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Difference between revisions of "Twinning by merohedry"

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<Font color ="blue"> Maclage par mériédrie </Font> (''Fr''). <Font color="green"> Macla por meriedria </Font> (''Sp''). <Font color="black"> Geminazione per meroedria</Font>(''It''). <Font color="purple"> 欠面双晶</Font>(''Ja'')
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<font color ="blue">Maclage par mériédrie</font> (''Fr''). <font color="red">Meroedrische Verzwillingung</font> (''Ge''). <font color="black">Geminazione per meroedria</font>(''It''). <font color="purple">欠面双晶</Font> (''Ja''). <font color="green">Macla por meroedría</font> (''Sp'').
  
  
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The twin operation belongs to the point group of the lattice but not to the point group of the crystal. Therefore, the point group of the crystal must be a subgroup of the point group of the lattice, ''i.e.'' the crystal shows only a part (''[[merohedry]]'') of the symmetry operations belonging to its lattice which, instead, shows ''[[holohedry]]'' (complete symmetry). The twinning element of symmetry may (''Class I of twins by merohedry'') or may not belong to the Laue class of the crystal (''Class II of twins by merohedry'').
  
= [[Twinning]] by merohedry=
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== Examples ==
  
The twin operation belongs to to the point group of the lattice but not to the point group of the crystal. Therefore, the point group of the crystal must be a subgroup of the point group of the lattice, i.e. the crystal shows only a part (''[[merohedry]]'') of the symmetry elements belonging to the its lattice which, instead, shows ''[[holohedry]]'' (complete symmetry). The twinning element of symmetry may (''Class I of twins by merohedry'') or may not belong to the Laue class of the crystal (''Class II of twins by merohedry''): consequences are discussed under ''solving the crystal structure of twins''. - Examples - Class I: in crystals with point group 2 (Laue group 2/''m'') the mirror plane ''m'' acts as twinning operator .
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Class I: in crystals with point group 2 (Laue group 2/''m'') the mirror plane ''m'' acts as twin element.
Class II: in crystals with point group 4 (Laue group 4/''m'') a mirror plane ''m'' parallel to the foufold axis 4 acts as twinning operator.
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Class II: in crystals with point group 4 (Laue group 4/''m'') a mirror plane ''m'' parallel to the fourfold axis 4 acts as twin element.
  
 
== See also ==
 
== See also ==
  
Chapter 3.3 of ''International Tables of Crystallography, Volume D''<br>
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*Chapter 3.3 of ''International Tables for Crystallography, Volume D''
  
 
[[Category:Twinning]]
 
[[Category:Twinning]]

Latest revision as of 13:43, 15 July 2021

Maclage par mériédrie (Fr). Meroedrische Verzwillingung (Ge). Geminazione per meroedria(It). 欠面双晶 (Ja). Macla por meroedría (Sp).


The twin operation belongs to the point group of the lattice but not to the point group of the crystal. Therefore, the point group of the crystal must be a subgroup of the point group of the lattice, i.e. the crystal shows only a part (merohedry) of the symmetry operations belonging to its lattice which, instead, shows holohedry (complete symmetry). The twinning element of symmetry may (Class I of twins by merohedry) or may not belong to the Laue class of the crystal (Class II of twins by merohedry).

Examples

Class I: in crystals with point group 2 (Laue group 2/m) the mirror plane m acts as twin element.

Class II: in crystals with point group 4 (Laue group 4/m) a mirror plane m parallel to the fourfold axis 4 acts as twin element.

See also

  • Chapter 3.3 of International Tables for Crystallography, Volume D