Department of Geology

1. Twinning of crystal
NAME:- Soumyaranjan Rout
CLASS :- M.Sc. 1st year
ROLL. NO. :- GEOL23-006
Dept. Of Geology, Sundargarh

2. CONTENTS
• Introduction of twinning.
• Components of twinning crystal.
• Types of twinning.
• Origin of twinning
• Common twin laws.
• References

3. INTRODUCTION
• Twinning crystal may be defined as the symmetrical intergrowth of
two or more individual crystals.
• Sometimes during the growth of a crystal, or if the crystal is subjected
to stress or temperature/pressure conditions different from those
under which it originally formed, two or more intergrown crystals are
formed in a symmetrical pattern are known as twinned crystals.
• In a twinned crystal lattice direction of one unit have a definite
crystallographic relation to the lattice direction of the other.
• Twinning never occurs in relation to the existing symmetry of the
crystals.

4. COMPONENTS OF TWINNED
CRYSTAL
TWIN PLANE:-
• The plane that divides the twin crystal into two halves which are mirror
image of each other, is called “Twin plane”.
• Twin plane is always perpendicular to the twin axis.
TWIN AXIS:-
• Rotation produces two or more crystal segments related to each other by
rotation on a crystallographic axis common to all segments called twin axis.
OR
• The crystal axis about which one individual of a twinned crystal may be
rotated(Usually 180 degrees)to coincide with the other individual.

5. COMPOSITION PLANE:-
• The plane on which two halves of a twin crystal meet is called
“composition plane”.
• Composition plane not always coincide with the twin plane.
• Rutile shows composition plane and twin plane on (011).
• If the composition plane is irregular, it is called the “composition surface”.
• TWIN LAW:-
• Twin law is a descriptive relation between the parts of a twin crystal.
• Twin laws are expressed as either form symbols to define the twin planes
or zone symbol to define the direction of twin axes.
• Twin law is never parallel to the existing planes of symmetry.

6. • REENTRANT ANGLES:-
• Twin crystals usually possess
reentrant angles.
• “Reentrant angles” are the v-shaped
depressions found at the junction
between the twin crystals.
• Twinning is commonly detected by
the presence of reentrant angles.

7. TYPES OF TWINNING
• There are commonly three types of twinning are found in the twin
crystals.
A. CONTACT TWINNING:-
• When two halves of a twinned crystal are symmetrical with respect to
the twin plane, is called “contact twin”.
• Contact twin have a definite composition plane which separates the
two halves and the twin is defined by a twin plane.
• EX:- Rutile, orthoclase and Calcite.
Orthoclase
twinned on
Braveno law
with (021)

8. B. PENETRATION TWIN:-
• When the two individual crystals penetrated to one
another, is called “penetration twin”.
• Penetration twins have an irregular composition surface
separating two individual crystals.
• Penetration twin can’t be divided into two separate
halves.
• In this figure twin crystal of orthoclase twinned on
Carlsbad law with (001)as the twin axis.
EX:- Staurolite and Fluorite.
Saturolite twin,
twin plane(032)
Orthoclase showing
Carlsbad twin

9. C. REPEATED TWIN :-
• When a twin crystal is composed of three or more parts which are
related to one another by the same twinning law, the twin is called
“repeated twinning “ or “multiple twinning”.
• Contact twin can also occur as repeated or multiple twins.
• Repeated twins are of two types:-
1. Polysynthetic twin
2.Cyclic twin

10. 1. POLYSYNTHETIC TWINS:-
• Polysynthetic twin is a type of repeated twin.
• If all the compositions surfaces are parallel to one
another that us called “polysynthetic twin”.
• In this figure [A] plagioclase show polysynthetic
twinning on (010) as the twin plane called “Albite
twining”.
2. CYCLIC TWINS :-
• Cyclic twin is also a type of repeated twin.
• If the composition surfaces are not parallel to one
another, they are called “Cyclic twin”.
• Chrysoberyl shows cyclic twin along (031) plane
which can be shown in the figure [B].
Albite twinning in
Plagioclase on (010) axis
[A]
[B]
Cyclic twinning
in chrysoberyl
on (031)plane.

11. ORIGIN OF TWINNING
Twinning can be originated in three different ways :
1. Growth twinning
2. Penetration twinning
3. Deformation twinning
(1.) GROWTH TWINNING :-
• When accident occurs 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 the orientation will be different from the
original crystal.
• Such growth twins can be contact twins or penetration twins.

12. (2.) TRANSFORMATION TWINS :-
• Transformation twin occurs when a pre-existing crystal undergoes
a transformation due to a change in pressure and temperature.
• This is commonly occurs in minerals that have different crystal
structures and different symmetry at different temperature and
pressure.
• When the temperature and 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
form an intergrowth of one or more crystals.
(3.) DEFORMATION TWINS :-
• During deformation disturbance occurs in the internal arrangement
of crystals.
• If the disturbance able to produce a symmetrical arrangement, it
produces deformation twins.

13. COMMON TWIN LAWS
1.TRICLINIC SYSTEM :-
• In triclinic system the twinning exhibited by the plagioclase feldspar and microcline
are the important twinning.
ALBITE LAW (010) :-
• Plagioclase commonly shows albite polysynthetic twinning.
• The twin law (010) indicates that the twinning occurs perpendicular to the b-
crystallographic axis.
• Albite law commonly found in plagioclase.
PERICLINE LAW (010) :-
• The pericline law has (010) as the twin axis.
• Pericline twinning occurs as the result of monoclinic orthoclase or sanidine
transforming to microcline.
• The combination of pericline and albite twinning produces a cross hatched pattern
called tartan twinning.

14. MONOCLINIC SYSTEM :-
• The most common twins in the monoclinic system occur on the planes (100)
and (001).
• The orthoclase-feldspar and sanidine are the most commonly twinned
minerals in the monoclinic system.
• Both contact twin and penetration twin occurs and both types result from
accidents during growth.
MANEBACH LAW (001):-
• It shows contact twinning in orthoclase on (001).
CARLSBAD LAW (001) :-
• Carlsbad law forms a penetration twin in the orthoclase on (001).
• Crystal twinned under the Carlsbad law show two intergrown crystals,
one rotated 180°from the other on (001) axis.
• It is the most common type of twinning in orthoclase.

15. BRAVENO LAW (021) :-
• It forms a contact twin in the mineral orthoclase on (021).
SHALLOW TAIL TWINS (100) :-
• This twinning occurs in gypsum on (100).
ORTHORHOMBIC SYSTEM (110) :-
• In the orthorhombic system the twinning on (110) is most common.
• Contact twin of aragonite and cyclic twin of cerussite are all twinned on (110).
HEXAGONAL SYSTEM :-
• In this system important twins are found in the class calcite(CaCo3) and
quartz(SiO2).
CALCITE TWIN :-
• The two most common twin laws that are (0001) and the rhombohedron(011bar2)
• Both are contact twins, but the(011bar2)twins can also occur as polysynthetic
twins that results from deformation(Due to change in temp. and pressure).

16. Quartz shows three other hexagonal twins:
BRAZIL LAW (112bar0):-
• It is a penetration twin that results from transformation.
DAUPHINE LAW (0001):-
• It is also a penetration twin that results from
transformation.
JAPANESE LAW (112bar2) :-
• It is a contact twin that results from accidents during
growth.
TETRAGONAL SYSTEM (011):-
• Twinning in the tetrahedral system usually occurs
on(011)forming Cyclic contact twins.
• The mineral rutile(Tio2) and cassiterite(SnO2)commonly
show this type of twinning.

17. ISOMETRIC SYSTEM :-
SPINEL LAW (111):-
• In “galena type” class of the cubic system, the most
common twin has (111) as the twin plane.
• This type of twin is very common in spinel crystal, so
it is called as Spinel law.
• IRON CROSS (001):-
• In “pyrite type” class two pyritohedron may form a
penetration twin with a 90° rotation about the twin
axis and this twin is called iron-cross twinning.

18. THANK YOU