1. Twinning
GLY 4200
Fall, 2018

2. Twinning Illustration
The twin plane cannot be a part of the normal symmetry of a crystal
Diagram:

3. Twinning Operations
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

4. Twin Law Must define two things: The type of twin operation
The orientation of the twin element associated with the operation

5. Types of Twinning
Twins may also be classified on the basis of their physical properties
There are two basic types of twin
Contact twins
Penetration twins

6. Contact Twinning
Have a planar composition surface separating two individual crystals
These are usually defined by a twin law that expresses a twin plane
Orthoclase twinned on the Braveno Law

7. Polysynthetic Twinning
A type of multiple contact twinning is called polysynthetic
The compositions surfaces are parallel to one another, they are called polysynthetic
Plagioclase commonly shows this type of twinning, called the Albite Twin Law, with {010} as the twin plane
Such twinning is one of the most diagnostic features of plagioclase

8. Albite Twinning Videoclip showing albite twinning under the microscope
Source:
Running time: 11 seconds
Videoclip showing albite twinning under the microscope

9. Cyclic Twinning Another type of contact twinning
If the composition surfaces are not parallel to one another, they are called cyclical twins
Shown here is the cyclical twin that occurs in chrysoberyl
Photo:

10. Penetration Twins
Have an irregular composition surface separating 2 individual crystals
Are defined by a twin center or twin axis
Twinned crystal of orthoclase twinned on the Carlsbad Law with [001] as the twin axis
Left Photo:
Right Photo:

11. Origin of Twinning Twinning can originate in 3 different ways
Growth twins
Transformation twins
Glide or deformation twins

12. Growth Twins
When accidents occur 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 has an orientation different from the original crystal
Such growth twins can be contact twins or penetration twins
Both Carlsbad and Albite twins are growth twins

13. Transformation Twins
Occurs when a preexisting crystal undergoes a transformation due to a change in pressure or temperature
This commonly occurs in minerals that have different crystal structures and different symmetry at different temperatures or pressures
When the temperature or 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 thus form an intergrowth of one or more crystals

14. Brazil Twin
Dauphiné and Brazil twinning in quartz commonly forms this way during a decrease in temperature   
Photomicrograph shows a Brazil twin in amythyst in crossed polars
Photo:
Running time: 14 seconds
Click to play video

15. Tartan Twinning
Combination of albite twinning and pericline twinning in alkali feldspar results when high temperature sanidine (monoclinic)  transforms to low temperature microcline (triclinic) is known as "tartan“, “gridiron, or “cross-hatch” twinning pattern
One of the most characteristic diagnostic properties for the identification of microcline

16. Deformation Twins During deformation atoms can be pushed out of place
If this happens to produce a symmetrical arrangement, it produces deformation twins

17. Examples of Deformation Twinning
Plagioclase and calcite are two common minerals exhibiting this behavior
Deformational albite twins characteristically "pinch out" rather than running throughout the grain
They may also be bent
Calcite deformation enhances ductility in some rocks, such as marble

18. Deformational Albite Twins
Source:
More common in metamorphic rocks

19. Deformation Twinning in Calcite
The mineral calcite can be easily twinned during deformation, producing polysynthetic twins on {01bar12}
Photomicrograph:

20. Manebach Twinning
A type of contact twinning seen in orthoclase on the {001} plane
Diagnostic of orthoclase when it occurs
Monoclinic system 

21. Gypsum
Swallow tail twins {100} are commonly observed in the mineral gypsum CaSO4●2H2O)
Photo:

22. Orthorhombic Cyclic Twinning
Cyclical Twins - The minerals aragonite (CaCO3) , chrysoberyl (BeAl2O4), and cerrusite (PbCO3) commonly develop twinning on {110}, which results in a cyclical twin that gives these minerals a pseudo-hexagonal appearance
Diagram illustrates {110} in aragonite

23. Twinning in Staurolite
Two types of interpenetration twins occur in staurolite
{031} twins from a right-angled cross
{231} twins form a cross at about 60°

24. “Fairy Cross” Twins The staurolite crystal shown has twinning on {031}
The small crystals on the surface are garnets
Photo:

25. Staurolite Twin The common 60° twin on {231}
Photo:

26. Tetragonal System
Twinning usually occurs on {011} forming cyclical contact twins
Rutile (TiO2 - left) and cassiterite (SnO2 – below) often show this type of twinning
Image:

27. Calcite Twinning
Most common twin laws that are observed in calcite crystals are {0001} and the rhombohedron {01bar12}
Both are contact twins, but the {01bar12} twins can also occur as polysynthetic twins that result from deformation
Photo shows {01bar12} twin
Photo:

28. Quartz Twinning
Brazil Law - {11bar20} - is a penetration twin that results from transformation
Dauphiné Law - [0001] - is also a penetration twin that results from transformation
Japanese Law - {11bar22} - is a contact twin that results from accidents during growth

29. Quartz Twin Pictures Japanese Law Dauphiné twin
Upper photo:
Lower photo:

30. Isometric System Spinel law – Twin reflection on (bar1 bar1 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

31. Spinel Law Photo Twin plane clearly visible

32. Iron Cross Twin Iron Cross in Pyrite
Photo: