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Crystal

2007 Schools Wikipedia Selection. Related subjects: Materials science

   Quartz crystal
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   Quartz crystal

   In chemistry and mineralogy, a crystal is a solid in which the
   constituent atoms, molecules, or ions are packed in a regularly
   ordered, repeating pattern extending in all three spatial dimensions.

   Generally, crystals form when they undergo a process of solidification.
   Under ideal conditions, the result may be a single crystal, where all
   of the atoms in the solid fit into the same crystal structure. However,
   generally, many crystals form simultaneously during solidification,
   leading to a polycrystalline solid. For example, most metals
   encountered in everyday life are polycrystals. Crystals are often
   symmetrically intergrown to form crystal twins.

   Which crystal structure the fluid will form depends on the chemistry of
   the fluid, the conditions under which it is being solidified, and also
   on the ambient pressure. The process of forming a crystalline structure
   is often referred to as crystallization.
   Synthetic bismuth crystal
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   Synthetic bismuth crystal

   While the cooling process usually results in the generation of a
   crystalline material, under certain conditions, the fluid may be frozen
   in a noncrystalline state. In most cases, this involves cooling the
   fluid so rapidly that atoms cannot travel to their lattice sites before
   they lose mobility. A noncrystalline material, which has no long-range
   order, is called an amorphous, vitreous, or glassy material. It is also
   often referred to as an amorphous solid, although there are distinct
   differences between solids and glasses: most notably, the process of
   forming a glass does not release the latent heat of fusion. For this
   reason, many scientists consider glassy materials to be viscous liquids
   rather than solids, although this is a controversial topic; see the
   entry on glass for more details.
   Insulin crystals
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   Insulin crystals

   Crystalline structures occur in all classes of materials, with all
   types of chemical bonds. Almost all metal exists in a polycrystalline
   state; amorphous or single-crystal metals must be produced
   synthetically, often with great difficulty. Ionically bonded crystals
   can form upon solidification of salts, either from a molten fluid or
   when it condenses from a solution. Covalently bonded crystals are also
   very common, notable examples being diamond, silica, and graphite.
   Polymer materials generally will form crystalline regions, but the
   lengths of the molecules usually prevents complete crystallization.
   Weak Van der Waals forces can also play a role in a crystal structure;
   for example, this type of bonding loosely holds together the
   hexagonal-patterned sheets in graphite.

   Most crystalline materials have a variety of crystallographic defects.
   The types and structures of these defects can have a profound effect on
   the properties of the materials.
   Gallium, a metal that easily forms large single crystals
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   Gallium, a metal that easily forms large single crystals
   A huge monocrystal of potassium dihydrogen phosphate grown from
   solution by Saint-Gobain for the megajoule laser of CEA.
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   A huge monocrystal of potassium dihydrogen phosphate grown from
   solution by Saint-Gobain for the megajoule laser of CEA.

   While the term "crystal" has a precise meaning within materials science
   and solid-state physics, colloquially "crystal" refers to solid objects
   that exhibit well-defined and often pleasing geometric shapes. In this
   sense of the word, many types of crystals are found in nature. The
   shape of these crystals is dependent on the types of molecular bonds
   between the atoms to determine the structure, as well as on the
   conditions under which they formed. Snowflakes, diamonds, and common
   salt are common examples of crystals.

   Some crystalline materials may exhibit special electrical properties
   such as the ferroelectric effect or the piezoelectric effect.
   Additionally, light passing through a crystal is often refracted or
   bent in different directions, producing an array of colors; crystal
   optics is the study of these effects. In periodic dielectric structures
   a range of unique optical properties can be expected as described in
   photonic crystals.

   Crystallography is the scientific study of crystals and crystal
   formation.

Historical and mythical uses

   According to Rebbenu Bachya, the word "Achlmah" in the verse Exodus
   28:19 means "Crystal" and was the stone on the Ephod representing the
   tribe of Gad.

   Crystals also figure or figured prominently as healing tools in a
   number of mythologies.

   Retrieved from " http://en.wikipedia.org/wiki/Crystal"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
