Editing Crystal (section)

== Crystal structure (microscopic) ==
{{multiple image
| align     = right
| direction = horizontal
| width     = 200
| header    = Halite (table salt, NaCl): Microscopic and macroscopic
| image1    = Sodium-chloride-3D-ionic.png
| width1    = 100
| alt1      = Halite crystal (microscopic)
| caption1  = Microscopic structure of a [[halite]] crystal. (Purple is [[sodium]] ion, green is [[chlorine]] ion). There is [[cubic crystal system|cubic symmetry]] in the atoms' arrangement
| image2    =Selpologne.jpg
| width2    = 100
| alt2      = Halite crystal (Macroscopic)
| caption2  = Macroscopic (~16 cm) halite crystal. The right-angles between crystal faces are due to the cubic symmetry of the atoms' arrangement}}
{{Main article|Crystal structure}}
The scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the [[crystal structure]]. A crystal is a solid where the atoms form a periodic arrangement. ([[Quasicrystal]]s are an exception, see [[#Quasicrystals|below]]).

Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a ''[[polycrystalline]]'' structure. In the final block of ice, each of the small crystals (called "[[crystallite]]s" or "grains") is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does ''not'' have a periodic arrangement of atoms, because the periodic pattern is broken at the [[grain boundaries]]. Most macroscopic [[inorganic]] solids are polycrystalline, including almost all [[metal]]s, [[ceramic]]s, [[ice]], [[rocks]], etc. Solids that are neither crystalline nor polycrystalline, such as [[glass]], are called ''[[amorphous solid]]s'', also called [[glass]]y, vitreous, or noncrystalline. These have no periodic order, even microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the [[latent heat of fusion]], but forming a crystal does.

A crystal structure (an arrangement of atoms in a crystal) is characterized by its ''unit cell'', a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are [[honeycomb (geometry)|stacked]] in three-dimensional space to form the crystal.

The [[crystal structure|symmetry of a crystal]] is constrained by the requirement that the unit cells stack perfectly with no gaps. There are 219 possible crystal symmetries (230 is commonly cited, but this treats chiral equivalents as separate entities), called [[Space group|crystallographic space groups]].<ref>{{Citation|last=|first=|editor-first1=T. R|editor-last1=Welberry|title=International Tables for Crystallography|url=https://doi.org/10.1107/97809553602060000001|work=|year=2021|volume=A|pages=|place=Chester, England|publisher=International Union of Crystallography|doi=10.1107/97809553602060000001|isbn=978-1-119-95235-0|s2cid=146060934|access-date=}}</ref> These are grouped into 7 [[crystal system]]s, such as [[cubic crystal system]] (where the crystals may form cubes or rectangular boxes, such as [[halite]] shown at right) or [[hexagonal crystal system]] (where the crystals may form hexagons, such as [[Ice Ih|ordinary water ice]]).