twin crystal

In crystallography, twinning refers to the regular intergrowth of two or more crystal grains. These crystals are symmetrically joined, often appearing in starlike or crosslike shapes. Twinning can occur during crystal growth, where the individual crystals have different atomic orientations but share common planes or directions.

There are several types of twin crystals, including penetration twins, where complete crystals pass through each other. Twinning involves a symmetry operation, such as reflection across a twin plane or rotation around a twin axis, which is not present in a single, untwinned crystal. These twin elements have simple relationships to the crystal's crystallographic axes.12