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Crystallinity refers to the degree of structural order in asolid. In acrystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a large influence onhardness,density,transparency anddiffusion. In anideal gas, the relative positions of the atoms or molecules are completely random. Amorphous materials, such asliquids andglasses, represent an intermediate case, having order over short distances (a few atomic or molecular spacings) but not over longer distances.
Many materials, such asglass-ceramics and somepolymers, can be prepared in such a way as to produce a mixture of crystalline andamorphous 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 metallicalloys are crystalline, but they usually comprise many independent crystalline regions (grains orcrystallites) in various orientations separated bygrain boundaries;[1] furthermore, they contain othercrystallographic defects (notablydislocations) that reduce the degree of structural perfection. The most highly perfect crystals are siliconboules produced for semiconductor electronics; these are largesingle crystals (so they have no grain boundaries), are nearly free of dislocations, and have precisely controlled concentrations of defect atoms.
Crystallinity can be measured usingx-ray diffraction, but calorimetric techniques are also commonly used.
In geochemistry, mineral crystallinity plays a pivotal role in regulating water-rock interactions, particularly during the early stages of mineral crystallization. For example, in the case of goethite, dissolved lithium can be taken up by poorly crystallized goethite, whereas little sorption occurs in well-crystallized goethite.[2] Additionally, lithium isotope fractionation during low-temperature water-rock interactions is controlled by reactions occurring at defect sites in poorly crystallized minerals, such as hectorite[3] and goethite.[2]
Geologists describe four qualitative levels of crystallinity: