Difference between revisions of "Merohedral"
From Online Dictionary of Crystallography
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| + | <font color="blue">Mérièdre</font> (''Fe''). <font color="red">Meroedrisch</font> (''Ge''). <font color="black">Meroedrico</font> (''It''). <font color="green">Meroédrico</font> (''Sp''). | ||
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'''Merohedral''' is the adjectival form of [[merohedry]] and indicates a crystal that does not possess the full point symmetry of its lattice. | '''Merohedral''' is the adjectival form of [[merohedry]] and indicates a crystal that does not possess the full point symmetry of its lattice. | ||
==Discussion== | ==Discussion== | ||
| − | In the literature, the term ''merohedral twinning'' is often improperly used instead of [[twinning by merohedry]]. A merohedral crystal may undergo several different types of twinning and for this reason the term | + | In the literature, the term ''merohedral twinning'' is often improperly used instead of [[twinning by merohedry]]. A merohedral crystal may undergo several different types of twinning and for this reason the term 'merohedral twinning' is misleading, as the following example shows. |
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| + | A crystal belonging to the [[geometric crystal class]] 2 is merohedral because its lattice has at least symmetry 2/''m''. There are three minimal supergroups of order four of the point group 2 which correspond to three different twins. | ||
| + | #Twinning by reflection across the (010) plane or by inversion: this corresponds to [[twinning by merohedry]], twin point group 2/''m'''. | ||
| + | #Twinning by reflection across the (100) or (001) plane: this corresponds to [[twinning by pseudomerohedry]], [[twinning by reticular merohedry]], or [[twinning by reticular pseudomerohedry]] if β ≠ 90º, or to [[twinning by metric merohedry]] if β = 90º; the twin point group is ''m''′2''m''′. | ||
| + | #Twinning by rotation about the [100] or [001] direction: this corresponds to the same types of twinning as case 2 above but the twin point group is 2′22′. | ||
| + | Case 1 above would be a 'merohedral twin of a merohedral crystal' while cases 2 and 3 would be 'non-merohedral twins of a merohedral crystal'. | ||
| − | + | To avoid any terminological awkwardness, the adjective '''merohedric''' has been suggested with reference to twins, but the use of the category names like [[twinning by merohedry]] remains preferable. | |
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| − | To avoid any terminological awkwardness, the adjective '''merohedric''' has been suggested with reference to twins, but the use of the category names like [[twinning by merohedry]] remains preferable | ||
==See also== | ==See also== | ||
Latest revision as of 14:50, 30 November 2018
Mérièdre (Fe). Meroedrisch (Ge). Meroedrico (It). Meroédrico (Sp).
Merohedral is the adjectival form of merohedry and indicates a crystal that does not possess the full point symmetry of its lattice.
Discussion
In the literature, the term merohedral twinning is often improperly used instead of twinning by merohedry. A merohedral crystal may undergo several different types of twinning and for this reason the term 'merohedral twinning' is misleading, as the following example shows.
A crystal belonging to the geometric crystal class 2 is merohedral because its lattice has at least symmetry 2/m. There are three minimal supergroups of order four of the point group 2 which correspond to three different twins.
- Twinning by reflection across the (010) plane or by inversion: this corresponds to twinning by merohedry, twin point group 2/m'.
- Twinning by reflection across the (100) or (001) plane: this corresponds to twinning by pseudomerohedry, twinning by reticular merohedry, or twinning by reticular pseudomerohedry if β ≠ 90º, or to twinning by metric merohedry if β = 90º; the twin point group is m′2m′.
- Twinning by rotation about the [100] or [001] direction: this corresponds to the same types of twinning as case 2 above but the twin point group is 2′22′.
Case 1 above would be a 'merohedral twin of a merohedral crystal' while cases 2 and 3 would be 'non-merohedral twins of a merohedral crystal'.
To avoid any terminological awkwardness, the adjective merohedric has been suggested with reference to twins, but the use of the category names like twinning by merohedry remains preferable.