crystal

A crystal is a solid material in which the atoms or molecules are arranged in an orderly, repeating pattern extending in all directions, forming a crystal lattice. The external form reflects this internal order, giving a characteristic crystal habit. Crystals occur in many materials, including minerals, metals, salts, and certain organic compounds, and can form naturally or be grown synthetically in laboratories.

The internal arrangement is described by a unit cell and a lattice; crystals are classified into seven crystal systems and 14 Bravais lattices based on symmetry and cell parameters. Long-range order differentiates crystals from amorphous solids, where atoms lack a repeating pattern.

Crystals form by solidification from a melt, precipitation from a solution, or deposition from a gas. Growth rate, temperature, concentration of constituents, and impurities affect size, shape, and defect structure. Defects such as vacancies or dislocations can influence properties.

Physical properties include cleavage planes, hardness, density, optical properties such as refractive index and birefringence, and color, which can be influenced by impurities. Notable examples include quartz (SiO2), halite (NaCl), calcite (CaCO3), diamond (carbon), and ice (H2O).

Crystallography studies the arrangement of atoms using methods like X-ray diffraction, electron diffraction, and neutron diffraction. Applications span gemstones, electronics (semiconductors and piezoelectrics), optics, catalysis, and pharmaceuticals, where crystal form can affect stability and solubility.