Geo 3500 - Lecture 11

1. Twins and
Defects

2. Twinning
A twin is a symmetrical intergrowth of two or more
crystals of the same substance
Twinning can be considered a type of planar defect
The surface on which two individuals are united is
known as the composition surface. If this surface is
a plane, it is called the composition plane and can
be irregular or regular

3. Twinning
The operations that relate a crystal to its twinned
counterpart are symmetry operations:
‣ twin plane - reflection by a mirror plane
‣ twin axis - rotation about a crystal direction common
to both, with the angular rotation normally 180°
‣ twin center - inversion about a point

4. Twin Classification
4 types of twins:
✦ Contact ✦ Polysynthetic
✦ Penetration ✦ Cyclic

5. Contact & Penetration
Contact twins have a regular composition surface
separating the two individuals; the twin is defined by
a twin plane
Penetration twins are made up of interpenetrating
individuals having an irregular composition surface,
and the twin law is usually defined by a twin axis
direction

6. Polysynthetic & Cyclic
Multiple twins that have three or more individual
crystals twinned according to the same law and if all
the composition surfaces are parallel, the resulting
group is a polysynthetic twin
If successive composition planes are not parallel, a
cyclic twin results

7. Common Twins
Swallowtail Polysynthethic Penetration
Twin Gypsum Albite Twin Twin Galena

8. Common Twins
Penetration Penetration Carlsbad Twin
Twin Pyrite Twins Orthoclase
Staurolite

11. Causes of Twinning
Growth twins - error in stacking of unit cells when
crystal first start to grow
Transformation twin - variations in temperature and
pressure stresses the lattice - lattice relieves the
stress by “warping”
Deformation twin - some kind of stress is put on the
crystal structure - crystal relieves stress by twinning

12. Structural Defects
As a crystal grows, structural defects are
incorporated into the mineral
Such imperfections affect growth rate, crystal
morphology, and basic properties of crystalline
materials, such as strength, conductivity,
mechanical deformation, and color

13. Structural Defects
3 Types:
point
line
plane

14. Point Defects

15. Frenkel & Shottky
Defects
Frenkel Shottky

16. Line Defects
Edge
Screw

17. Edge Dislocation

18. Screw Dislocation

19. Planar Defects
Grain Boundaries
Twin Planes
Sub-grain Boundaries

20. Grain Boundaries

21. Twin Planes

22. Subgrain Boundaries

23. Crystal Deformation

24. Crystal Deformation
Dislocation Slip

25. Migration of Vacancies thru Crystal Lattice

26. Dislocations Quartz
Transmission Electron Microscope Photos

27. Dislocations Quartz
Transmission Electron Microscope Photos

28. Dislocations Quartz
Strain-free Neoblasts

29. Subgrain Boundaries

30. Order-Disorder