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Mineralogy

Mineralogy

Mineralogy is the study of minerals . Rocks in the earth's crust are composed of one or more minerals. A mineral in the geologic sense is a naturally occurring, inorganic, crystalline solid. A particular mineral has a specific chemical composition. Each mineral has its own physical properties such as color, hardness, and density.

Most minerals are chemical compounds that are made of two or more different elements. The composition of a mineral is shown by its chemical formula, which states each of the chemical elements present in the mineral as well as the ratios of each element. For example, the mineral quartz has the chemical formula SiO2. This means that quartz is made of the elements silicon (Si) and oxygen (O). The formula also shows that for every one silicon atom , two oxygen atoms are present. The mineral orthoclase has the chemical formula KAlSi3O8. A molecule of orthoclase contains one potassium (K) atom, one aluminum (Al) atom, three silicon atoms, and eight oxygen atoms. Some minerals always have the same chemical formula. Quartz always is composed of SiO2 and halite is always made of sodium (Na) and chlorine (Cl), with the chemical formula NaCl. Some minerals can have more than one chemical formula, depending on their composition. Sometimes an element can substitute for another in a mineral. This occurs when the atoms of two elements are the same charge and close to the same size. For example, an iron (Fe) atom and a magnesium (Mg) atom are both about the same size, so they can substitute for each other. The chemical formula for the mineral olivine is (Mg,Fe)2SiO4. The (Mg,Fe) indicates that either magnesium, iron, or a combination of the two may be present in an olivine sample.

Native elements are minerals that are composed of only one element. These are the substances that dietitians call minerals. Examples of native elements include gold (Au), silver (Ag), and platinum (Pt). Two other native elements are graphite and diamond , both of which are entirely made of carbon (C).

All minerals are crystalline solids. A crystalline solid is a solid consisting of atoms arranged in an orderly three-dimensional matrix. This matrix is called a crystal lattice. A crystalline solid is composed of molecules with a large amount of order. The molecules in a crystalline solid occupy a specific place in the arrangement of the solid and do not move. The molecules not only occupy a certain place in the solid, but they are also oriented in a specific manner. The molecules in a crystalline solid vibrate a bit, but they maintain this highly ordered arrangement. The molecules in a crystalline solid can be thought of as balls connected with springs. The balls can vibrate due to the contractions and expansions of the springs between them, but overall they stay in the same place with the same orientation. It is not easy to deform a crystalline solid because of the strong attractive forces at work within the structure. Crystalline solids tend to be hard, highly ordered, and very stable.

Under ideal conditions, mineral crystals will grow and form perfect crystals. An ideal condition would be in a place where the crystals are allowed to grow slowly without disturbances, such as in a cavity. A perfect crystal has crystal faces (planar surfaces), sharp corners, and straight edges. The external crystal form is controlled by the internal structure. When the atoms in a crystal are arranged in a perfectly orderly fashion, the crystal will also be formed in a perfect orderly fashion. Even if a perfect crystal is not formed, the internal crystalline structure can be shown. Many minerals exhibit a property called cleavage. A mineral that has cleavage will break or split along planes. If the internal structure is formed in an orderly crystal arrangement, then the breaks will occur along the planes of the internal crystal structure.

There have been over 3,500 minerals identified and described. Only about two dozen of these are actually common. There are a limited number of minerals, mainly because there are only a certain number of chemical elements that can combine to form chemical compounds. Some combinations of elements are unstable, such as a potassium-sodium or a silicon-iron compound. In addition, only eight elements are found abundantly in the earth's crust, where minerals are formed. Oxygen and silicon alone account for more than 74% of the earth's crust. These factors place a limit on the number of possible minerals.

The minerals that have been discovered and studied can be placed into one of five groups. These groups are the silicate minerals, carbonate minerals, oxides, sulfides, and halides. The silicate minerals are those that contain silica, a combination of silicon and oxygen. Examples of silicates include quartz, orthoclase, and olivine. The silicate minerals are the most common, making up approximately one-third of all known minerals. They are composed of building blocks called the silica tetrahedron. A silica tetrahedron is one silicon atom and four oxygen atoms. The atoms are arranged in a four-faced pyramidal structure (the tetrahedron) with the silicon atom in the center. The silicon atom has a +4 charge, and each of the four oxygen atoms have a 2 charge. As a result, a silicon tetrahedron has a net charge of 4. Because of this charge, it does not occur in isolation in nature. A silica tetrahedron is always bound to other atoms or molecules.

There are two types of silicate minerals, the ferromagnesian and the nonferromagnesian silicates. Ferromagnesian silicates are those containing iron, magnesium, or both. These minerals tend to be dark colored and more dense than the nonferromagnesian silicates. An example of a ferromagnesian silicate is olivine. Nonferromagnesian silicates do not have iron and magnesium. These minerals are light colored and less dense. The most common nonferromagnesian silicates are the feldspars.

The carbonate minerals contain the carbonate ion, (CO3)2. Calcite (CaCO3), the main component of limestone , is an example of a carbonate mineral. The oxides are minerals that contain an element combined with oxygen. An example of an oxide is hematite, Fe2O3. The sulfides contain a cation combined with sulfur (S2). An example of a sulfide is galena (PbS), which is lead (Pb) combined with sulfur. The halides all contain halogen elements, such as chlorine and fluorine (F). Examples of halite minerals include halite (NaCl) and fluorite (CaF2).

All minerals posses specific physical properties such as color, luster, crystal form, cleavage, fracture, hardness, and specific gravity . The physical characteristics of a mineral depend on its internal structure and chemical composition. The physical properties of minerals can be used for identification purposes by mineralogists. Color is the least reliable of the physical properties. Many minerals display a variety of colors due to impurities. Some generalizations can be made, however. Ferromagnesian silicates, for example, are usually black, brown, or dark green. The luster of a mineral refers to the way in which light is reflected off the mineral. Two types of luster can be displayed: metallic or nonmetallic.

The crystal form of a mineral is also a physical property specific for the type of mineral being observed. This property is most easily observed when the mineral has formed a perfect crystal. Cleavage is the tendency of a mineral to break or split along planes. There are different types of cleavage that correspond to the different internal crystal structures that make up individual minerals. Fracture occurs when a mineral does not break along smooth planes, rather along irregular surfaces. Some minerals display cleavage, others display fracture.

The hardness of a mineral is its resistance to being scratched. The Mohs hardness scale can be used to determine how hard a mineral is by determining what will scratch its surface. The specific gravity of a mineral is the ratio of its density to the density of water . For example, a mineral with a specific gravity of 4.0 is four times as dense as water, meaning that an certain volume of the mineral would weigh four times as much as an equal volume of water. The specific gravity of a mineral is determined by its composition and structure.

Mineralogy is an interesting science that studies the nature of mineralsnaturally occurring, inorganic crystalline solids. There are many different minerals, each with its own properties determined by its chemical composition. Mineralogists continue to search for new, useful minerals in the earth's crust.

See also Chemical bonds and physical properties; Crystals and crystallography; Ferromagnetic; Mohs' scale

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Mineralogy

MINERALOGY

MINERALOGY. Observations on minerals in the New England and Virginia colonies appear in the writings of John Josselyn and other early-seventeenth-century travelers. In the mid-seventeenth century John Winthrop, son of the first governor of the Massachusetts Bay Colony, actively engaged in the search for and development of mineral deposits. His grandson, John Winthrop Jr., formed a notable mineral collection that was presented to the Royal Society of London in 1734 and later incorporated into the British Museum. Throughout the colonial period, questions about the nature and use of minerals and rocks and about the development of known mineral deposits were usually answered by sending specimens or trial shipments of ore abroad or by importing experts from Europe. The first professional study of mineralogy in America began after the Revolution—led by Adam Seybert, Gerard Troost, and the mineral chemist James Woodhouse, all of Philadelphia. The first mineral collections of scientific importance began to be acquired at about the same time. Most of the specimens were brought from Europe, chiefly by Americans traveling abroad for educational purposes and by immigrants of scientific or technological bent. It was the acquisition of these European collections, with their store of correctly identified and labeled material illustrating European textbooks, that provided the basis for American study instruction.

The formal teaching of mineralogy—the term then usually included earth history and other aspects of geology—began in American colleges shortly before 1800. Benjamin Waterhouse, a Rhode Island Quaker who had been trained in medicine and the natural sciences in Leyden and London, lectured on mineralogy and botany at Rhode Island College (later Brown University) in 1786 and at the medical school at Harvard between 1788 and 1812.

The first textbook on mineralogy written in the United States, Parker Cleaveland's Elementary Treatise on Mineralogy and Geology, was published in Boston in 1816. Cleaveland, a Harvard graduate of 1799, was self-taught in mineralogy. The work received good reviews in Europe and remained a standard text for many years. In 1837 James Dwight Dana of Yale brought out the System of Mineralogy, which became an international work of reference, reaching a sixth edition in 1892. Both of these books drew heavily on European works, especially those of the German Friedrich Mohs and of the French crystallographer R. J. Haüy.

The most rapid progress in mineralogy in the United States took place in the first three decades of the nineteenth century, as New England colleges sought to add the sciences to their theological and classical curricula. The leading figure was Benjamin Silliman, appointed professor of chemistry and natural science at Yale in 1802. Silliman was active as a teacher, editor, and public lecturer, rather than as a researcher. Among his students, Amos Eaton, Charles Upham Shepard, and Dana became important in the further development of the geological sciences.

The marked growth of the geological sciences in American colleges in the early nineteenth century was accompanied by the formation of numerous state and local academies, lyceums, and societies concerned with natural history. These organizations afforded public platforms from which such men as Silliman and Eaton and, later, Louis Agassiz spread scientific ideas. The Academy of Natural Sciences of Philadelphia, organized in 1812, was a leading factor; it began the publication of its journal in 1817 and of its proceedings in 1826. The Boston Society of Natural History, formed in 1830—the year the first state geological survey was begun, in Massachusetts—and the Lyceum of Natural History of New York, organized in 1817, also were important. The American Journal of Science and Arts, started by Silliman in 1818, published the bulk of American mineralogical contributions for the next five decades. (A forerunner, the American Mineralogical Journal, edited by Archibald Bruce of New York City, had published only four issues, 1810–1814.)

Toward the middle of the nineteenth century, courses in analytical chemistry, emphasizing ores, minerals, and agricultural materials, were introduced into many colleges and medical schools. Mineral chemistry and geochemistry developed strongly during the late 1800s, fostered especially by the U.S. Geological Survey, organized in 1879, and American work on minerals and rocks was outstanding in those fields. The publications of the U.S. Geological Survey and of the state geological surveys carried much descriptive mineralogical and petrographic material. Toward the end of the nineteenth century, as the organization and interests of science enlarged and specialized, the various academies and their attendant periodicals were joined and ultimately virtually supplanted by national and regional professional societies. The Mineralogical Society of America was founded in 1919 and continues today. The American Mineralogist, an independent journal first published in 1916, became its official journal.

The great private mineral collections, to which public museums and universities are deeply indebted, were developed during the last decades of the nineteenth century and the first decades of the twentieth century, a period coinciding with the major development of America's mineral resources and the accumulation of fortunes from mining in the West. Commercial dealing in mineral specimens, as by A. E. Foote of Philadelphia, developed on a large scale. Exhibits at national and international fairs, notably at the St. Louis World's Fair of 1904, also spread interest.

Crystallography, particularly in its theoretical aspects, did not attract much attention in the United States during the nineteenth century, when American interest in minerals was primarily concerned with chemical composition, occurrence, and use. It was not until the early twentieth century that advanced instruction and research in the formal aspects of crystallography became widespread. Charles Palache of Harvard was one of the leaders.

Over the course of the twentieth century, the boundaries of the discipline of mineralogy shifted with the change in economic and social priorities. As scientific interest in the traditional extractive resources—particularly iron ore and precious metals—declined in relation to research in oil exploration, nuclear waste storage, and earthquake and volcano prediction, mineralogy became an umbrella term for an array of highly technical fields, including petrology, crystallography, geochemistry, and geophysics.

BIBLIOGRAPHY

Chandos, Michael Brown. Benjamin Silliman: A Life in the Young Republic. Princeton, N.J.: Princeton University Press, 1989.

Greene, John C., and John G. Burke. The Science of Minerals in the Age of Jefferson. Philadelphia: American Philosophical Society, 1978.

Oldroyd, David R. Sciences of the Earth: Studies in the History of Mineralogy and Geology. Brookfield, Vt.: Ashgate, 1998.

CliffordFrondel/a. r.

See alsoGeological Survey, U.S. ; Geological Surveys, State ; Geology .

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mineralogy

min·er·al·o·gy / ˌminəˈräləjē; -ˈral-/ • n. the scientific study of minerals. DERIVATIVES: min·er·al·og·i·cal / ˌmin(ə)rəˈläjikəl/ adj. min·er·al·og·i·cal·ly / ˌmin(ə)rəˈläjik(ə)lē/ adv. min·er·al·o·gist / -jist/ n.

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mineralogy

mineralogy The scientific study of minerals, comprising crystallography, mineral chemistry, economic mineralogy, and determinative mineralogy (concerned mainly with physical properties).

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mineralogy

mineralogy Investigation of naturally occurring inorganic substances found on Earth and elsewhere in the Solar System. See geochemistry; mineral; petrology

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mineralogy

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