Crystals

Crystals are solid materials whose smallest components—molecules, ions, or atoms—are arranged in an orderly, repeating pattern that extends in all three spatial directions. There are many varieties of crystals, both natural and synthetic, and because of their varied properties, they have countless uses in technology, entertainment, and defense.

An Overview of Crystals

Crystals, both natural and synthetic, are ubiquitous. They can be found in rocks and ice, are produced naturally by some living organisms, and are produced artificially by humans for a variety of uses. Common examples of crystals include diamonds, snowflakes, and sugar, and common uses include clocks, semiconductors, and lasers.

Crystals are scientifically defined by their microscopic structure, although macroscopic descriptions are also regularly used. In scientific terms, crystals can be classified by their physical properties (the crystal lattice system, crystal systems, and crystal families) or by their chemical properties (such as types of bonds). Macroscopically, crystals are described by their “habit”—that is, their overall external appearance, shape, and features.

Crystals are abundant in nature, but not all solid materials are crystals. Most inorganic solids are actually polycrystals; they are composed of many smaller crystals (crystallites), but they themselves are not crystals because the crystallites are not organized in a periodic pattern. Other solids, such as glass and gels, are described as amorphous. They have no periodic arrangement at any level.

In 2022, researchers analyzed recently discovered zircon crystals, which changed their perception about Earth's geochemistry about 3.8 billion years ago. From the crystals, they theorized that land above sea level existed at this time and interacted with freshwater. This may have made the start of life possible.

Physical Properties of Crystals

The microscopic structure of a crystal (the pattern of atoms, molecules, or ions) provides its scientific classification. A crystal’s structure is described by a unit cell, an imaginary box showing the three-dimensional pattern of atoms. Unit cells are arranged in a lattice, and lattice parameters describe the length of the unit cell sides and the angles between them. Crystals are classified into seven different lattice systems: triclinic (the least symmetrical), monoclinic, orthorhombic, rhombohedral, tetragonal, hexagonal, and cubic (the most symmetrical and simplest).

Crystals also can be classified into seven different crystal systems, most of which correspond to the lattice systems. Crystal system classification refers to point groups (geometrical symmetries), and several of the classes contain two different lattice possibilities, explaining why the systems are not entirely the same. The triclinic, monoclinic, orthorhombic, tetragonal, and cubic classes are the same, but the trigonal crystal system contains both rhombohedral and hexagonal lattice classes. The hexagonal lattice class is also present in the hexagonal crystal system. The seven crystal systems are split further into thirty-two crystal types.

One additional classification, crystal families, is almost identical to crystal systems, except that there are six crystal families. The hexagonal family encompasses both the trigonal and the hexagonal crystal systems because they share the same lattice.

While it is important to understand that a crystal’s true scientific classification comes from its microscopic structure, the macroscopic features that humans can see with unaided eyes also provide a useful classification system. One general way of describing crystals macroscopically is by referring to the clarity of definition of the crystal’s flat faces. A crystal with perfectly or near-perfectly defined faces is called euhedral, while a crystal with poorly defined or undefined faces is called anhedral. Subhedral crystals are the middle ground, with moderately defined faces.

The term crystal habit is used to describe a crystal’s overall external appearance. The crystal habit may not be entirely dictated by the microscopic structure of the crystal; other factors will affect the overall external structure, such as heat, pressure, and impurities present while the crystal grows.

There are many names for different crystal habits; the names are derived from features such as the shape of a crystal’s faces, the shape of the overall crystal, or whether the crystal is actually an aggregate of multiple crystals. For example, dodecahedral refers to an aggregate of twelve-sided crystals, like garnet. Fibrous refers to very thin, fiber-like crystals, such as tremolite, which is a type of asbestos. Rosette refers to a rose-like aggregate of plate-shaped crystals, such as gypsum.

Aside from its physical structure, a crystal also may be characterized by physical properties. These properties include cleavage (its tendency to split along weak planes), transparency and refractive index (optical properties), and surface tension.

Chemical Properties of Crystals

In addition to all of the physical classifications, crystals can be classified by their chemical properties. In particular, they can be classified according to their bonds: covalent, ionic, or molecular.

Ionic crystals are made of positive and negative ions, held together by the electrostatic attraction between them (ionic bonds). They can form when salts crystallize from a solution or solidify from a molten fluid. These crystals tend to be hard and brittle, they cleave easily along planes, and they have a high melting point. When molten, ionic crystals are good conductors of electricity; when solid, they are poor conductors. Table salt (NaCl) is a common example of an ionic crystal. Other examples are sodium fluoride (NaF, used for cavity prevention) and potassium chloride (KCl, a component of fertilizer).

Covalent crystals are characterized by their strong covalent bonds, where electrons are shared between two atoms. These crystals tend to have very high melting points. Diamonds are covalent crystals, as are graphite and silica. Diamonds and graphite are crystalline forms of the same solid: carbon. Chemical elements that can exist in multiple forms like this are known as allotropes. In many cases, the properties of the different forms can be different. Diamond is the hardest known solid, while graphite is extremely soft.

Molecular crystals, which are solids composed of molecules held together by weak forces, can be made of polar molecules or nonpolar molecules. Polar molecular crystals are held together by dipole-dipole attractions and by the weak London dispersion force, a type of van der Waals force; nonpolar molecular crystals are held together by just the London dispersion force. Both types have a low melting point and are generally soft, but nonpolar molecular crystals can conduct electricity in some forms, while polar molecular crystals are poor or nonconductors.

Formation of Crystals

Crystals form through a process called crystallization, which generally involves a crystal forming from a fluid or the dissolved materials in a fluid. (A quartz vein, for example, is formed when minerals precipitate out of a fluid flowing through a rock mass.) Often, crystallization is spurred by the cooling and solidification of a molten liquid, particularly in magmatic and metamorphic processes. In some cases, crystals can form directly from gas. A single fluid can yield many different crystals and crystalline structures because of the influence of variable factors, such as temperature and pressure.

There are two main steps to crystallization: nucleation and growth. In nucleation, a phase change is initiated, and the new crystal forms slowly around a small nucleus as the atoms or molecules begin to become correctly oriented. Nucleation can be homogeneous (without the influence of impurities) or heterogeneous (with the influence of impurities). Heterogeneous nucleation is faster than homogeneous because the foreign particles provide a scaffold upon which the crystal can grow. Indeed, when crystals are created artificially, the process can be sped up by adding small scratches to the glassware or by placing string, a rock, or small, previously made crystals (called seed crystals) into the solution. Homogeneous nucleation is fairly rare, as a large amount of energy is required to catalyze the process with no foreign surfaces present.

In the growth stage, the crystal expands from the nucleation site as ions, atoms, or molecules are added to the lattice; this step is much faster than nucleation. A perfect crystal would build slowly, but the defects and impurities naturally present in most crystallization will help speed up the process, similar to what happens in heterogeneous nucleation. The growth step involves a transfer of heat, matter, or both to drive the process.

While many crystals are created by natural geologic processes, particularly the ones found in Earth’s bedrock, some are actually produced by animals. Mollusks, for example, produce calcite and aragonite. Their mantle secretes a shell of polysaccharides and glycerides, which then directs the completion of the shell by forming crystalline materials.

Various methods exist for creating crystals artificially, such as hydrothermal synthesis, where substances are crystallized at high temperature and pressure conditions. Another method, laser-heated pedestal growth, uses a laser beam as a heat source to grow crystals from the liquid/solid phase transition.

Industrial techniques also exist to create large synthetic single crystals called boules; these crystals are used in semiconductors and for many other purposes. The Czochralski process and the Bridgman-Stockbarger technique are two such processes. Huge single crystals can also be created naturally. The Cave of the Crystals in Mexico has many of the largest natural single crystals ever discovered, including one that is 11 meters (36 feet) long.

Uses of Crystals

One of the most common uses of crystals is keeping time. Some crystals, such as quartz, are piezoelectric, meaning that electricity can accumulate in them from mechanical stress or pressure.

Because of this characteristic, quartz can be used to create a crystal oscillator, a circuit that creates an electric signal with a precise frequency based on the mechanical resonance of the crystal as it vibrates. Crystal oscillators are used in wristwatches, cell phones, radios, laboratory, equipment, and machinery such as oscilloscopes and signal generators.

The use of boules (synthetic single crystals) is widespread. Most integrated circuits contain a silicon boule, and classic semiconductors are boules or other crystalline solids. The advantage of using large single crystals over crystalline aggregates is that the single crystals do not have grain boundaries, which is the area that occurs between single crystallites in polycrystals. Grain boundaries would be detrimental to the operation of various types of equipment because they decrease the crystal’s electrical and thermal conductivity and provide a weak spot that is susceptible to corrosion.

Most electronic displays, such as television screens and computer monitors, rely on liquid crystals, which have some liquid properties and some crystal properties. The key property relevant to electronic display use is that they modulate light. Because the liquid crystals are not emitting their own light, liquid crystal displays are quite energy efficient.

Crystals also have aesthetic and cultural significance, and are used in jewelry and in cultural practices such as feng shui. Also, some persons believe that crystals have healing powers, although there is no scientific evidence to support that belief.

Principal Terms

boule: a large, synthetically made single crystal with many industrial and technological applications

crystal growth: the second stage of crystallization; a transfer of heat, matter, or both drives the crystal to expand from the nucleation site by adding molecules, atoms, or ions to the lattice

crystal habit: the external appearance of a crystal, including its shape and visible physical features

crystallization: the process of crystal formation; generally includes two steps—nucleation and growth

crystallography: the scientific study of crystals and how they form

grain boundary: the interface between the small crystals (crystallites) that make up a larger polycrystal material

lattice: an infinite array of discrete points upon which a crystal is built; the crystal’s arrangement of atoms, molecules, or ions is repeated at each lattice point

nucleation: the first stage of crystallization; a new crystal forms around a nucleus, initiated by a phase change

periodic: a repeating pattern

piezoelectricity: the property of some crystals (and some other solids) to create an electric charge from mechanical stress or pressure

point group: a group of geometric symmetries with one or more fixed points

unit cell: an imaginary box that shows the three-dimensional pattern of the atoms, molecules, or ions of a crystal

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