Twinning
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--Doos 09:22, 6 December 2006 (PST) |
Contents
Basic
Minerals do not always grow under ideal conditions to form perfect crystals, instead during growth (or after) some crystals may form twins.
There are basically two types of twins:
- Contact twins
- Penetration twins
And there are typically 3 causes of twinning:
- During crystal growth
- Transformation
- Deformation
Types of twins
Contact twins
Contact twins share a single plane (or face).
Penetration twins
Penetration twins share a single axes (usually a rotation axis).
Causes of twinning
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.
Most twinning occurs under these circumstances.
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.
At temperatures above 573°C quartz is hexagonal and named beta-quartz. This beta-quartz will transform to alpha-quartz (which is trigonal) below that temperature and twinning will occur during the transformation.
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.
An example of this is calcite.