What is Merohedral twinning?

What is Merohedral twinning?

Merohedral twins – The lattices of all twin components coincide perfectly in both direct and reciprocal space. Non-merohedral twins – Some axes coincide in reciprocal space, but others do not. Therefore part of the reciprocal lattice is exactly overlapped while other parts are not.

What causes twinning in crystals?

Transformation Twins – Transformation twinning occurs when a preexisting crystal undergoes a transformation due to a change in pressure or temperature. This commonly occurs in minerals that have different crystal structures and different symmetry at different temperatures or pressures.

What are twin fraction?

Definition of twinning. Twin: Two or more crystals of the same species. are joined together in different orientation. α is the twin fraction: I.

What is non Merohedral twinning?

Non-merohedral twinning This usually means that all reflections are affected by overlap, although reflections from one domain may overlap with systematic absences from another. Twins in which only certain zones of reciprocal-lattice points overlap are classified as being non-merohedral.

What is Carlsbad twinning?

A twinned crystal in which the twinning axis is the c axis, the operation is a rotation of 180 degrees, and the contact surface is parallel to the side pinacoid. It is quite common in the alkali feldspars. An alternative spelling is Karlsbad twin.

What is anti twinning?

During anti-twinning, a shear displacement of 1/3〈111〉 occurs on every successive {112} plane, in contrast to an opposite shear displacement of 1/6〈ˉ1ˉ1ˉ1〉 by ordinary twinning. This asymmetry in the atomic-scale shear pathway leads to a much higher resistance to anti-twinning than ordinary twinning.

What is the function of twinning?

The mechanical work of deformation twinning is dissipative, resulting from the defect motion associated with shearing. The important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.

What is twin law?

The twin law is the set of twin operations mapping two individuals of a twin. It is obtained by coset decomposition of the point group of the twin lattice with respect to the intersection group of the point groups of the individuals in their respective orientations.

What is Pericline twinning?

Pericline twinning is a type of crystal twinning which show fine parallel twin laminae typically found in the alkali feldspars microcline. The twinning results from a structural transformation between high temperature and low temperature forms.

What is the hardness of anorthite?


Mohs scale hardness 6
Luster Vitreous
Diaphaneity Transparent to translucent
Specific gravity 2.72–2.75

Can a pseudo Merohedral twin be a single crystal?

As with merohedral twins, Pseudo-merohedral twinshave reciprocal lattices that can be indexed on a single lattice and hence appear to be single crystals. Also as with merohedral twins, data sets of these compounds show difficulty with structure solution and especially structure refinement.

Why are non merohedral twins usually recognized at the time of data collection?

Non-merohedral twinsare generally recognized at the time of data collection because of difficulties with indexing or because of obvious multiple sets of reciprocal lattices. Many crystals that are twinned employ this mode of twinning. These twins are sometimes described as having

When does twinning occur in a monoclinic crystal?

3. Twinning in crystals. Monoclinic crystal structures sometimes have β very close to 90°. If twinning occurs, the unit cells in one domain may be rotated by 180° about the a or c axes relative to those in the other domain in exactly the fashion described above for bricks.

What’s the difference between a merohedraltwin and a non-merohealtwin?

Non-merohedraltwins have two or more crystalline domains with reciprocal lattices that either do not overlap or only partially overlapped. In contrast, Merohedraltwins have domains with diffraction patterns that are completely overlapped.

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