A twin crystal consists of two intergrown crystals of the same chemical composition and crystal structure, which are legally grow together. When twinning is formed as an additional element of either a mirror - symmetry plane ( twin plane ) or a pivot axis ( twin axis ). For acentric crystals also a center of inversion can be caused by point reflection. In his situation to the two crystal individuals the additional symmetry element obeys the law of rationality.
If more than two individuals involved in the twinning, one also speaks of triplets, quadruplets or multi- Lingen and repetitive twinning occur repetition Much Linge or polysynthetic twins.
Most crystal twins are easy to recognize by the re-entrant angles that occur at the twin plane. Similarly, striped surfaces an indication of twinning.
The science of twinning is called Geminographie and is a specialized field of crystallography, which deals with the analysis, characterization and description of the twin symmetries.
Twinning and forms
By type of intergrowth
By type of intergrowth are three possible twinning can be distinguished:
" Japanese twin" in the quartz
Knee -shaped Rutilzwilling
Durchdringungsmehrling the fluorite
" Iron Cross " at the pyrite
Chrysoberyl - rotation twin
Crystal twins who pretend a higher symmetry, as it corresponds to the crystal structure of the single crystal, also referred to as a supplement twins or mimetic twins. Representatives of this type are found predominantly among the contact twins and multiples rotation.
According to the mode of origin
Depending on the type of training conditions also can distinguish different crystal twins:
Analysis and classification
For the analysis of crystal twins the concept of point groups is very helpful. In the description of the twin law symmetry elements are expressed by the Miller indices.
According to the rules of François Ernest Mallard the twin symmetry simulates a higher symmetry than that corresponding to the actual crystal structure, which is why one speaks also of twin lattice in contrast to the actual crystal lattice. The actual crystal lattice can be found by omitting the symmetry elements of the twin lattice. The crystal lattice is thus a subset of the point group of the twin- grid.
Those twin lattice and the crystal lattice at the same Laue group, then one speaks of a merohedral twin crystal. In all other cases, it is nichtmeroedrische crystal twins. In the triclinic, monoclinic and orthorhombic crystal system meroedrische crystal twins are always inversion twins.
Are classified according to the crystal twins usually introduced in 1926 by Georges Friedel rules that associates the twins on the basis of two criteria Meroedrie and skewness four different groups:
- Meroedrie - complete and exact overlap of the lattices of the twinned crystals
- Pseudomeroedrie - complete, but approximate overlap ( Mallard Law)
- Reticulare Meroedrie - but partially exact overlap
- Reticulare Pseudomeroedrie - partial and approximate overlap
Twin laws, ie the laws according to which two individuals can be oriented to one another are very varied and are usually characterized by specific name. Named the twin laws are mostly based on mineral species for which this twin law is characteristic or to the place at which a corresponding crystal twin was found for the first time.
- Albite and pericline law, see albite
- Twin laws during orthoclase: Bavenoer law, Carlsbad law, Manebacher law
- Pyrite law
- Twin laws in the quartz: Belodwa beacon twin, Brazilian twins, Cornish - twin, Dauphinee - twin (also Alpine or Swiss law ), Esterel twin, Japanese twin, Liebisch twin, Pierre- Levee - twin, Sardinia - twin, Wheal - Coats - twin