Background (Encyclopedia of Global Resources)
Crystals are solids that naturally display smooth planar exterior surfaces called “faces,” which form during the growth of the solid. These faces collectively produce a regular geometric form that mimics the orderly internal atomic arrangement of the elements present in the solid. Some scientists use the term “crystal” to refer to any solid having an ordered internal atomic structure regardless of whether the solid displays faces. However, most scientists use the word “crystalline” for such solids when no faces are present. Many solids display a cleavage, a flat planar surface formed when the solid is broken; cleavage fragments are sometimes mistaken for crystals.
Crystals are described and classified according to the symmetrical relationship existing between the faces. The fundamental way of describing a crystal is to list the “forms” that it displays. Scientists recognize a total of forty-eight different forms, many designated by common geometric terms such as cube, octahedron, tetrahedron, pyramid, and prism. Most crystals display multiple forms. For example, quartz crystals display one prism and at least two sets of pyramids. Considering every possible symmetrical arrangement of faces, every crystal can be placed into one of thirty-two groupings called crystal classes. These classes are further grouped into six crystal systems based on similar symmetry characteristics. The names of the six systems, from most...
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Where Crystals Are Formed (Encyclopedia of Global Resources)
Large crystals can develop when the faces growing in a melt, solution, or gas are unimpeded by other surrounding solids. This situation commonly occurs where open cracks and cavities exist in rock and the liquid or vapor from which the crystal is growing has free access to the open space. The largest crystals are found in igneous pegmatites. The Etta pegmatite in the Black Hills of South Dakota contained a 12-meter crystal weighing more than 18 metric tons. The largest known crystal was a single feldspar from a pegmatite in Karelia, Russia, that weighed several thousand metric tons. Crystals are also found along fault planes, in hot springs areas, around vents for volcanic gases, and in cavities within igneous and sedimentary rocks where underground water is circulating. Another mechanism for the growth of crystals occurs during the process of metamorphism. Preexisting rocks that are subjected to elevated temperatures and pressures within the Earth can recrystallize while still solid. During this metamorphism some of the new minerals that form have a strong surface energy and will develop faces even while in contact with other growing minerals.
The growth conditions discussed above are so common within the Earth that crystals can be found in almost every state in the United States and every country in the world. It is impossible to specify all the important occurrences of large, well-formed crystals. Some of the...
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Uses of Crystals (Encyclopedia of Global Resources)
Particularly well-formed crystals are highly prized by collectors and museums. Most crystals, however, are more valuable for their chemistry or as crystalline solids. Many crystals are crushed during the processing of ore minerals. It was a common practice for miners to save the larger, better-formed crystals from the crushing mill because they were worth more as specimens for collectors than they were worth as ore material. Most crystals of gem minerals are cut and faceted to make jewelry. A large diamond crystal, for example, is worth more as a well-faceted gemstone than as a crystal specimen.
There are a growing number of technological uses of “crystalline solids” where the systematic internal arrangement of atoms can produce a variety of desirable physical phenomena useful in the fields of electronics and optics. As an example, very pure untwinned quartz is called “optical grade crystal” even though it lacks faces.Quartz crystal is cut, ground, and made into lenses and prisms for optical instruments and is also used in radio oscillators, timing devices, and pressure gauges in the electronics industry.
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Crystal Defects and Growth Rates (Encyclopedia of Global Resources)
Crystal defects occur naturally as crystals are formed; they are also sometimes introduced artificially, as they have useful electrical, mechanical, and optical qualities. A growing crystal typically requires the proper placement of trillions of atoms per hour. About one atom in every one hundred thousand is misplaced to form a defect. These defects can be point disorders, or they can geometrically be combined to form line, plane, or three-dimensional disorders. The Schottky defect is a point disorder in which an atom is missing from the spot it should occupy, leaving a hole in the pattern. The Schottky defect results when a second layer of atoms is quickly deposited before all the positions can be filled in the first layer. The Frenkel defect occurs when an atom is out of its proper position and can be found nearby, inappropriately stuck between other atoms. The impurity defect is yet another point disorder, occurring when an atom of a foreign element (an impurity) either substitutes for the normal atom or is stuffed between the proper atoms of the structure.
Coloration can be caused by various point defects. When an electron is captured by the hole of a Frenkel defect it will absorb energy from passing light and become what is known as a “color center.” An abundance of Frenkel color centers in fluorite will give the crystal a purple color. An impurity defect can be accompanied by a shift in electrons,...
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Twinning (Encyclopedia of Global Resources)
During formation, a solid may produce a symmetrical intergrowth of two or more crystals. When the intergrowth is crystallographically controlled, the resulting composite is called a twinned crystal. The individual crystals within the twinned aggregate are related to one another by a different symmetrical element—one that is not seen in any of the individual parts. This often results in a symmetrical, exotically shaped aggregate that does not appear to belong to any single crystal class. Crystals displaying exceptional twins can be more valuable for their twinning than as mineral specimens.
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History of Crystals (Encyclopedia of Global Resources)
Crystals have a history that reaches back into the realm of legends and myths. An important early work that combined legend with the first sound science was the thirty-seven-volume Historia Naturalis, written by Pliny the Elder in the first century. Pliny described many real as well as nonexistent crystals, which he stated were formed by such exotic processes as “the light of the moon” or “the purge from the sea.” Nicolaus Steno established the first law of crystallography in 1669, known as the law of constancy of interfacial angles. The law holds that for all crystals of a given mineral the angles measured between similar faces are always exactly the same. This law allows for the positive identification of deformed or malformed crystals simply by measuring the angles between existing faces. In 1781, René-Just Haüy was the first to recognize that a crystal is composed of a large number of smaller particles arranged in a regular geometric order such that it fills space without gaps. This was a remarkable advance, considering that it preceded the concept of the atom in chemistry by more than twenty years. In 1830, based on graphical and mathematical considerations, Johann Hessel predicted the existence of thirty-two classes of symmetry corresponding to modern crystal classes. In the 1920’s, two crystallographers, C. H. Hermann and Charles-Victor Mauguin, developed the notation that is used to designate...
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Further Reading (Encyclopedia of Global Resources)
De Graef, Marc, and Michael E. McHenry. Structure of Materials: An Introduction to Crystallography, Diffraction, and Symmetry. Cambridge, England: Cambridge University Press, 2007.
Klein, Cornelis, and Barbara Dutrow. The Twenty-third Edition of the Manual of Mineral Science. 23d ed. Hoboken, N.J.: J. Wiley, 2008.
Read, P. G. Gemmology. 3d ed. Boston: Elsevier/Butterworth-Heinemann, 2005.
Smyth, Joseph R., and David L. Bish. Crystal Structures and Cation Sites of the Rock-Forming Minerals. Boston: Allen & Unwin, 1987.
Tilley, Richard J. D. Crystals and Crystal Structures. Hoboken, N.J.: John Wiley, 2006.
Wenk, Hans-Rudolf, and Andrei Bulakh. Minerals: Their Constitution and Origin. New York: Cambridge University Press, 2004.
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Crystals (Science Experiments)
Shapes and structures
Crystal Structure: Will varying shape crystals form from varying substances?
Design Your Own Experiment
Crystals affect your life in countless ways, from what you eat to how your computer works. Any solid matter whose particles are arranged in a regular and repeated pattern is called a . The type of particle and its geometric pattern determine the properties of the crystal. Salt, sugar, and rubies are all crystals, along with many metallic elements, such as iron. Both natural rock and artificial materials are often crystalline. Our bones even contain tiny crystals of a mineral called apatite.
All crystals have flat, smooth surfaces, called facesThe flat, smooth surfaces of a crystal.. Some crystals, such as diamonds, are formed over millions of years, while others, such as snowflakes, are formed in a matter of hours. Crystals of the same substance have the same geometric pattern between its particles. This pattern is called a . In crystals the smallest possible repeating structural unit is called a unit cellThe basic unit of the crystalline structure.. The unit cell is repeated...
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