Crystal twinning

Crystal twinning is a symmetrical intergrowth of two or more crystals of the same substance, related by a specific crystallographic operation. The twin parts share a boundary or interpenetrate while retaining a defined orientation relationship.

Twin law: A crystallographic rule specifying the orientation relationship between the parts of a twin. It states the symmetry operation that relates the intergrown crystal parts.
Carlsbad twin: A common feldspar twin formed by a 180-degree rotation around the crystallographic c-axis. It is a recognizable feldspar example of a rotation twin.
X-ray crystallography: A method for determining crystal structures from the diffraction of X-rays by periodic atomic arrangements. Diffraction patterns can reveal or be complicated by multiple twin orientations.
Crystal polymorphism: The ability of a substance to crystallize in more than one distinct crystal structure. Polymorphs differ in structure, whereas twin parts belong to the same substance and structure.
Twin boundary: An interface across which two crystal domains have orientations related by a twin law. It is the physical interface that separates adjacent twin domains.
Manebach twin: A feldspar twin in which two individuals meet across a composition plane parallel to (001). Its composition plane illustrates how a specific plane characterizes a twin.
Crystallographic texture: The statistical distribution of crystallographic orientations in a polycrystalline material. Twin formation changes the orientations measured in a textured material.
Epitaxy: The oriented growth of one crystalline material on the surface of another crystalline material. Epitaxy relates separate materials at an interface rather than twin domains of one crystal.
Crystal lattice: A periodic arrangement of points representing the repeating structure of a crystalline solid. Twin operations act on the periodic lattice shared by the crystal domains.
Baveno twin: A feldspar twin with a composition plane parallel to the {021} planes. It is another named feldspar twin distinguished by its twin plane.

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