Extensive Definition
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. In a gas, the
relative positions of the atoms or molecules are completely random.
Amorphous materials, such as liquids and glasses, 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-ceramics
and some polymers), can
be prepared in such a way as to produce a mixture of crystalline
and 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 alloys are crystalline, but they usually
comprise many independent crystalline regions (grains or
crystallites) in various orientations separated by grain
boundaries; furthermore, they contain other defects (notably
dislocations) that reduce the degree of structural perfection. The
most highly perfect crystals are silicon boules produced for
semiconductor electronics; these are large single crystals (so they
have no grain boundaries), are nearly free of dislocations, and
have precisely controlled concentrations of defect atoms.
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 are
partially glassy; and holohyaline rocks (such as obsidian) are completely
glassy.
Crystallinity can be measured using x-ray
diffraction, but calorimetric techniques are also commonly
used.