Quasicrystal is a solid composed of atoms arranged in an orderly pattern that differs from the pattern in a crystal. In a crystal, the atomic structure is composed of a single type of atom cluster called a unit cell. The unit cell repeats throughout the structure so that the distance between the centers of the unit cells is the same throughout. A quasicrystal is composed of two, or sometimes more, kinds of unit cells. The unit cells repeat with different spacings between the different types of unit cells. The ratio of the spacings is an irrational number—that is, a number that cannot be expressed as a fraction (see Rational number ).
Both crystals and quasicrystals display symmetry—that is, it is possible to rotate their atomic structure by certain angles and have the structure appear identical to the first view. However, because quasicrystals have more than one shape of unit cell, they have types of symmetry that are impossible in crystals. For example, many quasicrystals have fivefold symmetry, meaning that they look the same after every one-fifth rotation around a circle. Crystals cannot have this type of symmetry because it is not possible to pack together a single shape of unit cell so that there is both five-fold symmetry and equal distance between the unit cells. Physicists discovered quasicrystals in 1984. The only known natural quasicrystals were found in Russia. They are believed to be the remains of a meteorite.
