Last Update 19/ 9/ 2004
in English/ in Esperanto/ in Portuguese
During crystal growth matter assembles in uniform mode according to one of the seven crystal systems. This uniform mode may occur without any modified orientation, developing a periodical repetition of a structural unit in a single crystal. Suddenly for some reason part of the components may be assembled on one or more individual crystal according to an orientation modification guided by an allowed symmetry operation and naturally selected new direction according to the twin law. Meanwhile another part of the original crystal continues its growth in the initial mode. The result of this modification is observed on twin crystals composed of two or even of multiple parts, each one with its own orientation.
Any structure determination study with x-ray diffraction demands single crystal. Twin crystals are dismissed.
The chemical environment on the boundary between both twin crystals is different if compared with any of the considered adjacent crystal.
The application above represents crystals with perpendicular edges or
not prependicular edges. Twin crystals appear initially on the left side
of the figure or after a click on button A. On the right side a sketch
of the image that would be observed in a crystallographic
microscope with crossed polariser and analyser Nicol is presented.
A click on button B presents the same sample as in the previous button
turned 90o counter clockwise around an axis perpendicular
to the screen . A click on button C shows the model of a single crystal.
An action on button D will display the same single crystal after a rotation
of 90o. For didactical purposes the contents of the left side
models were amplified in higher scale in comparison to the size of its
crystal edges.
Eexercise
Inform yourself on twin crystals of the following minerals: quartz, gypsum, fluorite, aragonite and write a report.
Please send your comments.
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