Twinning in crystals(crystallography)

Twinning in crystals crystallography

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 I ST SEMESTER TWINNING IN A CRYSTAL Reg. No-Y18251017

2. Contents Introduction Elements of twinning D evelopment of twinning kinds of twinning Common types of twins in different crystal systems Why we study twinning 2

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 results two or more crystal of same substance shows symmetrical i ntergrowth and the individuals have different orientation related by a symmetry operation, the crystal are said to be twinned crystal . ex- chrsoberely 3

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 . 4

5. DEVELOPMENT OF TWINS The development of twins is governed by certain twins laws :- An axis of even fold symmetry can never become a twin axis ,only 3 -fold symmetry axis can serve as a twin axis . Twin axis is always normal to the twin plane but need not be normal to composition plane . Twinning plane never coincide with symmetry planes Twins that develop in classes with a center of symmetry have both twin axis and twinning plane perpendicular to it . 5

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 6

7. In a twinned crystal the two parts may appears as 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 . 7 e called

8. If the twins are composed of three or more individual crystals they ae called multiple twins Types of multiple twins :- 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 8

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. 9

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. 10

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. 11

12. Common tyes of twins in different crystal system : 1. Isometric system Spinel law – Twin reflection on ( 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.   12 spinel

13. Twinning usually occurs on {011} forming contact twins Rutile (TiO 2 - left) and casseterite (SnO 2 – below) often show this type of twinning 13 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 [01 2] Both are contact twins, but the {01bar12} twins can also occur as polysynthetic twins that result from deformation . Image shows[01 2 ] twin.   14

15. *Twinning in quartz* Brazil Law – [11 is a penetration twin that results from transformation. Dauphiné Law - [0001] - is also a penetration twin that results from transformation. Japanese Law -   [11 is a contact twin that results from accidents during growth.   15

16. 4. Orthorhombic System Cyclic Twins - The minerals aragonite (CaCO 3 ) , chrysoberyl (BeAl 2 O 4 ), and cerrusite (PbCO 3 ) commonly develop twinning on {110} plane . F igs show :- Sturolite twins 16 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. 17 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. 18

19. 6. TRICLINIC SYSTEM Albite Law - As described previously, plagioclase ( NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 ) 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 . 19

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 . 20

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