Macla

A twin is the symmetrical grouping of identical crystals. The symmetry can be specular with respect to the twinning plane or by the rotation of its elements around the twinning axis by 60°, 90°, 120° or 180°.

They are called "contact" when the elements are joined in a plane, and "penetration" if they are crossed and interpenetrated with each other. In general they have incoming dihedrals; Otherwise we speak of mimicry of an individual crystal.

Types of twins

Often the individuals that form the twin join together as if they had stuck to each other on the back, on one side or at the base, like the image that is formed of an object if we attach it to a mirror. In this case we talk about contact twins. In penetration twins it seems as if the individuals that form them have entered one by one into the interior of the other. Other times the crystals that join together apparently form a symmetry greater than their corresponding one. We talk, then, about mimetic twins. Multiple twins join more than two crystalline individuals. Among them we can distinguish the polysynthetic ones, which can group several crystals according to the direction of a single plane, and the cyclic ones, which when joining several individuals come to close on themselves in an annular shape.

In metallurgy, twinning is a deformation mechanism for single crystals. There is a uniform distribution of the deformation, a modification in the orientation of a crystal and a mirror image of the undeformed network.

Examples

• Calcite Macla
• Fluorite, contact or compenetration
• Plaster, in "lance mark"
• Typical deformations of BCC and HCP metals.
• Cutting maths formed after heat-collection treatments, typical in FCC.

Sources

• Monographic Encyclopedia of Natural Sciences (1971), vol. 1 Guide. Minerals and rocks. Fossils., pp. 86-87, Aguilar S.A. de Ediciones, Madrid.
• The content of this article incorporates material from a Entry of the Universal Free Encyclopedia, published in Spanish under the Creative Commons Share-Igual 3.0 license.