Difference between revisions of "Twinning"

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{{edit|--[[User:Doos|Doos]] 09:22, 6 December 2006 (PST)}}
 
 
 
==Basic==
 
==Basic==
  
Minerals do not always grow under ideal conditions to form perfect crystals, instead during growth (or after) some crystals may form twins.
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Minerals do not always grow under ideal conditions to form perfect crystals. During growth (or after), some crystals may form twins.
  
 
There are basically two types of twins:
 
There are basically two types of twins:
* Contact twins
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* contact twins
* Penetration twins
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* penetration twins
  
And there are typically 3 causes of twinning:
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There are typically 3 causes of twinning:
* During crystal growth
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* during crystal growth
* Transformation
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* from transformation
* Deformation
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* by deformation
  
 
===Types of twins===
 
===Types of twins===
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====During growth====
 
====During growth====
  
During the growth of a crystal there may be changes inside the magma (like temperature, pressure and flow) which causes elements of the magma not to keep building a crystal in the same orientation as it did.
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During the growth of a crystal there may be changes inside the magma (like temperature, pressure and flow) which cause elements of the magma to alter a crystal's orientation.
 
<br />Most twinning occurs under these circumstances.
 
<br />Most twinning occurs under these circumstances.
  
 
====Transformation====
 
====Transformation====
  
When a fully grown crystal experiences sudden change in tempearture and pressure the internal arrangement of atoms can shift. A good example of this is quartz.<br />
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When a fully grown crystal experiences sudden change in temperature and pressure, the internal arrangement of atoms can shift. A good example of this is quartz.<br />
At temperatures above 573°C quartz is hexagonal and named beta-quartz. This beta-quartz will transform to alpha-quartz below that temperature and twinning will occur during the transformation.  
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At temperatures above 573°C, quartz is hexagonal and is termed beta-quartz. Below that temperature, beta-quartz will transform into trigonal alpha-quartz, and twinning will occur during this transformation.
  
 
====Deformation====
 
====Deformation====
  
When a fully grown crystal undergoes mechanical stress (such as pressure) the crystal lattice can be distorted. One can imagine this by laying the shell of a matchbox on a table and apply slight pressure to it. The matchbox will shear and its form is thus distorted. When two matchbox shells are placed on top of eachother, the pressure will cause them to shear into an arrow head.<br />
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When a fully grown crystal undergoes mechanical stress (such as pressure), the crystal lattice may be distorted. One can imagine this by laying the shell of a matchbox on a table and applying slight pressure to it. The matchbox will shear, distorting its form. When two matchbox shells are placed next to each other, the same pressure will cause them to shear into an arrow head.<br />
 
An example of this is [[calcite]].
 
An example of this is [[calcite]].
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[[image:deformation_twin.jpg|left|200px|thumb|Deformation twin (contact)]]
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'''Next:[[Cleavage| Cleavage, Parting and Fracture]]'''
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'''[[Table_Of_Contents| Return to the Table of Contents]]'''

Latest revision as of 22:05, 30 May 2009

Basic

Minerals do not always grow under ideal conditions to form perfect crystals. During growth (or after), some crystals may form twins.

There are basically two types of twins:

  • contact twins
  • penetration twins

There are typically 3 causes of twinning:

  • during crystal growth
  • from transformation
  • by deformation

Types of twins

Contact twins

Contact twins share a single plane (or face).

Geneculate (knee-type) twinning in zircon


Penetration twins

Penetration twins share a single axes (usually a rotation axis).

Fluorite penetration twin (two cubes intergrown)
Staurolite penetration twin (two hexagonal prisms intergrown)


Causes of twinning

During growth

During the growth of a crystal there may be changes inside the magma (like temperature, pressure and flow) which cause elements of the magma to alter a crystal's orientation.
Most twinning occurs under these circumstances.

Transformation

When a fully grown crystal experiences sudden change in temperature and pressure, the internal arrangement of atoms can shift. A good example of this is quartz.
At temperatures above 573°C, quartz is hexagonal and is termed beta-quartz. Below that temperature, beta-quartz will transform into trigonal alpha-quartz, and twinning will occur during this transformation.

Deformation

When a fully grown crystal undergoes mechanical stress (such as pressure), the crystal lattice may be distorted. One can imagine this by laying the shell of a matchbox on a table and applying slight pressure to it. The matchbox will shear, distorting its form. When two matchbox shells are placed next to each other, the same pressure will cause them to shear into an arrow head.
An example of this is calcite.

Deformation twin (contact)



Next: Cleavage, Parting and Fracture

Return to the Table of Contents