mineral

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mineral,

inorganic substance occurring in nature, having a characteristic and homogeneous chemical composition, definite physical properties, and, usually, a definite crystalline form. A few of the minerals (e.g., carbon, arsenic, bismuth, antimony, gold, silver, copper, lead, mercury, platinum, and iron) are elements, but the vast majority are chemical compounds. A generalized formula can usually be assigned to each mineral that is a chemical compound, although sometimes one element in a mineral may be replaced by another without changing the species of the mineral (isomorphismisomorphism
, of minerals, similarity of crystal structure between two or more distinct substances. Sodium nitrate and calcium sulfate are isomorphous, as are the sulfates of barium, strontium, and lead. Crystals of isomorphous substances are almost identical.
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). Minerals combine with each other to make up rocks, which, as distinguished from minerals, are of heterogeneous composition. Minerals may occur in the massive state when conditions for the formation of crystals are unfavorable. Among the important physical properties of minerals are specific gravity, hardness, cleavage, fracture, luster, color, transparency, streak, striations, tenacity, fusibility, heat conductivity, taste, odor, feel, magnetism, and electrical properties. Minerals originate by precipitation from solution, by the cooling and hardening of magmas, by the condensation of gases or gaseous action on country rock, and by metamorphismmetamorphism,
in geology, process of change in the structure, texture, or composition of rocks caused by agents of heat, deforming pressure, shearing stress, hot, chemically active fluids, or a combination of these, acting while the rock being changed remains essentially in the
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. Minerals in rocks are frequently replaced by other minerals through the action of water or gases (metasomatism). Minerals, especially the metals, are of great economic importance to a highly industrialized civilization, entering into the composition of many manufactured articles. Many minerals which would otherwise be of no economic significance are highly valued as gems (see gemgem,
commonly, a mineral or organic substance, cut and polished and used as an ornament. Gems also are used as seals (items of assurance) and as talismans (good-luck charms). For birthstones, see month.
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). Mineralogy, a branch of geology, is the science of minerals.

Bibliography

See J. L. Gillson, Industrial Minerals and Rocks (1960); C. S. Hurlbut, Jr., Minerals and Man (1968); B. Mason and L. G. Berry, Elements of Mineralogy (1968); C. J. Morrissey, ed., Mineral Specimens (1968); J. D. Dana, Manual of Mineralogy (18th ed., rev. by C. S. Hurlbut, Jr., 1971); K. Frye, ed., The Encyclopedia of Mineralogy (1982).

Mineral

 

a naturally occurring material, approximately homogeneous in chemical composition and physical properties, formed as a result of the physicochemical processes that occur on the surface of or within the earth (and other celestial bodies); it is a basic constituent of rocks, ores, and meteorites.

Most minerals are solids and thereby governed by all the laws of solid-state physics; liquid minerals (for example, native mercury) are less common. The inclusion of water in the mineral category remains a topic of controversy, although ice is generally regarded as a mineral. Minerals are classed as crystalline, amorphous (mineraloids, such as opals, lechatelierite, limonite), or metamict; the last have an external crystalline structure but are actually in an amorphous, vitreous state.

Every mineral (mineral species) is a natural compound with a specified composition and a specified crystal structure. Modifications of minerals of identical composition (for example, diamond-graphite, calcite-aragonite) but different crystal structure are classified as different mineral species. In contrast, isomorphous series of minerals (for example, olivine, wolframite, columbite) with somewhat varying composition but a fixed structure are considered as a single mineral species. Minerals in which minor variations in chemical composition, certain properties (for example, color), or morphological characteristics do not produce any marked differences in structure (for example, in such varieties of quartz as rock crystal, amethyst, citrine, and chalcedony) are known as mineral varieties.

Single crystals, grains, and other mineral bodies that are physically separated from one another are classified as individual minerals. Assemblages of individual minerals form mineral aggregates.

Approximately 2,500 mineral species and an equal number of varieties have been discovered and studied. An average of 30 new mineral species are discovered each year.

Most minerals display ionic structures. Covalent and intermetallic structures are less common. Molecular lattices are extremely rare in nature (for example, realgar, AsS; native sulfur; bitumens; and resins). The actual structures of minerals are characterized by the presence of crystal defects (vacancies, extrinsic and interstitial atoms or ions) and dislocations. Formations described as disordered structures frequently occur in minerals, which are characterized by a disruption of the proper arrangement of ions in the lattice and subsequent redistribution resulting in increased degree of order (for example, in feldspar). Certain structural elements in the crystal lattices of minerals (sheets, bundles, chains) may be slightly displaced with respect to one another while fully preserving the internal structure of these elements. This process results in the formation of polytypic modifications (polytypes), which are characterized by identical unit cell parameters in two directions and different parameters in the third. Polytypes occur most frequently in lamellar minerals (mica, graphite, molybdenite, clay minerals).

Unlike the formation of polymorphic modifications, the transition of one polytype into another is a gradual process and is not accompanied by any significant thermal effect. This explains the natural occurrence of several polytypic modifications of minerals under identical thermodynamic conditions. If polymorphism is associated with changes in temperature and pressure, then it would seem that mineral polytypism is primarily dependent upon the conditions of crystal growth. By studying the phenomena of ordering, structural defects, polytypism, and other deviations from ideal structures in the composition of real minerals, it is possible to determine the thermodynamic conditions necessary for mineral formation.

Chemical composition, formulas, and classification. Chemical elements play various roles in the structure of minerals. Some are the principal constituents and determine the basic composition of the mineral itself, while others, similar to the principal elements in properties and atomic (ionic) structure, usually occur as isomorphous admixtures, for example, Pd, Ge, In, Cd, Ga, Tl, Se, I, Br, Re, Rb, and many rare-earth elements. The chemical composition of minerals is characterized by the predominance of compounds of varying composition that form homogeneous mixed crystals (solid solutions), known as isomorphous series of minerals. This feature and the varied degree of order in the structure determine the internal variations in physical and chemical properties of a given mineral species, for example, density, hardness, color, refractive index, magnetic susceptibility, and melting point. The complexity and poor stability of the mineral composition is due to isomorphism, the presence of submicroscopic inclusions, and sorption; the last takes place during mineral formation from colloidal systems (for example, limonite, montmorillonite, opals containing uranium). Submicroscopic inclusions may occur in minerals under the following conditions: (1) after the entrapment of dispersed admixtures during crystallization from a melt, solution, or other medium (for example, gas-liquid inclusions in quartz, hematite inclusions in feldspar); (2) upon decomposition of solid solutions as a result of variations in temperature (the formation of perthites in feldspar, decomposition of complex sulfides and complex oxides); (3) during metamict transformation; and (4) as a result of substitution of one mineral by another, or secondary changes. Many mineral species, for example, magnetite, maintain a fixed content of various microscopic inclusions.

Silicates are the most widely occurring class of minerals, constituting approximately 25 percent of all minerals. Oxides and hydroxides constitute about 12 percent, sulfides and their analogs constitute about 13 percent, and phosphates and arsenates (vanadates) constitute about 18 percent. The rest, 32 percent, consists of other natural chemical compounds. Silicates, oxides, and hydroxides make up 92 percent of the earth’s crust. Minerals are subdivided into simple (native elements) and compound (binary and other) substances, according to the type of chemical compound. Along with such simple anions as S2-, O2-, OH-, and Cl-, complex salt-forming anion radicals, such as [CO3]2-, [SiO4]4-, and [PO4]3-, are also often found in the structures. Depending on the composition of the simple or complex anion, we distinguish among the minerals the sulfides and their analogues, the oxides, the halides, and the oxyacid salts.

The modern system of mineral classification (see Table 1) is based on the differences in the types of chemical compounds and crystal lattices. Both the composition of a mineral with a specified structure and the mineral’s natural isomorphic variations are determined by the structure and crystallochemical properties of the component atoms (ions), the radii and coordination numbers of these atoms (ions), and the type of chemical bond.

The constitutions (composition and structure) of minerals are represented by crystallochemical formulas. These formulas indicate the following: (1) ionic valence, if elements with varying degrees of valence are present; (2) complex anions, given in brackets, for example, [SiO4]4- and [AlO4]5-; (3) isomorphous groups of elements, enclosed in parentheses and separated by commas, with the elements constituting the largest quantities being written first; (4) supplementary anions, such as OH-, F-, Cl-, and O2-, placed after the anion radical; (5) water molecules in crystalline hydrates, placed at the end of the formula and connected by a dot; (6) zeolitic or adsorbed water, also inserted at the end of the formula and connected by a dot, written as nH2O; and (7) the number of atoms missing in defective structures, denoted by the letter x. If the cations occupy a different position in the mineral structure, they appear separately in the formula; their coordination number is designated by roman numerals in the exponent to the right of the element symbol. The following are examples of expanded crystallochemical mineral formulas: magnetite, Fe2+Fe23+O4; andalusite, AlVI Alv [SiO4]O; gypsum, Ca [SO4]-2H2O; pyrrhotite, Fe1-xS; phlogopite, K[Mg, Fe]3 [AlSi3O10](OH, F)2; and opal, SiO2 · nH2O.

Morphology. The morphology of minerals is determined by the internal structure and conditions of formation of the minerals. The size of an individual crystal varies considerably, from 1–100 nm (colloidal minerals) to 10 m (for example, spodumene crystals in pegmatites). Depending on the crystal structure and the conditions of growth, mineral crystals occur in different habits. Crystal habits are classed as isometric (halite, galena, sphalerite, olivine), foliated and lamellar (molybdenite, mica, talc), tabular (barite), and columnar and acicular (rutile, actinolite, tourmaline). Some mineral crystals exhibit characteristic patterns of striation, growth, and dissolution. It is possible to reconstruct the history of the formation of a single crystal by carefully studying the morphology of the mineral and the sculpture of the faces, that is, through a crystallomorphological study. Mineral assemblages exist in addition to individual mineral crystals. These may be naturally oriented with respect to one another (twins, parallel and epitaxial growths) or have no mutual orientation (mineral aggregates).

Morphological studies of aggregates have revealed the following types: druses; dendrites; grainy, solid, and earthy masses; oolites and spherolites; secretions and concretions; and various sintered mineral aggregates, which are particularly characteristic of exogenous minerals. The morphological study of mineral aggregates constitutes a separate branch of mineralogy called ontogenetic mineral analysis. Knowledge of the morphological characteristics of various minerals facilitates identification of minerals.

Physical properties. The physical properties of minerals are determined by their crystal structure and chemical composition. These properties generally vary somewhat because of isomorphism, microscopic inhomogeneity, disordering, existing defects, and other features characteristic of natural mineral crystals. The physical properties of minerals are divided into scalar properties (for example, density) and vectorial properties, which exhibit different values depending on the crystallographic direction (hardness, crystallooptic properties). Physical properties include density, radioactivity, and mechanical, optical, luminescent, magnetic, electric, and thermal properties. Both the physical properties and the forms of development serve as the basis for mineral identification.

According to density, minerals are classed as light (up to 2,500 kg/m3), medium (2,500 to 4,000 kg/m3; the majority of minerals belong to this group), heavy (4,000 to 8,000 kg/m3), or very heavy (more than 8,000 kg/m3). Mineral density is dependent on the mass of atoms or ions constituting the crystal structure and on the type of packing, as well as on the presence of supplementary anions (for example, OH-, F-) and water in the mineral.

Mechanical properties include hardness, brittleness, ductility, cleavage, parting, fracture, flexibility, and elasticity. The relative hardness of a mineral is usually determined by comparison with Mohs’ scale.

Cleavage is classified as perfect, good, distinct, imperfect, or no cleavage. It is the ability of minerals to break along specific lines of direction, that is, parallel crystal lattice networks with highest reticular atomic density and lowest interatomic bonding force. Fracture (even, irregular, hackly, conchoidal) is characterized by the surfaces of the break, which do not follow the cleavage planes of the mineral.

Optical properties, such as color, luster, degree of diaphaneity,

Table 1. Classification of minerals1
Principal types of chemical compoundsClasses2Subclasses, divisions3
1Mineral groups are distinguished according to composition and structure 2By principal anion 3By complexity of composition or of structure or spatial association of complex anions. Classification within subclasses and divisions is based on grouping minerals with identical composition (for example, supplementary anions, water content) or on grouping minerals according to the principal types of structural pattern (for example, coordination, chains, sheets, rings) formed by the distribution of cations and anions in the mineral structure.
Simple substancesNative elementsMetals,
Submetals
Nonmetals
Binary compounds with the following anions: S2 ,S22-, Se2-, Ag-3, and other anionsSulfides and their analogues (arsenides, selenides, etc.)Simple Disulfides, diarsenides, etc. Complex (including sulfosalts)
O2-, (OH)-Oxides, hydrous oxides, and oxyhydratesSimple
Complex
Hydrous oxides and oxyhydrates (simple and complex)
F-, Cr, Br-, I-HalidesSimple
Complex (containing water, supplementary 02- anions, etc.)
Saltlike with complex anions of the type [Memz+ On2-](2n - mz)Silicates (aluminosilicates and others) BoratesMineral
 Phosphates
Arsenates
Vanadates
Chromates
Molybdates
Tungstates
Titanates
Sulfates
Carbonates
Nitrates
Mineral
Organic compoundsOrganic acid salts Resins, bitumensNone

refraction, reflection, and pleochroism, can be studied in certain sections of mineral grains using optical microscopy in the visible, ultraviolet, and infrared regions of the spectrum.

Luster is caused by the quantity of light reflected from the mineral surface and depends on the refractive index. It is divided into metallic, submetallic, and nonmetallic. The terms adamantine, vitreous, greasy, resinous, silky, and pearly are used to describe the luster of nonmetallic minerals.

Minerals are subdivided according to the degree of diaphaneity into transparent, translucent, translucent in splinters, and opaque. Refraction, reflection, and pleochroism in minerals can be determined only through microscopic analysis. Most of the other physical properties (luminescence, magnetic and electric properties, radioactivity) are examined in separate entries. Recent advances in modern mineralogy have given rise to a new and rapidly developing specialized field of research known as mineralogical physics.

Identification. Minerals are generally first tentatively identified in the field through external physical characteristics: the form of development and color, crystal habit and symmetry, streak, luster, cleavage, fracture, and relative hardness. A magnetic compass needle is used to identify ferromagnetic minerals (magnetite, pyrrhotite). Carbonates are easily recognized by their effervescence when treated with HC1. Qualitative chemical reactions are sometimes used in diagnostic studies. There are special determinative tables which make it possible to identify an unknown mineral from its physical characteristics.

Many minerals, for example, clay minerals, cannot be identified in the field. Classical methods of chemical analysis and emission or atomic-adsorption spectrochemical analysis are used in determining the elements in such minerals in the laboratory. Transparent and translucent minerals are examined in transmitted light with a polarizing microscope, while opaque minerals are studied in reflected light using special microscopes. X-ray photographs facilitate the precise identification of many minerals. Finely dispersed minerals, which show indistinct lines on X-ray powder patterns (Debye or diffractometer patterns), are examined under an electron microscope using electrographic methods. Special devices called luminoscopes are employed for rapid identification of certain fluorescent minerals (for example, scheelite). Thermal analysis (differential heat curves, weight loss curves), infrared spectroscopy, and nuclear magnetic resonance make it possible to identify the form in which water is present in the mineral; the form of admixtures of elements is determined by means of an X-ray microanalyzer with an electron probe and by electron paramagnetic resonance. Luminescent and radio-graphic methods (for U and Th) are also sometimes used.

X-ray and electrographic methods facilitate the investigation of structural ordering and polytypism in minerals.

Occurrence and formation. All minerals are divided according to natural distribution into the following types: rock-forming and ore-forming (basic constituents of rocks or ores), secondary or accessory (mineral content not exceeding 1 percent), and rarely occurring and extremely rare (found to exist only under certain unique conditions). This subdivision is conditional, because minerals that are but rarely formed by one natural process may attain (or show) broad distribution under another geological condition.

Every mineral follows a unique pattern of development, emerging under specific geological and physicochemical conditions as a result of certain natural geochemical processes. It passes through stages of origin, growth, and change. The development of mineral crystals and crystal aggregates through all the given stages was unified under the general term “mineral ontogenesis” by the Soviet scientist D. P. Grigor’ev in 1961.

A mineral can be formed from media of varying phase states (melt, solution, gas) in a suspension or on a substrate. During growth, the mineral entraps, through isomorphous or mechanical processes, admixtures that are present in the mineral-forming medium (resulting in the zonal structure of minerals), as well as liquid, gas-liquid, and gas inclusions occurring in the medium itself. The following phenomena may occur during such changes in the physicochemical environment as drop in temperature, increase in pressure, and influx of new solutions: (1) deformations, giving rise to mechanical twinning, dislocations, and mosaic or domain structures; (2) dissolution of the mineral, indicated by the existence of specific patterns on the crystal faces; (3) polymorphic transformations; (4) decomposition of solid solutions; (5) recrystallization; and (6) chemical changes resulting in the substitution of certain minerals by others. If the external form of the original mineral is preserved during such changes, then pseudomorphism occurs (for example, limonite after pyrite). Pseudomorphism in which the primary (original) mineral and the resulting secondary mineral are polymorphic modifications of identical composition is known as paramorphism (sphalerite after wurtzite, graphite after diamond).

Because a mineral is the product of many diverse reactions, it never exists by itself but always in association with other minerals. Mineral associations are formed during the course of a single process, limited in space and time, under specific physicochemical conditions; this process is known as paragenesis, and paragenetic associations are formed. The number of possible stable minerals present in a paragenetic association is determined by the mineralogical phase rule. Since natural processes take place at varying temperatures, pressures, and component concentrations, one paragenetic association is regularly replaced by another during a mineral’s development. The study of resulting mineral associations with the aid of physicochemical diagrams (composition-paragenesis) forms the basis of paragenetic analysis, developed by the Soviet scientist D. S. Korzhinskii. This method makes it possible to predict the occurrence of a mineral in a particular association and to differentiate the various stages of mineral formation.

A mineral can occur in different paragenetic associations distributed throughout a given deposit; that is, it may be found in various stages of development. These depositions of one particular mineral at different times are called generations. Since minerals are products of natural reactions, a causal relationship exists between a mineral and its surrounding formation medium, the phase state of the medium, and physicochemical conditions. All the given factors directly affect the composition and properties of a mineral, which tends to acquire specific typomorphic characteristics as it passes through each stage. Subtypomorphism is defined as the aggregate of all the chemical, structural, and physical properties of a mineral that are in a cause-and-effect relationship with the medium in which the mineral has formed. Minerals or their parageneses, as well as their individual characteristic features, may be typomorphic. The typomorphic properties of a mineral can be used to establish the origin of the mineral and can also serve as prospecting criteria in geological fieldwork.

Minerals are formed through endogenous, exogenous, and metamorphogenetic processes. The modern conception of mineral genesis takes into account the characteristics of several phenomena that determine the origin of a mineral, including (1) the chemical processes of mineral formation; (2) the phase state of the medium of the developing mineral; (3) the physicochemical parameters of the system in which the mineral formed (temperature, pressure, component activity, oxygen potential, acid-base regime); (4) the mechanism of mineral origin, growth, and development, especially the method of the mineral’s formation (free crystallization, metasomatic development, recrystallization, crystallization of gels); (5) the subsequent changes in the mineral and the occurrence of metamorphism; and (6) the source of the mineral material.

The principal methods of determining mineral origin are (1) examination of the geological conditions where the mineral was found, (2) identification of typomorphic features in the mineral, (3) paragenetic analysis, (4) ontogenetic research, (5) investigation of gas-liquid inclusions in the mineral, (6) calculation of thermodynamic characteristics in natural reactions, (7) determination of thermodynamic parameters using various types of geothermometers and geobarometers, (8) study of physicochemical systems, (9) experimental simulation of possible natural processes of mineral formation, and (10) analysis of the isotopic composition of the given mineral. Once objective quantitative data on mineral origin are obtained, it then becomes possible to reconstruct the geological processes and course of development characterizing various mineral deposits and, at the same time, to create a scientific basis for the location, exploration, and commercial evaluation of deposits.

Application. The properties of minerals determine their uses in industry. Thus, for example, very hard minerals, such as diamond, corundum, and granite, serve as abrasives, while minerals with piezoelectric properties are used in radio electronics. Ore-dressing methods and geophysical techniques for exploring commercial mineral deposits are based on various physical properties of minerals, primarily on density, elasticity, and magnetic, electric, surface, and radioactive properties. In view of this, special importance is given to the comprehensive study of mineral properties and characteristics. The most promising of the possible future advancements is the directed alteration of mineral properties by “healing” crystal lattice defects through the following methods: mechanical, acoustic (ultrasonic processing), thermal (heating and subsequent rapid or slow cooling), chemical (pickling, treatment with reagents capable of “healing” the mineral surface through the action of extrinsic ions), and radiation (irradiation with X rays, gamma rays, high-speed particle flux).

No more than 15 percent of all known minerals are presently used in industry. The detailed investigation of mineral distribution, composition, and properties makes possible the practical application of the increasing number of new mineral species. This would include nearly all the elements in the Mendeleev system of elements that occur as basic constituents (ores of ferrous, nonferrous, and, partly, rare metals) or admixtures (diffuse elements) in different minerals. Single crystals of some minerals and their synthetic analogues have found wide application in optics, radio electronics, and power engineering. Some minerals are classed as gem and ornamental stones. At present, mineralogists are showing increasingly greater interest in analyzing minerals found on the moon and other celestial bodies, as well as those found in the earth’s mantle.

REFERENCES

Vernadskii, V. I. Istoriia mineralov zemnoi koiy, vol. 1, fascs. 1–2. Leningrad, 1923–27.
Deer, W. A., R. A. Howie, and J. Zussman. Porodoobrazuiushchie mineraly, vols. 1–5. Moscow, 1965–66. (Translated from English.)
Sovremennye melody mineralogicheskogo issledovaniia: Sbomik, parts 1–2. Moscow, 1969.
Osnovnye problemy v uchenii o magmatogennykh rudnykh mestorozhdeniiakh, 2nd ed. Moscow, 1955.
Fersman, A. E. Pegmatity, vol. 1. In Izbrannye trudy, vol. 6. Moscow, 1960.
Betekhtin, A. G. Kurs mineralogii, 3rd ed. Moscow, 1961.
Rostov, I. Minerahgiia. Moscow, 1971. (Translated from English.)
Lazarenko, E. K. Kurs mineralogii. Moscow, 1971.
Smol’ianinov, N. A. Prakticheskoe rukovodstvo po mineralogii, 2nd ed. Moscow, 1972.
Voprosy odnorodnosti i neodnorodnosti mineralov: Sbornik. Moscow, 1971.
Mineraly: Spravochnik, vols. 1–3. Moscow, 1960–72.
Grigor’ev, D. P. Ontogeniia mineralov. L’vov, 1961.
Shafranovskii, I. I. Kristally mineralov. Moscow, 1961.
Tipomorfizm mineralov i ego prakticheskoe znachenie. Moscow, 1972. (Collection of articles.)
Korzhinskii, D. S. Teoreticheskie osnovy analizaparagenezisov mineralov. Moscow, 1973.

G. P. BARSANOV and A. I. GINZBURG

mineral

[′min·rəl]
(geology)
A naturally occurring substance with a characteristic chemical composition expressed by a chemical formula; may occur as individual crystals or may be disseminated in some other mineral or rock; most mineralogists include the requirements of inorganic origin and internal crystalline structure.

mineral

any of a class of naturally occurring solid inorganic substances with a characteristic crystalline form and a homogeneous chemical composition
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