In mineralogy and crystallography, a crystal (from the greek κρύσταλλος, krýstallos, ice) is defined as the atomic or molecular structure that matter in the solid state presents, chemically and physically homogeneous. In other words atoms, molecules or ions have a regular three-dimensional geometric arrangement, which is repeated indefinitely in the three spatial dimensions, called crystal lattice or Bravais lattice.

The regular arrangement of the three-dimensional structure is characteristic of the type of material and produces the characteristic shape of crystals (morphology) defined by characteristic faces and dihedral angles; consequently, the relative properties of crystalline substances also depend on their particular intimate structure and not on the polyhedral shape they assume. Crystalline solids exhibit planes of flaking that are related to the shape of the crystal building.

The same substance sometimes has more than one crystalline form, depending on the temperature and pressure at which it solidifies. This property is called polymorphism when referred to a compound (e.g. calcite, aragonite) and allotropy when referred to an element (e.g. diamond, graphite, fullerenes).

There are many cases in which different substances, but able to give crystals with the same structure, even at the molecular level, show a mutual and complete miscibility in the state (mixed crystals). This property is called isomorphism.

Typical characteristic of crystalline solids is the anisotropy: property of a substance for which the values of its physical quantities (refractive index, electrical and thermal conductivity, hardness, etc.) depend on the direction that is considered.

There are materials that have a lower degree of crystallinity, such as certain polymers, which have only a two-dimensional order, and many synthetic or natural fibers, which are ordered only along the axis of the fiber.

Some organic crystals, when properly heated, go to an intermediate state between solid and liquid, which is called the mesomorphic or liquid crystal state. Materials with this behavior also include coordination compounds.

Most minerals are polycrystalline, that is, they are composed of many crystals (crystallites), although this is not usually visible to the naked eye, because the individual crystals are microscopic in size. In contrast, solids consisting of a single crystal (called monocrystals) are very rare. Non-crystalline solids (such as glass) are called amorphous.

A crystal is a solid formation that has a periodic and ordered arrangement of atoms at the vertices of a lattice structure, the crystal lattice; the presence of such atomic organization can give the crystal a definite geometric shape. Crystals are formed by the gradual solidification of a liquid or by the frosting of a gas. Such crystallization can occur spontaneously in nature or be artificially reproduced.

The type of structure assumed by the crystal plays a determining role in many of its properties, such as flaking. Depending on the symmetries of their structure, many properties, such as electrical, optical and mechanical (e.g. Young’s and Poisson’s moduli), can be anisotropic, i.e. dependent on their orientation in space. In contrast, only some crystals are isotropic.

The formation and characteristics of a crystal depend on the rate and conditions of solidification (also called “crystallization”). For example, the liquids that form granite are erupted to the surface as volcanic lava and cool relatively slowly. If cooling is more rapid, an aphanitic rock is formed, with crystals not visible to the naked eye; even slower cooling leads to the formation of large crystals.

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