbad Twin in Orthoclase
Contact Twin in Orthoclase with Braveno Law {021} as twin plane
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Twin Laws
Twin laws are expressed as either form symbols to define twin planes (i.e. {hkl}) or zone symbols to define the direction of twin axes (i.e. [hkl]).
The surface along which the lattice points are shared in twinned crystals is called a composition surface.
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Twin TypesPotential Twin Plane (111)
MirrorReflection
Composition Plane
Contact TwinsContact TwinsComposition planescorrespond to twin planes (mirrors)
Penetration TwinsPenetration TwinsComposition planes are irregular; formed by rotational twin axis operation
TwinAxes
Have a planar composition surface separating 2 individual crystals.
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Multiple Twins Formed from 3 or more twinned crystals
Polysynthetic Polysynthetic twinstwins where composition planes are parallel
Cyclic twinsCyclic twins where composition planes are not parallel
Contact twins can also occur as
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Albite LawPlagioclase is a solid solution of Anorthite CaAl2Si2O8 to Albite NaAlSi3O8. It very commonly shows Albite polysynthetic twinning. The Albite twin law {010} indicates that the twins make a form, the faces are parallel to the mirror plane (010), i.e. perpendicular to the b-axis.
Albite twinning is so common in plagioclase, that it's presence is a diagnostic property for identification of plagioclase when seen with crossed polarizers.
XPL (crossed polarizers)
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Pericline Law - The Pericline law has faces in the zone
around [010], so b is the twin axis. The result of monoclinic Orthoclase or
Sanidine transforming to Microcline (all KAlSi3O8).
Pericline twinning usually occurs with Albite twinning in Microcline; then a cross-hatched pattern, called tartan twinning, distinguishes Microcline from the other feldspars under the microscope.
Microcline XPL
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Microcline:Albite and Pericline Laws
The Albite twin law {010} indicates that the twins are the form symmetrical about the mirror plane (010), i.e. perpendicular to the b-axis.
The Pericline law has faces in the zone [010], parallel to the b axis.
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Twins in the Triclinic System
Albite Law{010} form twin plane
Albite twins combined with Pericline twinningparallel [010] zone axisresults in “tartan twinning” in microcline (K-feldspar)
Form: refers to a group of faces which have the same relation to the elements of symmetry.
A zone is a set of planes that share one common direction, the zone axis.
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Twins in the Monoclinic System
Note that twin planes do not align with mirror planes
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Twins in the Orthorhombic Systemcontact & cyclic twinning
Penetrative
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Twins in the Tetragonal System
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Twins in the Hexagonal System
{0112} is most common and may produce single contact or polysynthetic twins
exception: twin axis || C
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Twins in the Isometric System
Penetrative Penetrative twins with twins with twin axes twin axes parallel to parallel to rotational rotational axesaxes
Spinel MgAl2O4
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Twinning
Aragonite twin
Note atoms in composition plane common to each twin
Redrawn from Fig 2-69 of Berry, Mason and Dietrich, Mineralogy, Freeman & Co.
Although aragonite is orthorhombic, the twin looks hexagonal due to the 120o O-C-O angle in the CO3 group
Composition Plane
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Twinning• Twin Operation is the symmetry operation which relates the
two (or more) parts (twin mirror, rot. axis)
1) Reflection (twin plane)Example: gypsum “fish-tail”
2) Rotation (usually 180o) about an axis common to both (twin axis): normal and parallel twins.
Example: carlsbad twin3) Inversion (twin center)
• The twin element cannot be a symmetry element of the individuals. Twin plane can't be a mirror plane of the crystal
• Twin Law is a more exact description for a given type (including operation, plane/axis, mineral…)
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Contact & Penetration twins Both are simple twins only two parts
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Multiple twins (> 2 segments repeated by same law) Cyclic twins - successive planes not parallel
if parallel Polysynthetic twins
Albite Law
in plagioclase
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TwinningMechanisms:1) Growth
Feldspars: Plagioclase: Triclinic Albite-law-striations
bb
a-ca-c
bb
a-ca-c
If several crystals are aligned by the same twin law they are referred to as multiple twins. If these multiple twins are aligned in parallel they are called polysynthetic twins
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TwinningMechanisms:1) Growth
Feldspars: Plagioclase: Triclinic Albite-law-striations
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TwinningMechanisms:2) Transformation (secondary twins)
Feldspars: Hi-T K-spar (monoclinic) Lo-T K-spar microcline (triclinic)
MonoclinicMonoclinic(high-T)(high-T)
bb
a-ca-c TriclinicTriclinic(low-T)(low-T)
bb
a-ca-c
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TwinningMechanisms:2) Transformation (secondary) twins
K-Feldspars: high temperature Sanidine (monoclinic) transforms to low temperature microcline (triclinic).
“tartan twins” combine
Albite and Pericline twins
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TwinningMechanisms:3) Deformation (secondary)
Results from shear stress
greater stress gliding, and finally rupture Also in feldspars. Looks like transformation, but the difference in interpretation is tremendous
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Mechanisms:3) Deformation (secondary)
Results from shear stress. Plagioclase
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Mechanisms:3) Deformation (secondary) Results from shear stress. Calcite