Crystal Twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner.The result is an intergrowth of two separate crystals in a variety of specific configurations.

1. DR. HARISINGH G0UR VISHWAVIDYALAYA
SAGAR, M.P.
(A CENTRAL UNIVERSITY)
DEPARTMENT OF APPLIED GEOLOGY
UNDER GUIDANCE OF:
PROF. R.K. TRIVEDI
PRESENTED BY:
PRADEEP KUMAR
M.Tech IST SEMESTER
Reg. No-Y18251017

2. Contents
 Introduction
 Elements of twinning
 Development of twinning
 kinds of twinning
 Common types of twins in different crystal
systems
 Why we study twinning
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3. INTRODUCTION
Well formed single crystal are seldom in nature, where as
aggregates of crystal are common .
TWINNING:
During the growth of crystal if it is subjected to
stress or temperature /pressure , crystallization
two or more crystal of same substance shows
symmetrical intergrowth and the individuals have
different orientation related by a symmetry operation,
the crystal are said to
be twinned crystal .
ex- chrsoberely
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4. ELEMENTS OF TWINNING
 TWIN PLANE: Reflection across a lattice plane .
 TWIN AXIS: An axis about which Rotation is imagined
to have occurred to produce the twinned crystal .
 COMPOSITION PLANE : Surface along which the lattice
point are shared in twinned crystal .
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5. DEVELOPMENT OF TWINS
The development of twins is governed by certain twins laws
:-
1. An axis of even fold symmetry can never become a twin
axis ,only 3 -fold symmetry axis can serve as a twin axis .
2. Twin axis is always normal to the twin plane but need
not be normal to composition plane .
3. Twinning plane never coincide with symmetry planes
4. Twins that develop in classes with a center of symmetry
have both twin axis and twinning plane perpendicular to
it .
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6. KINDS OF TWINNING
In a twinned crystal the two parts may
appears as :-
 Contact Twins
 Penetration twins
 Simple twins
 Multiple twins
GENETIC KINDS OF TWINS:
 Growth twins
 Transformation twins
 Gliding twins
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7. In a twinned crystal the two parts
may appears as
1. United along a regular composition plane merely in contact relationship
with other are called contact twinning
 An example shown here is a crystal of orthoclase twinning.
2. Two halves of a crystals of twin interpenetrate more or less
they are called as penetration twins.
 The composition plane of it is slightly irregular surface.
 orthoclase crystal twinned with [001] as twin axis .
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8.  If the twins are composed of three or more
individual crystals they ae called multiple twins
Types of multiple twins :-
1. The composition surfaces are parallel to one another, they are
called polysynthetic twinning.
 Plagioclase commonly shows this type of twinning, such
twinning is one of the most diagnostic features of plagioclase.
2. The composition plane is not parallel and tends to turn in circle
are called cyclic twin .
 T he cyclic twin is beautifully shown by aragonite
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9. Genetic types of twinning
GROWTH TWINS
 During the growth of a crystal there may be changes
inside the magma (like temperature , pressure and flow)
which cause elements of magma to alter a crystal’s
orientation.
 penetration twin in orthoclase and albite are growth
twins.
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10. Deformation/GLIDE TWINS
In the Gliding twins certain atoms shift from their lowest
energy state to the twinned position of the moderately low
energy.
 A FCC metal like aluminium experiences extreme stress it
will experience twinning as seen in the case of explosions.
 Calcite shows gliding twins.
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11. Transformation Twins
This is occurs when a high temperature crystal is transformed into low
temperature polymorph on change of condition
 The right handed and left handed quartz are the best example.
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12.  Common tyes of twins in different crystal system :
1.Isometric system
 Spinel law – Twin reflection on (1 1 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

13.  Twinning usually occurs on {011}
forming contact twins
 Rutile (TiO2 - left) and casseterite
(SnO2 – below) often show this
type of twinning
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2. Tetragonal system:
* knee shaped or geniculated twins

14. 3. HEXAGONAL SYSTEM
* Calcite Twinning
 Most common twin laws that are observed in
calcite crystals are {0001} and the
rhombohedron[011 2]
 Both are contact twins, but the {01bar12} twins
can also occur as polysynthetic twins that
result from deformation.
 Image shows[0112] twin.
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15. *Twinning in quartz*
 Brazil Law – [11 𝟐0]is a penetration twin that results from
transformation.
 Dauphiné Law - [0001] - is also a penetration twin that
results from transformation.
 Japanese Law - [112𝟐]is a contact twin that results from
accidents during growth.
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16. 4. Orthorhombic System
 Cyclic Twins - The minerals aragonite
(CaCO3) , chrysoberyl (BeAl2O4), and
cerrusite (PbCO3) commonly develop
twinning on {110} plane.
Figs show :- Sturolite twins
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Aragonite

17. 5. MONOCLINIC SYSTEM
 Manebach LAW-A type of contact
twinning seen in orthoclase on
the {001} plane.
Diagnostic of orthoclase when it
occurs.
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swallow tail twins
orthoclase

18.  Carlsbad law - [010] - forms
a penetration twin in the
mineral orthoclase. Crystals
twinned under the carlsbad
law show two inter-grown
crystals, one rotated 180
degree from the other about
the [010] axis.
 Baveno law - {021} - forms a
contact twin in the mineral
orthoclase.
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19. 6. TRICLINIC SYSTEM
 Albite Law - As described previously, plagioclase
(NaAlSi3O8 – CaAl2Si2O8) very commonly shows albite
polysynthetic twinning. The twin law - {010} indicates
that the twinning occurs perpendicular to the C
crystallographic axis.
 Pericline Law - The pericline law has [010] as the twin axis
. Pericline twinning occurs as the result of monoclinic
orthoclase or sanidine transforming to microcline.
Pericline twinning usually occurs in the combination with
Albite twinning.
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20.  WHY WE STUDY TWINNING ?
 Twinning is a distuinguishing feature in thin section of many
minerals like in plagioclase (polysynthetic twinning),
microcline(cross hatched twinning) , orthoclase (simple
twinning) or in many other minerals.
 Twinning or twinning tendencies may either promote or
preclude the use of mineral in industry, like twinning in quartz
preclude the use of quartz either lens or oscillator.
 Twinning is one of the causes of parting which is closely
resembles cleavage .
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