View source for Crystallinity

{{No footnotes|date=December 2019}}

{{Short description|Degree of structural order in a solid}}
{{wiktionary|crystallinity|crystalline}}
'''Crystallinity''' refers to the degree of structural order in a [[solid]]. In a [[crystal]], the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on [[hardness]], [[density]], [[Transparency and translucency|transparency]] and [[diffusion]]. In an [[ideal gas]], the relative positions of the atoms or molecules are completely random. Amorphous materials, such as [[liquids]] and [[glass]]es, represent an intermediate case, having order over short distances (a few atomic or molecular spacings) but not over longer distances.

Many materials, such as [[glass-ceramic]]s and some [[polymer]]s, can be prepared in such a way as to produce a mixture of crystalline and [[Amorphous solid|amorphous]] regions. In such cases, crystallinity is usually specified as a percentage of the volume of the material that is crystalline. Even within materials that are completely crystalline, however, the degree of structural perfection can vary. For instance, most metallic [[alloy]]s are crystalline, but they usually comprise many independent crystalline regions (grains or [[crystallite]]s) in various orientations separated by [[grain boundary|grain boundaries]]; furthermore, they contain other [[crystallographic defect]]s (notably [[dislocation]]s) that reduce the degree of structural perfection. The most highly perfect crystals are silicon [[Boule (crystal)|boules]] produced for semiconductor electronics; these are large [[single crystal]]s (so they have no grain boundaries), are nearly free of dislocations, and have precisely controlled concentrations of defect atoms.

Crystallinity can be measured using [[x-ray crystallography]], but calorimetric techniques are also commonly used.

== Rock crystallinity ==
Geologists describe four qualitative levels of crystallinity:
* '''holocrystalline''' rocks are completely crystalline;
* '''hypocrystalline''' rocks are partially crystalline, with crystals embedded in an amorphous or glassy matrix;
* '''hypohyaline''' rocks are partially glassy;
* '''holohyaline''' rocks (such as [[obsidian]]) are completely glassy.

==References==
*''[[iarchive:dictionaryofscie0000unse_h7c5|Oxford dictionary of science]]'', 1999, {{ISBN|0-19-280098-1}}.

[[Category:Crystals]]
[[Category:Physical quantities]]      
[[Category:Phases of matter]]