HEXAGONAL SYSTEM

one of the principal categories of structures to which a given crystalline solid can be assigned. Components of crystals in this system are located by reference to four axes-three of equal lengths set at 120 to one another and a fourth, perpendicular to the plane of the other three. If the atoms or atomic groups in the solid are represented by points and the points are connected by line segments, the resulting lattice will define the edges of an orderly stacking of blocks, or unit cells. The hexagonal unit cell is distinguished by the presence of a single line, called an axis of 6-fold symmetry, about which the cell can be rotated by either 60 or 120 without changing its appearance. A type of unit cell commonly included in the hexagonal system contains two additional lattice points. Its shape is that of a regular rhombus, with internal angles of 60 and 120. Some crystallographers, however, prefer to assign this unit cell to a separate primary crystal system, the rhombohedral or trigonal system. Among the primary crystal systems, the hexagonal system has the fewest substances assigned to it, including arsenic, calcite, dolomite, quartz, apatite, tourmaline, emerald, ruby, cinnabar, and graphite. All crystals in the hexagonal system are classed as optically uniaxial; each crystal exhibits double refraction and yields two refractive indices for light of each colour, one parallel to the optical axis and one perpendicular to the optical axis.