crystal

Crystal

Crystal is defined as a homogeneous (uniform or the same throughout) portion of matter that has a definite, orderly atomic structure, and an outward form bounded by smooth, plane surfaces, symmetrically arranged, and is produced whenever a solid is formed gradually from a fluid, whether the formation results from the freezing of a liquid, the deposition of dissolved matter, or the direct condensation of a gas into solid form. The angles between corresponding faces of any two crystals of the same substance, regardless of size or superficial difference of form, are always identical. Most solid matter displays orderly atomic arrangement and is of crystalline structure. Solids that have no crystalline structure, such as glass, are called amorphous. In structure they show greater similarity to liquids than to solids, and are known as supercooled liquids.

If a solution remains undisturbed as it is cooled, it often passes the point at which it would normally crystallize, reaching a supercooled state. The same is true for a solution containing a maximum amount of solute; if it assimilates additional solute, it is termed supersaturated. In both instances, if a tiny crystal, called a seed crystal, is added to the solution, a sudden chain reaction occurs, as shown here, and crystal growth is dramatic.

Cooling rate

The same liquids that gradually freeze deep within the earth to form granite are sometimes ejected at the surface as volcanic lava and cool quickly, forming a glassy rock called obsidian. If the cooling is slightly slower, a rock called felsite is formed; it is crystalline, but the crystals are too small to be seen with the naked eye. Such a structure is called cryptocrystalline, or aphanitic. Still slower cooling results in a rock of porphyritic structure, in which some of the crystals are large enough to be visible; this rock, which may be of identical composition with obsidian, felsite, or granite, is called rhyolite. All of these minerals are the same atomic structure.

Crystal growth

Crystal growth is attained when a microscopic crystal that has formed condences more of the same element from its environment. Sometimes, in the absence of this first minute crystal, or seed, crystallization does not take place, and the solution becomes supersaturated (more highly concentrated than is normally possible under given temperature and pressure), just as a liquid below its freezing point becomes supercooled. When a new organic chemical is prepared, it is often difficult to make the first crystal unless a substance with simular form can be found. The tendency to crystallize decreases with increasing viscosity (high resistance to flow) of the liquid; if a solution becomes considerably supersaturated or supercooled it becomes very viscous, and crystallization becomes almost impossible. Further cooling or evaporation of the solvent produces first a syrup and then a glass.

Some substances have a strong tendency to form seed crystals. If a solution of such a substance is cooled slowly, a few seeds grow into large crystals; but if it is cooled rapidly, numerous seeds form and grow only into tiny crystals. Table salt, purified at a factory by recrystallization, is composed of numerous perfect cubic crystals, which are barely visible to the naked eye; rock salt, formed by the slow processes of geology, contains enormous crystals of the same cubic form.

Crystallography

The study of the growth, shape, and geometric character of crystals is called crystallography. When conditions are favorable, each chemical element and compound tends to crystallize in a definite and characteristic form. Thus, salt tends to form cubic crystals; but garnet, which also occasionally forms cubes, more commonly occurs in dodecahedrons (solids with 12 faces) or trisoctahedrons (solids with 24 faces). Despite their differences in habit (shape of crystallization), salt and garnet always crystallize in the same class and system. Thirty-two classes of crystal are theoretically possible; almost all common minerals fall into one of about twelve classes, and some classes have never been observed. The thirty-two classes are grouped into six crystal systems, based on the length and position of the crystal axes, imaginary lines passing through the center of the crystal, intersecting the faces, and bearing definite relations to the symmetry of the crystal. Minerals in each system share certain details of symmetry and crystal form and many important optical properties.

The Six Crystal Systems

Isometric

This system comprises crystals with three axes, all perpendicular to one another and all equal in length.

Isometric crystals, such as the pyrite shown here, have three perpendicular axes of equal length. The cubic-isometric structure is the most symmetrical of all the crystals. The pyrite crystal system forms rocks that are fairly hard, but quite brittle. Pyrite is also known as fool's gold because of its yellow color and metallic luster.

Tetragonal

This system comprises crystals with three axes, all perpendicular to one another; two are of equal length.

This Siberian idocrase has a tetragonal crystal structure. Its axes are all perpendicular and two are of equal length. Idocrase is grouped with rocks such as zircon, rutile, and wulfenite, which are rocks of medium hardness that may possess a diamond-like fire.

Orthorhombic

This system comprises crystals with three mutually perpendicular axes, all of different lengths.

Barite, from which barium comes, has an orthorhombic crystal structure. It has three mutually perpendicular axes of different lengths. Barite exhibits perfect cleavage, which means it splits easily along specific, intersecting planes.

Monoclinic

This system comprises crystals with three axes of unequal lengths, two of which are oblique (not perpendicular) to one another, but both of which are perpendicular to the third.

Gypsum is an example of a mineral exhibiting a monoclinic crystal structure. Monoclinic crystals have three axes of unequal length, two of which are perpendicular to the third axis, but not to each other. A soft, sedimentary rock, gypsum is the source of plaster of Paris and also has applications in agriculture and construction.

Triclinic

This system comprises crystals with three axes, all unequal in length and oblique to one another.

Triclinic crystals exhibit the least symmetry of the crystal systems. Their axes are unequal and do not intersect at right angles anywhere. This Brazilian axinite is an example of a triclinic crystal.

Hexagonal

This system comprises crystals with four axes. Three of these axes are in a single plane, symmetrically spaced, and of equal length. The fourth axis is perpendicular to the other three

A hexagonal crystal such as beryl, shown here, has four axes of symmetry. Three of the axes are of equal length, and are symmetrically placed in one plane. The fourth axis is perpendicular to the others.

Some crystallographers split the hexagonal system in two, calling the seventh system thus formed trigonal or rhombohedral.

A few elements and compounds can crystallize in two different systems, giving rise to substances which, although identical in chemical composition, are different in virtually all their physical properties. For example, carbon crystallizes in the isometric system to form diamond, and in the hexagonal system to form graphite. Although diamond is in the same system as salt and garnet, it is in a different class. It crystallizes in tetrahedrons (solids with 4 faces) or octahedrons (solids with 8 faces); the latter is possible in the garnet-salt class, the former is not.

Other Crystal Properties

The habit of any mineral includes many other properties based on crystalline structure. For example, argentite, a common silver ore, crystallizes in the same class as garnet and salt, but usually occurs in irregular cryptocrystalline masses. Fluorite, another common mineral, also crystallizes in the same class and usually forms cubes; when fractured, florite tends to cleave into perfect octahedral fragments. Salt forms cubic cleavage fragments, and garnet has no well-developed cleavage planes. Some substances tend to form multiple crystals growing through one another.

Some crystals, when compressed, develop electrical charges at their ends; other crystals develop similar charges when heated. These properties, called piezoelectricity and pyroelectricity respectively, are both shown to a marked degree by quartz. For this reason, quartz crystals are used in sonar and in many types of radio apparatus. In the transistor special properties of germanium and silicon crystals are utilized for amplifying electric current. Another electronic device, the solar battery, utilizes a silicon or cadmium sulfide crystal to convert sunlight into electrical energy.

Much work has been done in recent years on the preparation of single crystals of substances that are normally cryptocrystalline. Large, single crystals of metals, for example, can be grown by a number of methods, the simplest of which is to melt the metal in a conical vessel, and then lower the vessel slowly from the furnace, point first. If the proper conditions exist, a single seed forms at the point of the cone and continues to grow until it has filled the vessel. Such single crystals often differ markedly from metals in their usual form. Pure and specially designed crystals are now also produced by advanced techniques such as molecular-beam epitaxy for use in semiconducting devices, integrated circuits, and various other important systems of modern technology.

When X rays pass through the symmetrically arranged atoms of a crystal, the atoms act as a diffraction grating, deflecting the rays in regular patterns. Photographs taken of these patterns give scientists a basis for deducing many facts concerning the nature of the crystal. The actual arrangements of atoms in crystals can be revealed in images produced by transmission electron microscopes (see Microscope) and field-ion emission devices.

One basic rule of crystallography has long been that no crystal structure can have fivefold, or pentagonal, symmetry. It was thought that such symmetry could not exhibit the translational periodicity required of crystals. In 1984, however, a group of scientists found an alloy of aluminum and magnesium that seems to break that rule. The alloy's diffraction pattern shows it to have the rotational symmetry of an icosahedron, or 20-faced solid, with 10 threefold axes and 6 fivefold axes of rotational symmetry. This discovery opens up the possibility that a whole new phase of organized solid matter may exist, distinct from crystals and glasses.