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What Are Crystals?
In physics, the term "crystal" describes a solid substance with internal symmetry and a related, regular surface pattern. This configuration, called the crystal structure, recurs so regularly that you can use it to predict the organization of atoms throughout the crystal [sources: Encyclopaedia Britannica; Isaacs et al.].
If this arrangement carries on beyond a few neighboring atoms it is called long-range order, akin to a half-time band marching in formation. Liquid crystals, like those found in LCD monitors, usually fall into short-range order (picture the marching band scatter-drilling into smaller subunits). Solid crystals can assume either pattern. Here's how: As crystalline substances melt, they become amorphous, meaning they display only short-range order. As they cool, they can either fall back into a long-rage formation or remain amorphous, like silicon-based glass [sources: Arfken et al.; Encyclopaedia Britannica; Isaacs et al.].
Cast in the role of our band members are ions (positively or negatively charged atoms) linked up by ionic or covalent bonds. These bonds pack up into various compact, stable shapes called coordination polyhedra [sources: Banfield; Dutch].
To better picture these coordination polyhedra, forget the marching band and instead picture a geometric mosaic like those found in the Alhambra. Now visualize that mosaic in three dimensions so that its tesserae (tiles) consist of cubes, pyramids and diamond-shaped solids, each of which describes the arrangement of the atoms in a given type of crystal.
In a silica crystal, a small central ion of silicon might be surrounded by four larger ions of oxygen, forming a triangular pyramid, or tetrahedron. In manganese(II) oxide, a small central manganese ion lies within six larger oxygen ions -- one above, one below and four in a square around the middle, forming a three-dimensional diamond, or octahedron [sources: Banfield; Dutch; Purdue].
These 3-D mosaic tiles can pack into several different patterns, or lattices, sharing atomic bonds at their corners, along their edges or along their faces. The same elements can assume different arrangements, both in terms of their "tile shapes" (coordination polyhedra) and their mosaic patterns (lattices). These variations are called polymorphs, and they play a key role in determining a crystal's properties. Take carbon: Arranged tetrahedrally, it forms famously hard, clear diamonds; arranged in a layered honeycomb, it forms soft, gray graphite [sources: Dutch; Purdue; UCSB].
Crystallization doesn't always produce single crystals. Sometimes, the self-ordering process begins at a number of sites that grow together, forming a patchwork of lattices aligned along different directions. These polycrystals, which often develop during rapid cooling, tend to be stronger than single crystals [sources: Encyclopaedia Britannica; Encyclopaedia Britannica; University of Virginia]. When heated, larger crystals can absorb smaller ones. So temperature and pressure, stress and strain can influence crystals' characteristics, whether in their transformation -- or their creation.