crystal

Regular-shaped solid that reflects light. Examples include diamonds, grains of salt, and sugar. Particles forming a crystal are packed in an exact and ordered pattern. When this pattern is repeated many millions of times, the crystal is formed. Such an arrangement of particles, that is regular and repeating, is called a giant molecular structure.

In ionic compounds, such as sodium chloride (NaCl), the ions are arranged in a giant ionic lattice, with alternate positive and negative ions in a three-dimensional arrangement. The natural shape of the crystal is the same as the arrangement of ions in the lattice. In sodium chloride the ions form a cubic lattice. Hence sodium chloride crystals are cubic. In diamond there is a giant atomic structure made up of carbon atoms covalently bonded to each other in a regular, repeating arrangement throughout the whole of the structure. In metals the atoms are also packed tightly together in a regular pattern. This gives metals like copper a crystalline structure. Metal crystals are called grains.

A mineral can often be identified by the shape of its crystals and the system of crystallization determined. For example, extrusive igneous rock such as basalt contains very small crystals compared with an intrusive igneous rock such as granite. A single crystal can vary in size from a submicroscopic particle to a structure some 30 m/100 ft in length. Crystals fall into seven crystal systems or groups, classified on the basis of the relationship of three or four imaginary axes that intersect at the centre of any perfect, undistorted crystal.