A mineral is a natural, homogeneous, inorganic solid with a crystalline atomic structure. Crystallinity implies that a mineral has a definite and limited range of composition, and that the composition is expressible as a chemical formula. Some substances that do not satisfy all these conditions, such as metallic liquid mercury, are commonly considered in the mineral realm but should more properly be designated mineraloids. The word mineral may have different meanings in nonmineralogical sciences. In nutrition, it may mean any nonorganic element. In economics and economic geology, minerals may be practically anything of value extracted from the Earth, including petroleum and natural gas (which are not minerals according to the above geological definition, being neither inorganic nor solid). Minerals comprise the vast majority of the material of the solid Earth. Aside from air, water, and organic matter, practically the only nonminerals in the Earth as a whole are molten rocks (magmas) and their solid glassy equivalents. Crystalline rocks themselves, and even soils for the most part, consist of aggregates of minerals. Almost all inorganic substances that are used by or of value to humans are derived from minerals. Over 3,000 minerals are currently known, and about 50 new ones are now discovered each year. Most \Tgems\t are minerals, though some, such as opal, are mineraloids. HISTORY OF MINERALOGY Mineralogy is the study of the nature and origin of minerals. Although it is not one of the fundamental sciences, it was one of the first scientific fields to be developed, and curiosity about minerals led to many discoveries in physics and chemistry. Georgius \TAgricola\t (Latinized name of Georg Bauer), sometimes considered the founder of mineralogy, summarized most early knowledge of minerals in his works De Natura Fossilium (On the Nature of Fossils, 1546; Eng. trans., 1955) and De Re Metallica (On Metals, 1556; Eng. trans., 1912). The discovery by Nicolaus \TSteno\t, Arnould Carangeot (1742-1806), and Jean Baptiste Rome de l'Isle (1736-90) in the 17th and 18th centuries that interfacial angles of \Lcrystal\ls of a given mineral are constant, laid the basis for the science of crystallography. The founder of crystallography, however, is considered to be Rene Just \THauy\t, whose most important works were published in 1784 and 1801. He proposed correctly that crystals are formed by stacking of identical structural blocks, now called unit cells, and he showed that, as a consequence, the intercepts that the crystal faces make on a set of carefully chosen axes are always rational numbers when divided by an appropriate common factor. Other early-19th-century scientists who contributed to the development of crystallography include Johann F. C. Hessel (1796-1872), William H. Miller (1801-80), and Auguste Bravais (1811-63). Hauy's contemporary Abraham Gottlob \TWerner\t and his followers developed methods for identifying crystals by physical characteristics. The science of chemistry developed rapidly in the early 19th century, and the determination of the compositions of minerals proceeded apace, especially through the efforts of Jons Jakob \TBerzelius\t and his followers. The first lasting classification of minerals according to chemical composition was published by James Dwight \TDana\t in 1837. The elucidation of the optical properties of crystals took place in the early 19th century, with noteworthy work being done by Jean Baptiste \TBiot\t and Sir David Brewster. The invention of the polarizing microscope and the use of thin sections by William Nicol (1768-1851) made it possible to identify minerals by means of their optical properties, and Henry Clifton \TSorby\t, who is considered to be the founder of petrography, later applied these methods to the study of rocks. The simultaneous discovery in the late 19th century by Arthur M. Schoenflies (1853-1928), William Barlow (1845-1934), and E. S. Fedorov (1853-1919) of the 230 space groups (see below) essentially completed the development of classical crystallography. With the discovery (1912) of X-RAY \Tdiffraction\t by Max von \TLaue\t, Walter Friedrich, and Paul Knipping, the determination of the internal atomic structures of crystals became possible. Sir William H. \TBragg\t, Sir William L. \TBragg\t, and others immediately began to use this tool to determine crystal structures of minerals, and this work continues to the present. X-ray diffraction also provided a rapid and objective means for identifying minerals. The chemical analysis of minerals advanced importantly with the invention (1949) of the electron microprobe by R. Castaing. The development of high-speed electronic computers has greatly facilitated the determination of the crystal structures of minerals and other substances. CRYSTALLOGRAPHY AND CRYSTAL CHEMISTRY Crystals consist of unit cells in the shape of parallelepipeds, stacked like bricks to form a homogeneous solid. The unit cell is not a physically separable entity, such as a molecule; it simply describes the repeat pattern of the structure. In fact, most minerals cannot be separated into molecules; the chemical bonds in the structure form a continuous network. The dimensions of unit cells are on the order of a few angstroms (10(-8) cm), and the number of atoms contained in the volume of a unit cell is usually between two and a few hundred. The lattice is the collection of unit-cell corners, and the crystal axes are usually considered to be parallel to the unit-cell edges. Crystals have varying amounts of symmetry, that is, repetition of the basic structural elements by symmetry operators such as rotation axes and mirror planes. All crystals may be assigned to one of 230 space groups, or possible combinations of symmetry elements in crystal lattices. In terms of the symmetry of the external faces and the physical properties, however, only 32 combinations, called crystal classes, are possible. These are further divided into six (or seven) crystal systems--\Ttriclinic\t, \Tmonoclinic\t, \Torthorhombic\t, \Ttetragonal\t, \Thexagonal\t (including trigonal), and \Tisometric\t (cubic)-- for each of which a different set of crystal axes is required. For example, in the triclinic system, of lowest symmetry, the three crystal axes are of unequal length and make no special angles with one another; in the isometric system, the three axes are all the same length and make 90 deg angles with one another, so that the unit cell has the shape of a cube. The objectives of mineralogical crystallography are to determine the crystal symmetry, the size and shape of the unit cell, and the arrangement of atoms within the unit cell of each mineral. This is now done almost entirely by X-ray diffraction, although it is still often necessary to consult the external morphology to determine the symmetry. The principal tools of the crystallographer are the various single-crystal X-ray cameras, for determining unit-cell size and shape, and the single-crystal X-ray diffractometer, for measuring the intensities of the diffraction maxima, from which the atomic arrangement can be deduced. Determining the structure is a complex, indirect procedure for which computers are now almost essential. The reflecting \Tgoniometer\t is used to measure external morphology. Crystal chemistry is the study of the principles of the atomic structures of crystals. Different principles are applicable depending on the type of \Lchemical bond\ling in the mineral. Oxides and silicates have predominantly ionic bonding, with some admixture of covalent bonding; the important silicon-oxygen bond is considered to be about half ionic and half covalent. For these minerals the rules devised by Linus Carl \TPauling\t for the structure of complex ionic crystals have been useful in predicting and rationalizing the structures. The first rule concerns the geometric possibilities for packing different-sized spheres, which ions may be considered to be. It states that each cation (positive ion) will be surrounded by a coordination polyhedron of anions (negative ions), the coordination number being determined by the radius ratio r (cation)/r (anion). The coordinating anions are considered to be at the corners of the polyhedron, so that four anions constitute a tetrahedron, six an octahedron, and so on. The second rule provides for the minimization of electrostatic potential energy, on the principle that ions with like charges should be separated as much as possible. The electrostatic-bond strength contributed by a cation is defined as the charge of the cation divided by its coordination number. The sum of the electrostatic bonds reaching an anion (contributed by the cations in contact with it) should equal the formal charge of the anion (with sign reversed). Crystal-field theory also is important for ionic minerals containing transition-metal ions. For sulfide minerals, which are primarily covalent, the dominant principles are the stereochemical aspects (see \Tstereochemistry\t) of covalent bonds. Each atom can form only a limited number of covalent bonds at certain angles to each other. CHEMICAL MINERALOGY The compositions of minerals were formerly determined by so-called wet-chemical analysis, in which the mineral is dissolved, usually in an acid solution, and either its constituent elements are separately precipitated out and weighed, or else their concentrations in solution are determined by spectroscopic methods. For this type of analysis, a pure sample of the mineral must be separated from the rock in which it occurs; this separation may require the use of heavy liquids. A more versatile method is the electron microprobe, in which a beam of electrons is directed onto a polished surface of the mineral sample. This bombardment by electrons causes each element in the sample to emit X rays of characteristic wavelength. The intensity of each of these different X rays is then measured and compared with those given off by a standard material of known composition, under the same conditions of bombardment. If the standard is a natural mineral, it must have been analyzed previously by wet-chemical or some other method. The electron beam can be focused to a diameter of about 1 micrometer; thus exceedingly small mineral grains, or different parts of the same grain, can be analyzed without separating them from the rock in which they occur. The composition of most minerals varies within limits. This variation is known as isomorphous \Tsolid solution\t between two or more end members. The term isomorphous (see \Tisomorph\t) signifies that the end members have essentially the same crystal structure. For example, olivine is a solid solution between the end members forsterite and fayalite. The elements Mg and Fe are said to substitute isomorphically for one another; they occupy the same position structurally in the mineral. The composition of solid solutions can often be determined by measurement of physical properties, such as refractive index, density and unit-cell size, as well as by direct analysis. Because they are often dependent on the conditions of crystallization, particularly temperature, the compositions of minerals can be important in determining the geologic history of a rock. Minerals forming solid solutions at high temperatures may unmix, or exsolve, at lower temperatures. The presence or absence of exsolution is also a clue to the geologic history of a mineral. Not only can a given mineral structure have variable composition, but also a given composition may have several different crystal structures, or \Lpolymorph\ls. Each structural modification is usually given a separate mineral name, as each usually is formed under different conditions. For example, the most common structural modification, or polymorph, of silica is quartz, but two polymorphs, tridymite and cristobalite, can form at higher temperatures, and two others, coesite and stishovite, can form at higher pressures. MINERAL IDENTIFICATION The description of the physical properties of minerals is usually made with a view toward identification. There are three principal approaches to identification of minerals: hand specimen methods, optical methods, and X-ray diffraction. Hand Specimen Methods Essentially all minerals possess attributes that can be determined by sight or with simple tools. These attributes include color; \Tluster\t, the general appearance of the surface in reflected light, such as metallic, resinous, vitreous, or greasy; \Tstreak\t, the color of the powder when the mineral is rubbed on a white porcelain plate; habit, the general shape of the crystal or aggregates of crystals, such as acicular, tabular, or bladed; morphology, the external structure of mineral crystals, such as a cube, octahedron, or prism; twinning, the intergrowth of two or more crystals of the same mineral with certain regular geometric relationships (see TWIN \Tlaw\t); \Tcleavage\t, the preferential breaking of a mineral on crystallographic planes of weakness; parting, the breakage along a plane of junction between closely related crystals, such as exsolution or twin planes; and fracture, the character of a broken, noncleavage surface, such as conchoidal (shell-like or curving), fibrous, or hackly. \Thardness\t is usually measured comparatively, by means of a scale of ten minerals--the Mohs scale--ranging from talc (1, the softest) to diamond (10, the hardest). For example, if a mineral is scratched by topaz (7) but scratches quartz (6), its hardness is 6.5. Some properties, such as \Tfluorescence\t, \Tmagnetism\t, asterism, chatoyancy, and iridescence, are not present in all minerals, but may be important diagnostics in individual cases. Other physical properties of minerals, such as \Tpiezoelectricity\t, heat conduction (see \Theat and heat transfer\t), electrical conductivity (see \Tconduction, electric\t), and \Tmelting point\t, are important for geological and geophysical reasons and for industrial applications but are usually not useful for identification purposes because of the difficulty of measurement. Optical Methods Every mineral has one or more characteristic refractive indices (see \Tindex of refraction\t). The refractive index of a material is v/c, where c is the velocity of light in a vacuum and v is the velocity of light in the material (always less than c). If two transparent materials have greatly differing refractive indices, not only is a beam of light refracted as it passes through the interface, but also any irregularities on the mutual surface are accentuated by light and shadow, or brought into relief. Refractive indices can be measured accurately by the immersion method, in which small grains of a transparent mineral are observed under the microscope in liquids of various known refractive indices, until a match is obtained between liquid and mineral. This match is confirmed when the relief vanishes; that is, the mineral grains seem to disappear in the liquid. The refractive index of minerals not belonging to the isometric system varies with the direction of oscillation of the incident light rays. This variation gives rise to more complicated phenomena, such as birefringence, interference colors, and interference figures, which may be observed with a polarizing petrographic microscope. These effects are also characteristic for each mineral. In the thin-section method, slices of rock 0.03 mm (0.0012 in) thick are mounted on glass slides. Most common minerals can be identified rapidly from an estimate of the refractive index on the basis of the surface relief shown, the interference color, and other properties. Nontransparent minerals, such as most sulfide and oxide minerals, may also be identified optically through the use of the reflecting microscope. Important properties of minerals in this mode of study include reflectivity (see \Treflection\t), color, hardness (judged by the size of an indentation made in a sample by a needle), and the reactivity of a mineral sample to tiny drops of acid or other chemicals. X-Ray Diffraction The X-ray powder method is also a powerful tool for identification. The atoms in the crystalline structure of the powdered sample diffract, or reflect, the incident X-ray beam, but only at certain angles, depending on the size and shape of the unit cell. The diffracted rays can be recorded on film, or they can be measured directly with a radiation counter in a powder diffractometer. Each mineral has a characteristic pattern with respect to angles and intensity of each diffracted ray. Identification is made by means of an index of X-ray patterns compiled by the Joint Committee on Powder Diffraction Standards. TAXONOMY AND MINERAL CLASSIFICATION The names of minerals may be derived from physical properties, often expressed in Latin or Greek; from chemical composition; or from the name of a person or locality. The validity of mineral names is judged by a special commission of the International Mineralogical Association. Each mineral with a definite crystal structure and a limited range of composition has a species name, but varietal names are also sometimes used, to denote minor differences in composition, color, habit, or other characteristics. Many schemes for classifying minerals have been devised, but the one most prevalent, attributable to Dana, is primarily a chemical classification based on the dominant anion or anion group. Classes are arranged in order of increasing complexity of the anion group. Within each class, minerals are often grouped according to structure type. This is especially important in silicate minerals, which are by far the most common minerals. Silicon is always bonded to or coordinated by four oxygen atoms. In some silicates, called nesosilicates, or island silicates, each oxygen is bonded to only one silicon. In other groups, some or all of the oxygen atoms are bonded to two silicon atoms, so that the silicon and oxygen form larger units, or polymers (see \Tpolymerization\t), which may take the form of "sisters" (sorosilicates), rings (cyclosilicates), chains (inosilicates), sheets (phyllosilicates), or continuous frameworks (tectosilicates). The group names are generally derived from the Greek words describing the type of polymer, but some alternate, occasionally used names are derived from the older chemical classification of Berzelius, based on fictitious silicic acids: for example, orthosilicates, from orthosilicic acid. SYNTHETIC MINERALOGY A very important part of mineralogy in the 20th century has been the reproduction of minerals in the laboratory, with the principal objective of ascertaining the conditions of temperature and pressure and the nature of liquid or gaseous phases present during the formation of natural minerals. Other reasons for synthesizing minerals are to obtain pure end-member specimens for determination of physical properties, and to supply substitutes for scarce minerals of economic importance. Synthesis has traditionally been achieved using furnaces with tungsten- or platinum-wire heating elements, and large steel presses or hydraulic apparatus for attaining high pressure. The sample is taken to high pressure and temperature, held for a time until the reaction is complete, and then quenched and examined. Extremely high pressures corresponding to those in the Earth's mantle have recently been attained with the diamond anvil, a compact apparatus in which the sample is placed between the faces of two diamond crystals and compressed with a thumbscrew. The device is small enough to fit on a microscope stage, and the sample can be observed through the transparent diamond faces. A laser beam can be projected through the microscope to heat the sample. MINERAL OCCURRENCE The core of the Earth consists of iron-nickel metal, as deduced from the presence of a magnetic field and from the overall density of the Earth. The upper mantle, as judged by fragments brought up in volcanic rocks, consists primarily of olivine and pyroxene. In the lower mantle, high pressures are thought to cause the breakdown of these complex silicates to simpler oxides. The oceanic crust is of basaltic composition, consisting primarily of pyroxene and plagioclase feldspar. The continental crust also has abundant plagioclase but also contains quartz, alkali feldspars, and other minerals. Surface Mineral Environments Minerals in \Ligneous rock\ls crystallize from molten silicate liquids, or \Lmagma\ls. \Lbasalt\ls normally consist of plagioclase feldspar and pyroxene with subsidiary ilmenite and apatite. Zeolites often form from low-temperature alteration of basalts. \Lgranite\ls consist primarily of feldspar--both plagioclase and alkali feldspar (orthoclase and albite) and quartz--with subsidiary mica and/or amphibole, zircon, minor iron oxides, and other minerals. \Lpegmatite\ls represent the last liquids to crystallize from granitic magmas and often contain large crystals of rare minerals. Alkalic igneous rocks, which have an abundance of sodium and potassium and a deficiency of silicon with respect to more-normal rocks, are very rare but nevertheless furnish an abundance of mineral species. \Tmetamorphic\t \Lrock\ls represent solid-state recrystallization of sedimentary or igneous rocks during deep burial in the crust. The most common minerals include quartz, feldspars, micas, amphiboles, epidote, garnet, and aluminum silicates such as andalusite, sillimanite, and kyanite. Metamorphosed siliceous carbonate rocks (\Tmarble\t) provide an especially rich variety of minerals, most of which contain calcium and silicon. Hydrothermal \Tore deposits\t of minerals precipitate from hot aqueous solutions. The sulfide minerals, such as chalcopyrite, galena, molybdenite, pyrite, and sphalerite, are found in this environment, although small amounts of pyrite may be found in any environment. Nonsulfide gangue minerals, such as barite, calcite, and fluorite, also occur here. On exposure to surface conditions, the sulfides are oxidized, resulting in the formation of metal sulfate minerals and oxides. Minerals in \Tsedimentary\t \Lrock\ls and soils are of two types: detrital, grains remnant from weathered igneous or metamorphic rocks; and authigenic, grains that actually form in the sediment, normally as precipitates from low-temperature aqueous solutions. Clay minerals are authigenic and are usually important constituents of soils, marine sediments, and sedimentary rocks, although detrital quartz and feldspar may often dominate. Some tropical soils, called \Llaterite\ls, are so deeply weathered that the only remaining minerals are the insoluble aluminum and/or iron oxides and hydroxides. These soils (bauxite) may be ores of aluminum. Halide minerals and the sulfates gypsum and anhydrite, as well as borate minerals, are found primarily in \Tevaporite\t deposits formed either from desert lakes or from receding seas. \Tlimestone\t consists of the mineral calcite, which has most often been precipitated by marine organisms, as evidenced by the presence of abundant fossils, which are themselves usually calcite or aragonite. The mineral dolomite also forms carbonate rocks, the precise origin of which is uncertain. Most economic concentrations of iron are sedimentary deposits consisting of the minerals hematite and magnetite. Other minerals of economic importance, such as gold, platinum, rutile, cassiterite, and ilmenite, are often concentrated in \Lplacer deposit\ls. Extraterrestrial Minerals Based on the study of meteorites, many of which are thought to have been derived from a planet or planets that once existed between the orbits of Mars and Jupiter, and on the samples returned from lunar missions, the minerals of the Moon and the terrestrial planets Mercury, Venus, and Mars are believed to be generally similar to those of the Earth. There are, however, differences. The principal minerals on the lunar surface are plagioclase feldspar and pyroxene, as on the Earth's surface, but Moon appears to lack a metallic core, and to be deficient in volatile materials such as alkalis and water, so that minerals containing these materials, such as micas, are absent. RELATED SCIENCES Some knowledge of mineralogy is fundamental in most branches of geology. Those sciences most closely allied to mineralogy are \Tpetrography\t and \Tpetrology\t, the study of the characteristics and origin of rocks (which are composed of minerals); \Tgeochemistry\t, the study of the natural distributions of the elements (which are contained for the most part in minerals); and economic geology, the study of the occurrence of economically important Earth materials (which, except for the organic fuels, are minerals or mineral derivatives). \TERIC\t DOWTY Bibliography: Berry, Leonard G., and Mason, Brian, Mineralogy: Concepts, Descriptions, Determinations (1959); Chesterman, Charles W., The Audubon Society Field Guide to North American Rocks and Minerals (1979); Cox, K. G., et al., An Introduction to the Practical Study of Crystals, Minerals and Rocks, rev. ed. (1975); Deer, W. A., et al., An Introduction to the Rock-Forming Minerals (1966); Desautels, Paul E., The Mineral Kingdom (1968); Hurlbut, Cornelius, Minerals and Man (1975); Hurlbut, Cornelius, and Klein, Cornelis, Manual of Mineralogy (After James D. Dana), 19th ed. (1977); Palache, Charles, et al., A System of Mineralogy of James Dwight Dana and Edward Salisbury Dana, 3 vols., 7th ed. (1944-1962); Pough, F. H., A Field Guide to Rocks and Minerals, 3d ed. (1960); Rapp, G. R., Jr., et al., Encyclopedia of Minerals (1974); Sinkankas, John, Mineralogy: A First Course (1966); Shaub, Benjamin, Treasures from the Earth (1975). See also: \Toceanic mineral resources\t.