Editing Crystal structure (section)

== Polymorphism ==
[[File:Quartz, Tibet.jpg|thumb|[[Quartz]] is one of the several [[crystalline]] forms of [[silica]], SiO<sub>2</sub>. The most important forms of silica include: [[quartz|α-quartz]], [[quartz|β-quartz]], [[tridymite]], [[cristobalite]], [[coesite]], and [[stishovite]].]]
[[Polymorphism (materials science)|Polymorphism]] is the occurrence of multiple crystalline forms of a material. It is found in many crystalline materials including [[polymer]]s, [[mineral]]s, and [[metal]]s.  According to Gibbs' rules of phase equilibria, these unique crystalline phases are dependent on intensive variables such as pressure and temperature.  Polymorphism is related to [[allotropy]], which refers to [[chemical elements|elemental solids]]. The complete morphology of a material is described by polymorphism and other variables such as [[crystal habit]], [[Amorphous solid|amorphous fraction]] or [[crystallographic defect]]s. Polymorphs have different stabilities and may spontaneously and irreversibly transform from a metastable form (or thermodynamically unstable form) to the [[Chemical stability|stable]] form at a particular temperature.<ref>{{cite journal | doi=10.1007/s10853-010-5113-0 | title=Review of the anatase to rutile phase transformation | date=2011 | last1=Hanaor | first1=Dorian A. H. | last2=Sorrell | first2=Charles C. | journal=Journal of Materials Science | volume=46 | issue=4 | pages=855–874 | doi-access=free | bibcode=2011JMatS..46..855H }}</ref> They also exhibit different [[melting points]], solubilities, and [[X-ray diffraction]] patterns.

One good example of this is the [[quartz]] form of [[silicon dioxide]], or SiO<sub>2</sub>. In the vast majority of [[silicates]], the Si atom shows tetrahedral coordination by 4 oxygens. All but one of the crystalline forms involve tetrahedral {SiO<sub>4</sub>} units linked together by shared vertices in different arrangements. In different minerals the tetrahedra show different degrees of networking and polymerization. For example, they occur singly, joined in pairs, in larger finite clusters including rings, in chains, double chains, sheets, and three-dimensional frameworks. The minerals are classified into groups based on these structures. In each of the 7 thermodynamically stable crystalline forms or polymorphs of crystalline quartz, only 2 out of 4 of each the edges of the {SiO<sub>4</sub>} tetrahedra are shared with others, yielding the net chemical formula for silica: SiO<sub>2</sub>.

Another example is elemental [[tin]] (Sn), which is malleable near ambient temperatures but is [[brittle]] when cooled. This change in mechanical properties due to existence of its two major [[allotrope]]s, α- and β-tin. The two [[allotrope]]s that are encountered at normal pressure and temperature, α-tin and β-tin, are more commonly known as ''gray tin'' and ''white tin'' respectively. Two more allotropes, γ and σ, exist at temperatures above 161&nbsp;°C and pressures above several GPa.<ref>{{cite journal|first = A. M.|last = Molodets|author2=Nabatov, S. S.|title = Thermodynamic Potentials, Diagram of State, and Phase Transitions of Tin on Shock Compression|journal = High Temperature|volume = 38|issue = 5|year = 2000|pages = 715–721|doi = 10.1007/BF02755923| bibcode=2000HTemp..38..715M |s2cid = 120417927}}</ref> White tin is metallic, and is the stable crystalline form at or above room temperature. Below 13.2&nbsp;°C, tin exists in the gray form, which has a [[diamond cubic]] crystal structure, similar to [[diamond]], [[silicon]] or [[germanium]]. Gray tin has no metallic properties at all, is a dull gray powdery material, and has few uses, other than a few specialized [[semiconductor]] applications.<ref>{{cite book|publisher = Walter de Gruyter|year = 1985|edition = 91–100|pages = 793–800|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first = Arnold F.|last = Holleman |author2=Wiberg, Egon |author3=Wiberg, Nils|chapter = Tin| language = de}}</ref> Although the α–β transformation temperature of tin is nominally 13.2&nbsp;°C, impurities (e.g. Al, Zn, etc.) lower the transition temperature well below 0&nbsp;°C, and upon addition of Sb or Bi the transformation may not occur at all.<ref>{{cite book|first = Mel|last = Schwartz |title = Encyclopedia of Materials, Parts and Finishes|edition = 2nd|chapter = Tin and Alloys, Properties|publisher = CRC Press|year = 2002|isbn= 978-1-56676-661-6}}</ref>