Tips and Tricks

What is Merohedral twinning?

What is Merohedral twinning?

Merohedral twins have a twin symmetry element that is not a symmetry element of the space group of the sample but is a symmetry element of the Laue group. Both the direct and reciprocal lattices of the different domains coincide with one another, making the crystal appear to be single until near the end of refinement.

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 is twinning plane for rhombohedral crystal?

7. What is twinning plane for rhombohedral crystals? Explanation: {0 1 1} are the twinning planes for rhombohedral crystals. In these crystals, twinning is only possible in these planes.

What minerals are twinning?

Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning. Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other.

What is non Merohedral twinning?

Merohedral twins – The lattices of all twin components coincide perfectly in both direct and reciprocal space. 159 of the 230 space groups belong to merohedral point groups. Non-merohedral twins – Some axes coincide in reciprocal space, but others do not.

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 twinning in plastic deformation?

Twinning is a prominent mechanism of plastic deformation in some metallic materials. It is a shear force that can produce atomic displacements. Twinning may be produced by mechanical deformation or as the result of annealing following plastic deformation. Twinning is also known as mechanical twinning.

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.

Does quartz have twinning?

There are six known twin laws for low-quartz (Friedel, 1926), namely, Dauphiné, Brazil, Japanese, Esterel, Sardinian, and Breithaupt. In addition, there is a complex type of twinning which is a combination of the Dauphiné and Brazil laws and has been variously called.

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 twinning in optical mineralogy?

twinning, in crystallography, regular intergrowth of two or more crystal grains so that each grain is a reflected image of its neighbour or is rotated with respect to it. Other grains added to the twin form crystals that often appear symmetrically joined, sometimes in a starlike or crosslike shape.