crystal a solid body bounded by natural plane faces that are the external expression of a regular internal arrangement of constituent atoms, molecules, or ions. The formation of a crystal by a substance passing from a gas or liquid to a solid state, or going out of solution (by precipitation or evaporation), is called crystallization.

Classification of Crystals

The particles in a crystal occupy positions with definite geometrical relationships to each other. The positions form a kind of scaffolding, called a crystalline lattice; the atomic occupancies of lattice positions are determined by the chemical composition of the substance. A crystalline substance is uniquely defined by the combination of its chemistry and the structural arrangement of its atoms. In all crystals of any specific substance the angles between corresponding faces are constant (Steno's Law, or the First Law of Crystallography, published by the Danish geologist Nicolaus Steno in 1669). Crystalline substances are grouped, according to the type of symmetry they display, into 32 classes. These in turn are grouped into seven systems on the basis of the relationships of their axes, i.e., imaginary straight lines passing through the ideal centers of the crystals.

Crystals may be symmetrical with relation to planes, axes, and centers of symmetry. Planes of symmetry divide crystals into equal parts (mirror images) that correspond point for point, angle for angle, and face for face. Axes of symmetry are imaginary lines about which the crystal may be considered to rotate, assuming, after passing through a rotation of 60°, 90°, 120°, or 180°, the identical position in space that it originally had. Centers of symmetry are points from which imaginary straight lines may be drawn to intersect identical points equidistant from the center on opposite sides.

The crystalline systems are cubic, or isometric (three equal axes, intersecting at right angles); hexagonal (three equal axes, intersecting at 60° angles in a horizontal plane, and a fourth, longer or shorter, axis, perpendicular to the plane of the other three); tetragonal (two equal, horizontal axes at right angles and one axis longer or shorter than the other two and perpendicular to their plane); orthorhombic (three unequal axes intersecting at right angles); monoclinic (three unequal axes, two intersecting at right angles and the third at an oblique angle to the plane of the other two); trigonal, or rhombohedral (three equal axes intersecting at oblique angles); and triclinic (three unequal axes intersecting at oblique angles). In all systems in which the axes are unequal there is a definite axial ratio for each crystal substance.

Physical Properties of Crystals

Crystals differ in physical properties, i.e., in hardness, cleavage, optical properties, heat conductivity, and electrical conductivity. These properties are important since they sometimes determine the use to which the crystals are put in industry. For example, crystalline substances that have special electrical properties are much used in communications equipment. These include quartz and Rochelle salt, which supply voltage on the application of mechanical force (see piezoelectric effect ), and germanium, silicon, galena, and silicon carbide, which carry current unequally in different crystallographic directions, as semiconductor rectifiers.

See solid-state physics .

Bibliography

See F. C. Phillips, An Introduction to Crystallography (1970); J. D. Dana, Manual of Mineralogy (18th ed., rev. by C. S. Hurlbut, Jr., 1971); B. K. Vainshtein, Modern Crystallography (2 vol., 1981–82).

Crystal

A crystal is a solid whose particles are arranged in an orderly, repeating, geometric pattern. Crystals come in all sizes and shapes. Regular table salt, for instance, consists of tiny cubic particles called salt crystals.

Crystals at the atomic level

The crystal shapes that we can see with our naked eye reflect a similar geometric pattern that exists at the level of atoms. For example, the substance we call salt is actually a chemical compound called sodium chloride. Sodium chloride is made of sodium ions and chloride ions. Each sodium ion is a tiny particle carrying a single positive electric charge. Each chloride ion is a tiny particle carrying a single negative charge.

A crystal of table salt consists of trillions and trillions of sodium ions and chloride ions. The way in which these ions arrange themselves in a salt crystal depends on two factors: the size of each ion and the electric charge on each ion. Recall that similar (or like) electric charges repel each other, and unlike charges attract each other. That fact means that all of the sodium ions tend to repel each other. They will get as far from each other as possible in a salt crystal. The same is true of the chloride ions.

On the other hand, positively charged sodium ions will be attracted by negatively charged chloride ions. Sodium ions and chloride ions will try to get as close to each other as possible. Obviously some kind of compromise position has to be found that allows these forces of attraction and repulsion to be balanced against each other. Size can make a difference, too. Chloride ions are much larger than sodium ions, and this affects the shape of salt crystals.

Unit cells and crystal lattice

The final compromise that best satisfies charge and size factors in sodium chloride is a cube. One sodium ion occurs at each of the alternate corners of the cube. One chloride ion occurs at the other alternate corners of the cube. In this arrangement, sodium ions and chloride ions are held together by forces of electrical attraction, but ions of the same kind are kept as far from each other as possible.

The basic shape that satisfies charge and size factors is known as a unit cell. Thus, for sodium chloride, the unit cell is a cube. Other geometric arrangements are also possible. For example, the problem of balancing electric charges for a crystal of calcium chloride is different than it is for sodium chloride. The calcium ions in calcium chloride each have a positive electric charge of two units. A different geometric arrangement is necessary to accommodate doubly charged calcium ions and singly charged chloride ions.

There are seven geometric shapes that crystals can assume. In a tetragonal crystal system, for example, ions are arranged at the corners of

a rectangular box whose end is a square. In an orthorhombic crystal system, ions are arranged at the corners of a rectangular box whose end is a rectangle.

Words to Know

Ion: A molecule or atom that has lost one or more electrons and is, therefore, electrically charged.

Lattice: A collection of unit cells that are all identical.

Unit cell: The simplest three-dimensional structure of which a crystal is made.

If you could look at a crystal of table salt with a microscope you would see a vast system of unit cells. That system is known as a crystal lattice. As shown in the photo of salt (on p. 602), a crystal lattice is simply many unit cells—all exactly alike—joined to each other. For this reason, the crystal shape that you can actually see for a crystal of table salt reflects exactly what the unit cell for sodium chloride looks like.

crys·tal / ˈkristl/ • n. 1. a piece of a homogeneous solid substance having a natural geometrically regular form with symmetrically arranged plane faces. ∎  Chem. any solid consisting of a symmetrical, ordered, three-dimensional aggregation of atoms or molecules. ∎  Electr. a crystalline piece of semiconductor used as an oscillator or transducer. ∎  a clear transparent mineral, esp. quartz. ∎  a piece of crystalline substance believed to have healing powers. ∎ inf. short for crystal meth (methamphetamine). 2. (also crystal glass) highly transparent glass with a high refractive index: [as adj.] a crystal chandelier. ∎  articles made of such glass: a collection of crystal. ∎  the glass over a watch face. • adj. clear and transparent like crystal: the clean crystal waters of the lake. PHRASES: crystal clear completely transparent and unclouded. ∎  unambiguous; easily understood: the house rules are crystal clear.

crystal Solid with a regular geometrical form and with characteristic angles between its faces, having limited chemical composition. There are 32 classifications of crystals using the combinations of symmetry. The external form is called the crystal habit. The structure of a crystal, such as common salt, is based upon a regular 3-D arrangement of atoms, ions or molecules (a crystal or ionic lattice). Crystals are produced when a substance passes from a gaseous or liquid phase to a solid state, or comes out of solution by evaporation or precipitation. The rate of crystallization determines the size of crystal formed. Slow cooling produces large crystals, whereas fast cooling produces small crystals.

Crystal ♀ 19th-century coinage, which has recently enjoyed some popularity. This is one of the group of names taken from or suggestive of gemstones. The word crystal, denoting high-quality cut glass, is derived from Greek krystallos ‘ice’. As a boy's name, Crystal originated as a Scottish pet form of Christopher, but it is rarely used today.

Variants: Krystal, Kristel, Krystle.

crystal †ice; pure quartz (resembling ice) OE.; piece of rock crystal, etc., XIV; highly transparent glass XVI; mineralogical form XVII. — (O)F. cristal — L. crystallum — Gr. krústallos ice, f. krustaínein freeze, krúos frost.
So crystalline XIV. — (O)F. cristallin — L. — Gr.; see -INE 1. crystallize XVI.

crystal A homogeneous, ordered solid, having naturally formed plane faces and a limited chemical composition. Crystals have definite geometric forms that reflect the arrangement in lattices of the atoms of which they are composed. See CRYSTAL CLASS; and CRYSTAL SYMMETRY.

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