Crystal is a solid that is composed of atoms arranged in an orderly pattern. Most nonliving substances are made up of crystals. For example, metals and rocks consist of crystals, as do snowflakes, salt, and sugar.
Well-developed crystals have a distinctly regular shape as a result of the orderly arrangement of atoms. Such crystals have smooth, flat surfaces, which intersect to form sharp edges. These surfaces, called crystal faces, show definite geometrical relationships. For example, the faces of crystals of the same substance always meet at the same angle regardless of the shape and size of the crystals.
The scientific study of crystals is called crystallography. Crystallographers measure angles between crystal faces and analyze arrangements of such surfaces. They also examine atomic structures of crystals with the aid of transmission electron microscopes and X-ray diffraction techniques (see X rays (In crystal research)).
Crystallization
is the process by which matter forms crystals. Crystals may form from vapors, solutions, or melts (molten materials). When either temperature or pressure is lowered or evaporation occurs, certain atoms in such substances move close together and join. In most cases, they do so on a crystallization nucleus, an impurity or a tiny piece of crystal consisting of a particle or cluster of atoms. The atoms collect on the nucleus and arrange themselves into structural units called unit cells to form a crystalline solid. A crystal grows by adding atoms to its surfaces in an expanding network of unit cells. See Mineral (Inside minerals).
In a few cases, crystals develop smooth, mirrorlike faces. Such crystals are said to be euhedral. Euhedral crystals grow only in a space where they cannot touch other crystals, so they rarely occur in nature. Most crystals are subhedral—that is, they have poorly formed faces that are rough or pitted. Some crystals, called anhedral crystals, have no faces at all. Most rocks are composed of anhedral crystals.
Classifying crystals.
Crystals are classified according to symmetry, a balanced arrangement of faces. There are three basic types of crystal symmetry—plane of symmetry, axis of symmetry, and center of symmetry.
A plane of symmetry is an imaginary plane that divides a crystal into identical halves. An axis of symmetry is an imaginary straight line drawn through the center of a crystal. When a crystal is rotated 360° about this axis, there appear from two to six faces or arrangements of faces that cannot be distinguished from one another. For example, a line through the center of a cube parallel to the line of intersection of any two faces is an axis of symmetry. This axis is called a fourfold axis of symmetry because four identical faces appear during a 360° rotation of the cube. Most crystals have a center of symmetry, which means their opposite sides are identical.
All crystals can be grouped into 32 combinations of symmetry. These combinations, in turn, can be classified into seven general crystal systems. These systems are (1) isometric, (2) tetragonal, (3) hexagonal, (4) rhombohedral, (5) orthorhombic, (6) monoclinic, and (7) triclinic. Each system may be described in terms of three imaginary axes, called crystallographic axes, which intersect in the center of a crystal.
Isometric system.
Crystals in this system have three axes of equal length that are perpendicular to one another. The simplest isometric crystal is a cube. Another form is the octahedron, which has eight sides consisting of equilateral triangles. Such minerals as galena, garnet, and pyrite crystallize in this system.
Tetragonal system.
Tetragonal crystals have three axes that intersect at right angles. Two of the axes are of equal length. The simplest form of tetragonal crystal is a prism in which the sides are rectangular and the top and bottom are square. Other tetragonal crystals resemble eight-sided pyramids. Their sides are made up of identical isosceles triangles, which are triangles with two equal sides. The minerals cassiterite, rutile, and zircon crystallize in the tetragonal system.
Hexagonal system.
Hexagonal crystals have four axes. Three of the axes are of equal length and lie in a horizontal plane with a 120° angle between one another. The fourth axis is perpendicular to the others and may be of any length. The simplest hexagonal crystal is a prism that has six rectangular faces parallel to the fourth axis. The minerals apatite, beryl, graphite, and molybdenite form in this system.
Rhombohedral system.
Some crystallographers consider the rhombohedral system a subdivision of the hexagonal system because both systems can be defined in terms of the same axes. However, there is one major difference between them. The vertical axis of a rhombohedral crystal is a threefold symmetry axis, but that of a hexagonal crystal is a sixfold axis. The simplest crystal in the rhombohedral system has six rhomboidal faces, each consisting of an equal parallelogram. This system includes crystals of calcite, dolomite, and quartz.
Orthorhombic system.
Orthorhombic crystals have three axes of unequal length that intersect at right angles. The simplest crystal of this type is an orthorhombic prism with three sets of unequal rectangular faces that meet at right angles. Aragonite, barite, topaz, and certain other minerals crystallize in this system.
Monoclinic system.
Monoclinic crystals have three axes of different lengths. Two of the axes are perpendicular to each other, but the third is inclined. A simple monoclinic crystal has two rhomboidal faces and four rectangular ones. The top and bottom surfaces are inclined. Many compounds, including the minerals gypsum, hornblende, orthoclase, and pyroxene, belong to this system.
Triclinic system.
Triclinic crystals have three axes of unequal length. None of the axes are perpendicular. The faces of these crystals are all different and do not meet at right angles. Plagioclase feldspars and a few other minerals form in this system.
See also Gem; Mineral; Quasicrystal; Snow; Symmetry; X-ray crystallography.
