A crystal is a solid in which the atoms have a definite, orderly atomic structure, and an outward form bounded by smooth, plane surfaces, symmetrically arranged. Crystals are 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 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
The process of forming a crystal from a fluid or from materials dissolved in the fluid is called crystallization. Which crystal structure the fluid will form depends on the chemistry of the fluid, the conditions under which it is being solidified, and also on the ambient pressure. While the cooling process usually results in the formation of a crystalline material, under certain conditions, the fluid may be frozen in a non-crystalline state. In most cases, this involves cooling the fluid so rapidly that atoms cannot travel to their lattice sites before they lose mobility. A non-crystalline material is called an amorphous, vitreous, or glassy material.
Polymorphism is the ability of a solid to exist in more than one crystal form. For example, water ice is ordinarily found in the hexagonal form but can also exist in the cubic form.
The shape of crystals is dependent on the types of molecular bonds between the atoms to determine the structure, as well as on the conditions under which they formed. Snowflakes, diamonds, and common salt are common examples of crystals.
Some crystalline materials may exhibit special electrical properties such as the piezoelectric effect such as Quartz does. Additionally, light passing through a crystal is often refracted or bent in different directions, producing an array of colors; crystal optics is the study of these effects Crystallography is the scientific study of crystals and crystal formation.
Inorganic matter, if free to take that physical state in which it is most stable, always tends to crystallize. Crystalline rocks form from aqueous solutions or from molten magma. The vast majority of igneous rocks belong to this group and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as granite, which have cooled very slowly and under great pressures, have completely crystallized, but many lavas were poured out at the surface and cooled very rapidly; in this latter group a small amount of amorphous or glassy matter is frequent. Other crystalline rocks, the evaporites such as rock salt, gypsum and some limestones have been deposited from aqueous solutions, mostly owing to evaporation in arid climates. Still another group, the metamorphic rocks which includes the marbles, mica-schists and quartzites; are re-crystallized, that is to say, they were at first fragmental rocks, like limestone, shale and sandstone and have never been in a molten condition nor entirely in solution. The high temperature and pressure conditions of metamorphism have acted on t he existing rocks to re-crystallize them into different rocks.
Conditions for Formation
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.
Granite, rhyolite, and felsite are not homogeneous and cannot be single crystals, but they are crystalline rocks. Each of the constituent minerals in these rocks is present in the form of small, but homogeneous crystals. Those substances that first solidified during the cooling of molten rock exhibit a normal arrangement of crystal faces. Those that, because of lower freezing points, solidified later were forced to occupy the remaining interstices, so that their external appearance is deformed.
Crystal growth is attained when a minute crystal that has formed abstracts more of the same mineral constituent from its environment. Sometimes, in the absence of this first minute crystal, or seed, crystallization does not take place, and the solution becomes supersaturated, just as a liquid below its freezing point becomes supercooled.
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
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. Despite their differences in habit (shape of crystallization), salt and garnet always crystallize in the same crystal system. Almost all minerals fall 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 are of great importance to mineralogists and gemologists; specification of the system is necessary in the description of any mineral.
This system comprises crystals with three axes, all perpendicular to one another and all equal in length.
This system comprises crystals with three axes, all perpendicular to one another; two are of equal length.
This system comprises crystals with three mutually perpendicular axes, all of different lengths.
This system comprises crystals with three axes of unequal lengths, two of which are oblique (that is, not perpendicular) to one another, but both of which are perpendicular to the third.
This system comprises crystals with three axes, all unequal in length and oblique to one another.
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 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
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.
References: Excerpts and general information were borrowed from Wikipedia and Encarta, as well as other sources.
Crystal Structure Crustalline Rocks Conditions For Formation
Crystallography Crystal Systems Other Crystal Properties