Crystals

Crystals are solids in which the atoms are arranged in a periodic repeating pattern that extends in three dimensions. When crystals are grown slowly and carefully they are normally bounded by plane faces that can be seen with the naked eye. This can be seen in the beautiful samples of many minerals that are exhibited in museums and stores that specialize in minerals and fossils or simply by looking at some table salt with a magnifying glass. The plane faces that are evident in the minerals (or the salt) are evidence of a regular internal structure. However, not all materials that are crystalline are so easy to distinguish. Many materials such as steel, concrete, bone, and teeth are made up of small crystals. Other materials such as wood, silk, hair, and many solid polymers are partially crystalline or have crystalline regions. The crystalline state of solids is the state of minimum free energy and is therefore their natural state. When a material is taken to a low temperature the molecules will always try to arrange themselves in a periodic structure. When they are unable to do so, because of the viscosity of the system or because the materials were cooled rapidly, noncrystalline solids can result. Solids such as glasses have no crystalline regions and no long-range order and are called amorphous solids.