Kristallijärjestelmät

Kristallijärjestelmät, the Finnish term for crystal systems, refer to the classification of crystalline solids based on their lattice symmetry and geometrical parameters. The concept is grounded in crystallography, where the arrangement of atoms or molecules in a crystal is described by a repeating unit cell. Each unit cell is defined by three lattice vectors and three interaxial angles, and the symmetry operations that leave the cell invariant determine the system to which a crystal belongs.

There are seven primary crystal systems: cubic, tetragonal, orthorhombic, hexagonal, rhombohedral (or trigonal), monoclinic, and triclinic. The cubic system features three equal lattice constants and all angles of 90°, giving rise to high symmetry crystals such as diamond and pyrite. The tetragonal system shares two equal axes and a distinct third, with right angles, exemplified by rutile and zircon. Orthorhombic crystals have all axes of different lengths yet right angles, found in minerals like olivine. Hexagonal crystals possess a fourfold rotational symmetry around one axis and include quartz and calcite. Rhombohedral systems have equal axes but angles deviating from 90°, seen in sodium chloride when expressed in a rhombohedral cell. Monoclinic crystals have one pair of axes inclined, with examples such as gypsum. Finally, triclinic systems lack equal axes and right angles, presenting the lowest symmetry as in chlorite.

These systems are part of a larger hierarchical classification that also includes crystal classes, space groups, and point groups, which further refine the symmetry description. Knowledge of crystal systems is essential for determining physical properties like optical anisotropy, cleavage planes, and mechanical behavior, and serves as a foundational tool in mineralogy, materials science, and solid-state physics.