Natural and artificial minerals. Primary and secondary minerals.

Mineral (from c.-century lat. minera - ore)- this is a natural body with a certain chemical composition and crystalline structure, formed as a result of natural physical and chemical processes occurring on the surface and in the depths of the Earth, the Moon and other planets, and possessing certain physical, mechanical and chemical properties; is usually a component rocks, ores and meteorites. A mineral is usually a natural chemical compound of elements, or a native element formed under certain physical and chemical environmental conditions.

Mineralogy is the study of minerals. Mineralogy studies the composition, chemical and physical properties of minerals, their origin, processes of change and transformation into other minerals, as well as the relationships of some minerals with others in mineral deposits or rocks.

The term “mineral” means a solid natural inorganic crystalline substance. But sometimes it is considered in a broader context, classifying some organic, amorphous and other natural products as minerals.

Minerals are also considered to be some natural substances that are normal conditions liquids (for example, native mercury, which comes to a crystalline state at a lower temperature). Water, on the contrary, is not classified as a mineral, considering it as a liquid state (melt) of the mineral ice.

Some organic substances - oil, asphalt, bitumen - are often mistakenly classified as minerals, or they are classified into a special class “organic minerals”, the feasibility of which is highly controversial.

Some minerals are in an amorphous state and do not have a crystalline structure. Minerals that have the external form of crystals, but are in an amorphous, glass-like state, are called metamict. For example, table salt is clearly crystalline, while opal is amorphous. In minerals with a crystalline structure, elementary particles (atoms, molecules) are located in a certain direction and at a certain distance from each other, forming a crystal lattice. In an amorphous substance, these particles are located chaotically. Its basic properties depend on the internal structure of the mineral (crystalline or amorphous). physical properties(hardness, cleavage, fragility, crystallographic external shape, etc.). And they, in turn, are among the most important diagnostic characteristics of minerals.

The composition of minerals is expressed by its chemical formula - empirical, semi-empirical, crystal chemical. The empirical formula reflects only the relationship between individual elements in minerals. In it, elements are arranged from left to right as the number of their groups in the periodic table increases, and for elements of one group - as their serial numbers decrease, i.e. as their strength characteristics increase.

Currently, more than 3 thousand minerals have been found and studied in nature, but they are not equally distributed. About 30 species of them are discovered every year, of which only a few dozen are widespread, the rest are rare. The most widespread are minerals containing oxygen, silicon and aluminum, since these elements predominate in the earth's crust - 82.58%.

Minerals are named after the place of their first discovery, in honor of major mineralogists, geologists and scientists of other specialties, famous collectors of minerals, travelers, astronauts, public and political figures of the past and present, according to some characteristic physical properties or chemical composition. The latter chemical principle is especially recommended, and most minerals discovered in recent decades carry information about their chemical composition in the name itself.

Attempts to systematize minerals on a different basis were made already in ancient world. In modern mineralogy there are many different variants of mineralogical taxonomy. Most of them are built on a structural-chemical principle. The most widely used classification is based on chemical composition and crystal structure. Substances of the same chemical type often have a similar structure, so minerals are first divided into classes based on chemical composition, and then into subclasses based on structural characteristics.

Minerals are classified depending on their origin. primary and secondary.

Primary minerals include those formed for the first time in the earth's crust or on its surface during the crystallization of magma. The primary most common minerals include quartz, feldspar, and mica, which make up granite or sulfur in volcanic craters.

Secondary minerals were formed under normal conditions from the products of destruction of primary minerals due to weathering, during precipitation and crystallization of salts from aqueous solutions, or as a result of the vital activity of living organisms. These are kitchen salt, gypsum, sylvite, brown iron ore and others.

No matter how rich and diverse the world of minerals is, it is not always you can get them in sufficient quantity and required quality. People often require not just any minerals, but only those that would meet the ever-growing demands of metallurgical, electrical and radio engineering, optical-mechanical, precision instrument making and other industries. Requirements national economy to minerals, are often very large: a high degree of chemical purity, transparency, perfect cutting, etc. And of course, nature is not always able to satisfy these requests. Therefore, not limiting ourselves to the extraction of natural minerals, man is constantly looking for ways and means of obtaining artificial minerals that are not only not inferior, but even superior in their properties to natural ones. The development of science and technology every year allows us to penetrate deeper into the secrets of the mineral world. Man has learned to create unique equipment that makes it possible to obtain minerals that are not only not inferior in quality to those born in the depths of the Earth, but also to produce new, previously unknown minerals, often with very valuable and original properties.

By artificial means (synthesis method) it is possible to obtain minerals that are found in natural conditions (diamond, corundum, quartz, etc.), and minerals that do not occur independently in natural conditions (alite, belite, etc.), but are included in various technical products such as cements, refractories, etc. Currently, a number of minerals that are rarely found in nature but have valuable properties (fluorite, corundum, etc.) have been obtained for industrial purposes.

Methods for the synthesis of natural minerals can be divided into two groups:

1) synthesis carried out under normal pressure conditions.

2) synthesis carried out at elevated pressures.

Currently, the production of artificial minerals comes down to the following processes:

1) melt crystallization;

2) reactions in which gas components participate;

3) obtaining minerals in the presence of aqueous solutions;

4) obtaining minerals by reaction in a solid medium.

The practical importance of mineral synthesis has increased dramatically in recent years. Nevertheless, the importance of artificial minerals is still relatively small. The main role belongs to natural minerals - the main suppliers of many metals for industry

Minerals are found widely application V modern world. About 15% of all known mineral species are used in technology and industry. Minerals are of practical value as sources of all metals and other chemical elements(ores of ferrous and non-ferrous metals, rare and trace elements, agronomic ores, raw materials for chemical industry). The technical applications of many minerals are based on their physical properties.

Hard minerals (diamond, corundum, garnet, agate, etc.) are used as abrasives and anti-abrasives; minerals with piezoelectric properties (quartz, etc.) - in radio electronics; mica (muscovite, phlogopite) - in electrical and radio engineering (due to their electrical insulating properties);

asbestos - as a heat insulator;

talc - in medicine and in lubricants;

quartz, fluorite, Iceland spar - in optics;

quartz, kaolinite, potassium feldspar, pyrophyllite - in ceramics;

magnesite, forsterite - as magnesia refractories, etc.

A number of minerals are precious and ornamental stones. In the practice of geological exploration, mineralogical prospecting and evaluation of mineral deposits are widely used.

Methods of ore enrichment and mineral separation, as well as geophysical and geochemical methods of prospecting and exploration of mineral deposits, are based on differences in the physical and chemical properties of minerals (density, magnetic, electrical, surface, radioactive, luminescent and other properties), as well as on color contrasts.

Industrial synthesis of single crystals of artificial analogues of a number of minerals for radio electronics, optics, abrasive and jewelry industries is carried out on a large scale.

To date, more than 4 thousand minerals are known. Every year, several dozen new mineral species are discovered and several are “closed” - they prove that such a mineral does not exist.

Four thousand minerals is not a lot compared to the number of known inorganic compounds (more than a million).

All processes of formation of minerals and rocks can be divided into three groups:

A. Endogenous (internal), or, as they are often called, hypogene (deep) processes occurring due to the internal thermal energy of the globe.

B. Exogenous (external) or hypergene (surface) processes occurring on the surface of the earth mainly under the influence of solar energy.

B. Metamorphic (metamorphogenic) processes associated with the degeneration of previously formed mineral associations (both exogenous and endogenous) as a result of changing physical and chemical conditions, among which the main place is occupied by changes in pressure and temperature.

Types and groups of minerals

Minerals: general characteristics
“Mineral” is a solid body consisting of chemical elements and possessing a number of individual physicochemical properties. In addition, it should be formed only naturally, under the influence of certain natural processes. Minerals can be formed either from simple substances (native) or from complex ones.

There are such processes that contribute to their formation:
Igneous
Hydrothermal
Sedimentary
Metamorphogenic
Biogenic
Large aggregates of minerals collected into single systems are called rocks. Therefore, these two concepts should not be confused. Rock minerals are mined precisely by crushing and processing entire pieces of rock. The chemical composition of the compounds in question may be different and contain a large number of various impurity substances. However, there is always one main thing that dominates the lineup. Therefore, it is this that is decisive, and impurities are not taken into account.
The structure of minerals
The structure of minerals is crystalline. There are several options for gratings with which it can be represented:
Cubic
Hexagonal
Rhombic
Tetragonal
Monoclinic
Trigonal
Triclinic

These compounds are classified according to the chemical composition of the determining substance.

Types of minerals
A classification that reflects the main part of the mineral's composition.

Native or simple substances. These are also minerals. For example: gold, iron, carbon in the form of diamond, coal, anthracite, sulfur, silver, selenium, cobalt, copper, arsenic, bismuth and many others.

Halides, which include chlorides, fluorides, bromides. For example: rock salt (sodium chloride) or halite, sylvite, fluorite.

Oxides and hydroxides. They are formed by oxides of metals and non-metals, that is, by combining them with oxygen. This group includes minerals - chalcedony, corundum (ruby, sapphire), magnetite, quartz, hematite, rutile, cassematite and others.

Nitrates. For example: potassium and sodium nitrate.

Borates: optical calcite, eremeyevite.

Carbonates are salts of carbonic acid. This group includes the following minerals: malachite, aragonite, magnesite, limestone, chalk, marble and others.

Sulfates: gypsum, barite, selenite.

Tungstates, molybdates, chromates, vanadates, arsenates, phosphates - all these are salts of the corresponding acids that form minerals of various structures. Names - nepheline, apatite and others.

Silicates. Salts of silicic acid containing the SiO4 group. For example: beryl, feldspar, topaz, garnets, kaolinite, talc, tourmaline, jadeite, lapis lazuli and others.

Also found organic compounds, forming entire natural deposits. For example, peat, coal, urkite, calcium and iron oxolates and others. As well as several carbides, silicides, phosphides, and nitrides.

Native elements

These are minerals that are formed by simple substances.
For example:
Gold in the form of sand and nuggets, bars
Diamond and graphite - allotropic modifications crystal lattice carbon
Copper
Silver
Iron
Sulfur
Group of platinum metals

Often these substances occur in the form of large aggregates with other minerals, pieces of rock and ores. Extraction and their use in industry have important. They are the basis, the raw material for obtaining materials from which the most various items household items, designs, decorations, appliances and much more.

Phosphates, arsenates, vanadates
This group includes rocks and minerals that are predominantly of exogenous origin, that is, found in the outer layers earth's crust. Only phosphates are formed inside. There are actually quite a lot of salts of phosphoric, arsenic and vanadic acids. But in general, their percentage in the bark is small.

Common crystals that belong to this group:

Apatite
Vivianite
Lindakerite
Rosenite
Carnotite
Pascoite

As already noted, these minerals form rocks of quite impressive size.

Oxides and hydroxides

This group of minerals includes all oxides, both simple and complex, which are formed by metals, nonmetals, intermetallic compounds and transition elements. The total percentage of these substances in the earth's crust is 5%. The only exception, which refers to silicates and not to the group under consideration, is silicon oxide SiO2 with all its varieties.

The most common:
Granite
Magnetite
Hematite
Ilmenite
Columbite
Spinel
Lime
Gibbsite
Romaneshit
Holfertitis
Corundum (ruby, sapphire)
Bauxite

Carbonates
This class of minerals includes a fairly wide variety of representatives, which also have important practical significance for a person.
Subclasses or groups:
calcite
dolomite
aragonite
malachite
soda minerals
bastnäsite

Each subclass includes from several units to dozens of representatives. In total there are about one hundred different mineral carbonates.

The most common of them:

marble
limestone
malachite
apatite
siderite
smithsonite
magnesite
carbonatite and others

Some are valued as a very common and important building material, others are used to create jewelry, and others are used in technology. However, all are important.

Silicates

The most diverse group of minerals in terms of external forms and number of representatives. This variation is due to the fact that silicon atoms, which underlie their chemical structure, are able to combine into different types structure, coordinating several oxygen atoms around itself.

Thus, the following types of structures can be formed:

island
chain
tape
leafy

These include:
topaz
pomegranate
chrysoprase
rhinestone
opal
chalcedony and others.
They are used in jewelry and are valued as durable structures for use in technology.

Important minerals in industry:
Datonite
Olivine
Murmanite
Chrysocol
Eudialyte
Beryl

Minerals- these are natural bodies, approximately homogeneous in chemical composition and physical properties, formed as a result of physical and chemical processes on the surface or in the depths of the Earth (or other cosmic bodies), mainly as component rocks, ores, meteorites, without human intervention in these processes.

This is the difference between minerals and artificial products obtained in laboratories, factories and factories.

More than 3 thousand minerals have been found and studied in nature. Currently, about 30 species of them are discovered annually, of which only a few dozen are widespread, the rest are rare.

Minerals are classified according to their physical state solid (quartz, feldspar, mica), liquid (water, oil, native mercury) and gaseous (hydrogen, oxygen, carbon dioxide, hydrogen sulfide, etc.). Some minerals, depending on conditions, can be either liquid or solid (for example, water).

Minerals are divided according to their internal structure into crystalline (kitchen salt) and amorphous (opal). In minerals with a crystalline structure, elementary particles (atoms, molecules) are located in a certain direction and at a certain distance from each other, forming a crystal lattice. In an amorphous substance, these particles are located chaotically.

Its basic physical properties (hardness, cleavage, crystallographic external shape, etc.) depend on the internal structure of the mineral (crystalline or amorphous).

Depending on their origin, primary and secondary minerals are distinguished.

Primary minerals include those formed for the first time in the earth's crust or on its surface during the crystallization of magma. The primary most common minerals include quartz, feldspar, and mica, which make up granite or sulfur in volcanic craters.

Secondary minerals were formed under normal conditions from the products of destruction of primary minerals due to weathering, during precipitation and crystallization of salts from aqueous solutions, or as a result of the vital activity of living organisms. These are kitchen salt, gypsum, sylvite, brown iron ore and others.

There are many processes that result in the formation of minerals in nature.. The following processes are distinguished: magmatic, supergene, or climatic, and metamorphic.

The main process is magmatic. It is associated with the cooling, differentiation and crystallization of molten magma during different pressures and temperature. Magma consists mainly of the following chemical components: Si02, Al203, FeO, CaO, MgO, K2O, it also contains others chemical compounds, but in smaller quantities.

Minerals are formed mainly at a temperature of 1000-1500°C and a pressure of several thousand atmospheres. All primary crystalline rocks are formed from minerals of igneous origin. Minerals whose origin is associated with magma and internal heat Earths are called primary. These include feldspars - orthoclase, albite, anorthite, orthosilicates - olivine and others.

Minerals are also formed from gases(gas phase of magma). The most common of them are pegmatites, or vein minerals, orthoclase with quartz, microcline, apatite, muscovite, biotite and many others. Such minerals are called pneumatogenic.

From the hot liquid of magma(liquid phase) hydrothermal minerals are formed - pyrite, gold, silver and many others.

Hypergenic processes occur on the surface of the Earth under normal conditions under the influence of water, temperature and other factors. As a result of this, various chemical compounds dissolve and move, and new (secondary) minerals appear, such as sylvite, quartz, calcite, brown iron ore and kaolinite. Minerals of the supergene cycle are formed at pressures up to 1 atm and temperatures below 100°C. High-quality composition the distribution of these minerals on the Earth's surface depends to a certain extent on geographic latitudes. It should be noted that the transformation of the same mineral under different conditions may not proceed in the same way. For example, hydromicas are formed not only from micas, but also artificially.

The main material for the formation of minerals of supergene origin are weathered primary rocks or those that have already undergone a transformation process. Living organisms also take part in this process. Minerals of the supergene cycle, formed under the influence of external processes, are part of sedimentary and soil-forming rocks.

Exogenous processes of mineral formation occur both on the Earth's surface and in the weathering crust. For the formation of minerals of exogenous origin, the processes of physical, chemical and biological weathering are important.

During the metamorphic process, minerals are formed at great depths from the Earth's surface when physical and chemical conditions change (temperature, pressure, concentration of chemically active components). Under these conditions, the transformation of many previously formed primary and secondary minerals occurs. Among them, the most common are hematite, graphite, quartz, hornblende, talc and many others.

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MINERALS AND MINERALOGY
Minerals are solid natural formations that are part of the rocks of the Earth, the Moon and some other planets, as well as meteorites and asteroids. Minerals, as a rule, are fairly homogeneous crystalline substances with an ordered internal structure and a certain composition, which can be expressed by an appropriate chemical formula. Minerals are not a mixture of tiny mineral particles, such as emery (consisting mainly of corundum and magnetite) or limonite (an aggregate of goethite and other iron hydroxides), but also compounds of elements with a disordered structure, like volcanic glasses (obsidian, etc.) .). Minerals are considered chemical elements or their compounds formed as a result of natural processes. The most important types of mineral raw materials of organic origin, such as coal and oil, are excluded from the list of minerals. Mineralogy is the science of minerals, their classification, chemical composition, features and patterns of structure (structure), origin, conditions in nature and practical application. For a deeper explanation of the internal structure of minerals and their connection with the history of the Earth, mineralogy involves mathematics, physics and chemistry. It uses quantitative data to a greater extent than other geological sciences, since fine chemical analysis and precise physical measurements are necessary to adequately describe minerals.
HISTORY OF MINERALOGY
Flint flakes with sharp edges were used by primitive man as tools already in the Paleolithic. Flint (a fine-grained variety of quartz) for a long time remained the main mineral resource. In ancient times, other minerals were also known to man. Some of them, such as cherry hematite, yellow-brown goethite, and black oxides of manganese, were used as paints for rock painting and body painting, while others, such as amber, jade, and native gold, were used to make ritual objects, jewelry, and amulets. In Egypt of the predynastic period (5000-3000 BC) many minerals were already known. Native copper, gold and silver were used for decoration. Somewhat later, tools and weapons began to be made from copper and its alloy, bronze. Many minerals were used as dyes, others for jewelry and signets (turquoise, jade, crystal, chalcedony, malachite, garnet, lapis lazuli and hematite). Currently, minerals serve as a source for the production of metals, building materials (cement, plaster, glass, etc.), raw materials for the chemical industry, etc. In the first known treatise on mineralogy, On Stones, by Aristotle’s student, the Greek Theophrastus (c. 372-287 BC). BC) minerals were divided into metals, earths and stones. About 400 years later, Pliny the Elder (23-79 AD) in five latest books Natural history summarized all the information on mineralogy available at that time. In the early Middle Ages in countries Arab East who adopted the knowledge of ancient Greece and ancient india, science flourished. The Central Asian scientist-encyclopedist Biruni (973 - ca. 1050) compiled descriptions of precious stones (Mineralogy) and invented a method for accurately measuring them specific gravity. Another outstanding scientist Ibn Sina (Avicenna) (c. 980-1037) in his treatise On Stones gave a classification of all known minerals, dividing them into four classes: stones and earths, fossil fuels, salts, metals. In the Middle Ages in Europe, practical information about minerals was accumulated. The miner and prospector, out of necessity, became practicing mineralogists and passed on their experience and knowledge to students and apprentices. The first set of factual information on practical mineralogy, mining and metallurgy was the work of G. Agricola On metals (De re metallica), published in 1556. Thanks to this treatise and an earlier work On the nature of fossils (De natura fossilium, 1546), which contains classification of minerals based on their physical properties, Agricola was known as the father of mineralogy. For 300 years after the publication of Agricola's works, research in the field of mineralogy was devoted to the study of natural crystals. In 1669, the Danish naturalist N. Stenon, summarizing his observations of hundreds of quartz crystals, established the law of constancy of angles between crystal faces. A century later (1772) Romé de Lisle confirmed Stenon's conclusions. In 1784, Abbot R. Gayuy laid the foundations modern ideas about crystal structure. In 1809, Wollaston invented a reflective goniometer, which made it possible to carry out more accurate measurements of angles between the faces of crystals, and in 1812 he put forward the concept of a spatial lattice as a law of the internal structure of crystals. In 1815 P. Cordier proposed studying optical properties fragments of crushed minerals under a microscope. Further development microscopic studies are associated with the invention in 1828 by W. Nicol of a device for producing polarized light (Nicol prism). The polarizing microscope was improved in 1849 by G. Sorby, who applied it to the study of transparent thin sections of rocks. There was a need to classify minerals. In 1735, C. Linnaeus published the work System of Nature (Systema naturae), in which minerals were classified according to external characteristics, i.e. just like plants and animals. Then Swedish scientists - A. Kronstedt in 1757 and J. Berzelius in 1815 and 1824 - proposed several options chemical classifications minerals. The second Berzelius classification, modified by K. Rammelsberg in 1841-1847, was firmly established after the American mineralogist J. Dana used it as the basis for the third edition of Dana's System of Mineralogy, 1850. Great contribution to the development of mineralogy in the 18 - the first half of the 19th century was introduced by German scientists A.G. Werner and I.A. Breithaupt and Russians - M.V. Lomonosov and V.M. Severgin. In the second half of the 19th century, improved polarization microscopes, optical goniometers and analytical methods made it possible to obtain more accurate data on individual mineral species. When crystals began to be studied using X-ray analysis, a deeper understanding of the structure of minerals came. In 1912, the German physicist M. Laue experimentally established that information about the internal structure of crystals can be obtained by passing X-rays through them rays. This method revolutionized mineralogy: the predominantly descriptive science became more accurate and mineralogists were able to link physical and Chemical properties minerals with their crystal structures. At the end of the 19th - beginning of the 20th century. The development of mineralogy was greatly facilitated by the work of outstanding Russian scientists N.I. Koksharov, V.I. Vernadsky, E.S. Fedorov, A.E. Fersman, A.K. Boldyrev and others. In the second half of the 20th century. mineralogy adopted new research methods solid state physics, in particular, infrared spectroscopy, a whole series of resonance methods (electron paramagnetic resonance, nuclear gamma resonance, etc.), luminescence spectroscopy, etc., as well as the latest analytical methods, including electron microprobe analysis, electron microscopy in combination with electron diffraction, etc. The use of these methods makes it possible to determine the chemical composition of minerals “at a point”, i.e. on individual grains of minerals, study the subtle features of their crystal structure, the content and distribution of impurity elements, the nature of color and luminescence. Implementation of accurate physical methods research produced a genuine revolution in mineralogy. The names of such Russian scientists as N.V. Belov, D.S. Korzhinsky, D.P. Grigoriev, I.I. Shafranovsky and others are associated with this stage in the development of mineralogy.
MAIN PROPERTIES OF MINERALS
For a long time, the main characteristics of minerals were the external shape of their crystals and other precipitates, as well as physical properties (color, shine, cleavage, hardness, density, etc.), which they still have today. great importance in their description and visual (in particular, field) diagnostics. These characteristics, as well as optical, chemical, electrical, magnetic and other properties, depend on the chemical composition and internal structure (crystalline structure) of minerals. The primary role of chemistry in mineralogy was recognized by the mid-19th century, but the importance of structure became apparent only with the introduction of radiography. The first decoding of crystal structures was carried out already in 1913 by English physicists W. G. Bragg and W. L. Bragg. Minerals are chemical compounds (with the exception of native elements). However, even colorless, optically transparent samples of these minerals almost always contain small amounts of impurities. Natural solutions or melts from which minerals crystallize usually consist of many elements. During the formation of compounds, a few atoms of less common elements can replace atoms of the main elements. Such substitution is so common that the chemical composition of many minerals only very rarely approaches that of the pure compound. For example, the composition of the common rock-forming mineral olivine varies within the compositions of two so-called. end members of the series: from forsterite, magnesium silicate Mg2SiO4, to fayalite, iron silicate Fe2SiO4. The ratio of Mg:Si:O in the first mineral and Fe:Si:O in the second is 2:1:4. In olivines of intermediate composition, the ratios are the same, i.e. (Mg + Fe):Si:O is 2:1:4, and the formula is written as (Mg,Fe)2SiO4. If relative quantities magnesium and iron are known, this can be reflected in the formula (Mg0.80Fe0.20)2SiO4, from which it can be seen that 80% of the metal atoms are represented by magnesium, and 20% by iron.
Structure. All minerals, with the exception of water (which - unlike ice - is not usually classified as minerals) and mercury, are present at ordinary temperatures solids. However, if water and mercury are greatly cooled, they solidify: water at 0 ° C, and mercury at -39 ° C. At these temperatures, water molecules and mercury atoms form a characteristic regular three-dimensional crystalline structure (the terms "crystalline" and "solid") " V in this case almost equivalent). Thus, minerals are crystalline substances whose properties are determined by the geometric arrangement of their constituent atoms and the type of chemical bond between them. The unit cell (the smallest unit of a crystal) is made up of regularly arranged atoms held together by electronic communications. These tiny cells, endlessly repeating in three-dimensional space, form a crystal. The sizes of unit cells in different minerals are different and depend on the size, number and relative arrangement of atoms within the cell. Cell parameters are expressed in angstroms () or nanometers (1 = 10-8 cm = 0.1 nm). The elementary cells of a crystal put together tightly, without gaps, fill the volume and form a crystal lattice. Crystals are divided based on the symmetry of the unit cell, which is characterized by the relationship between its edges and corners. Usually there are 7 systems (in order of increasing symmetry): triclinic, monoclinic, rhombic, tetragonal, trigonal, hexagonal and cubic (isometric). Sometimes trigonal and hexagonal systems are not separated and are described together under the name hexagonal system. Syngonies are divided into 32 crystal classes (types of symmetry), including 230 space groups. These groups were first identified in 1890 by the Russian scientist E.S. Fedorov. Using X-ray diffraction analysis, the dimensions of the unit cell of a mineral, its syngony, symmetry class and space group are determined, and the crystal structure is deciphered, i.e. mutual arrangement in the three-dimensional space of the atoms that make up the unit cell.
GEOMETRIC (MORPHOLOGICAL) CRYSTALLOGRAPHY
Crystals with their flat, smooth, shiny edges have long attracted human attention. Since the advent of mineralogy as a science, crystallography has become the basis for the study of the morphology and structure of minerals. It was found that the crystal faces have a symmetrical arrangement, which allows the crystal to be assigned to a certain system, and sometimes to one of the classes (symmetry) (see above). X-ray studies have shown that the external symmetry of crystals corresponds to the internal regular arrangement of atoms. The sizes of mineral crystals vary over a very wide range - from giants weighing 5 tons (the mass of a well-formed quartz crystal from Brazil) to so small that their faces can only be distinguished under an electron microscope. The crystal shape of even the same mineral may differ slightly in different samples; for example, quartz crystals are almost isometric, acicular or flattened. However, all quartz crystals, large and small, pointed and flat, are formed by the repetition of identical unit cells. If these cells are oriented in a certain direction, the crystal has an elongated shape; if in two directions to the detriment of the third, then the shape of the crystal is tabular. Since the angles between corresponding faces of the same crystal have constant value and are specific to each mineral species, this feature is necessarily included in the characteristics of the mineral. Minerals represented by individual well-cut crystals are rare. Much more often they occur in the form of irregular grains or crystalline aggregates. Often a mineral is characterized by a certain type of aggregate, which can serve as a diagnostic feature. There are several types of units. Dendritic branching aggregates resemble fern leaves or moss and are characteristic, for example, of pyrolusite. Fibrous aggregates consisting of densely packed parallel fibers are typical of chrysotile and amphibole asbestos. Collomorphic aggregates, which have a smooth, rounded surface, are constructed from fibers that extend radially from a common center. Large round masses are mastoid (malachite), while smaller ones are kidney-shaped (hematite) or grape-shaped (psilomelane).
Scaly aggregates consisting of small plate-like crystals are characteristic of mica and barite. Stalactites are drip-drip formations hanging in the form of icicles, tubes, cones or “curtains” in karst caves. They arise as a result of the evaporation of mineralized water seeping through limestone cracks, and are often composed of calcite (calcium carbonate) or aragonite. Oolites, aggregates consisting of small balls and resembling fish eggs, are found in some calcite (oolitic limestone), goethite (oolitic iron ore) and other similar formations.
CRYSTAL CHEMISTRY
After accumulating radiographic data and comparing them with the results chemical analyzes It became obvious that the features of the crystal structure of a mineral depend on its chemical composition. Thus the foundations were laid new science- crystal chemistry. Many seemingly unrelated properties of minerals can be explained by taking into account their crystal structure and chemical composition. Some chemical elements (gold, silver, copper) are found in native, i.e. pure, form. They are built from electrically neutral atoms (unlike most minerals, whose atoms carry electric charge and are called ions). An atom with a lack of electrons is positively charged and is called a cation; an atom with an excess of electrons has negative charge and is called an anion. The attraction between oppositely charged ions is called ionic bonding and serves as the main binding force in minerals. With another type of bond, outer electrons rotate around the nuclei in common orbits, connecting the atoms to each other. Covalent bond is the strongest type of bond. Minerals with covalent bonds usually have high hardness and melting points (for example, diamond). A much smaller role in minerals is played by the weak van der Waals bond that occurs between electrically neutral structural units. The binding energy of such structural units (layers or groups of atoms) is distributed unevenly. The van der Waals bond provides attraction between oppositely charged regions in larger structural units. This type of bond is observed between layers of graphite (one of the natural forms of carbon), formed due to the strong covalent bond of carbon atoms. Due to the weak bonds between the layers, graphite has low hardness and very perfect cleavage, parallel to the layers. Therefore, graphite is used as a lubricant. Oppositely charged ions approach each other to a distance at which the repulsive force balances the attractive force. For any particular cation-anion pair, this critical distance is equal to the sum of the “radii” of the two ions. By determining the critical distances between different ions, it was possible to determine the size of the radii of most ions (in nanometers, nm). Since most minerals are characterized by ionic bonds, their structures can be visualized in the form of touching balls. The structures of ionic crystals depend mainly on the magnitude and sign of the charge and the relative sizes of the ions. Since the crystal as a whole is electrically neutral, the sum of the positive charges of the ions must be equal to the sum of the negative ones. In sodium chloride (NaCl, the mineral halite), each sodium ion has a charge of +1, and each chloride ion -1 (Fig. 1), i.e. Each sodium ion corresponds to one chloride ion. However, in fluorite (calcium fluoride, CaF2), each calcium ion has a charge of +2, and each fluoride ion has a charge of -1. Therefore, to maintain the overall electrical neutrality of fluorine ions, it must be twice as much as calcium ions (Fig. 2).

The possibility of their inclusion in a given crystal structure also depends on the size of the ions. If the ions are the same size and are packed in such a way that each ion touches 12 others, then they are in appropriate coordination. There are two ways to pack balls of the same size (Fig. 3): cubic dense packing, in general case leading to the formation of isometric crystals, and hexagonal close packing, forming hexagonal crystals. As a rule, cations are smaller in size than anions, and their sizes are expressed in fractions of the anion radius, taken as one. Typically the ratio obtained by dividing the radius of the cation by the radius of the anion is used. If a cation is only slightly smaller than the anions with which it combines, it can be in contact with the eight anions surrounding it, or, as is commonly said, is in eight-fold coordination with respect to the anions, which are located, as it were, at the vertices of a cube around it. This coordination (also called cubic) is stable at ionic radius ratios from 1 to 0.732 (Fig. 4a). At a smaller ionic radius ratio, eight anions cannot be stacked to touch the cation. In such cases, the packaging geometry allows six-fold coordination of cations with anions located at six vertices of the octahedron (Fig. 4b), which will be stable at ratios of their radii from 0.732 to 0.416. With a further decrease in the relative size of the cation, a transition occurs to quadruple, or tetrahedral, coordination, stable at radius ratios from 0.414 to 0.225 (Fig. 4c), then to triple coordination within radius ratios from 0.225 to 0.155 (Fig. 4c). d) and double - with radius ratios less than 0.155 (Fig. 4,e). Although other factors also determine the type of coordination polyhedron, for most minerals the ion radius ratio principle is one of effective means crystal structure prediction.

Minerals of completely different chemical compositions can have similar structures that can be described using the same coordination polyhedra. For example, in sodium chloride NaCl, the ratio of the radius of the sodium ion to the radius of the chlorine ion is 0.535, indicating octahedral, or six-fold, coordination. If six anions cluster around each cation, then to maintain a 1:1 cation to anion ratio, there must be six cations around each anion. This produces a cubic structure known as the sodium chloride type structure. Although the ionic radii of lead and sulfur differ sharply from the ionic radii of sodium and chlorine, their ratio also determines the sixfold coordination, therefore PbS galena has a sodium chloride-type structure, i.e., halite and galena are isostructural. Impurities in minerals are usually present in the form of ions that replace those of the host mineral. Such substitutions greatly affect the sizes of ions. If the radii of two ions are equal or differ by less than 15%, they are easily substituted. If this difference is 15-30%, such substitution is limited; with a difference of more than 30%, substitution is practically impossible. There are many examples of pairs of isostructural minerals with similar chemical compositions between which ion substitution occurs. Thus, the carbonates siderite (FeCO3) and rhodochrosite (MnCO3) have similar structures, and iron and manganese can replace each other in any ratio, forming the so-called. solid solutions. There is a continuous series of solid solutions between these two minerals. In other pairs of minerals, ions have limited possibilities for mutual substitution. Since minerals are electrically neutral, the charge of the ions also affects their mutual substitution. If substitution occurs with an oppositely charged ion, then a second substitution must take place in some part of this structure, in which the charge of the substituting ion compensates for the violation of electrical neutrality caused by the first. Such conjugate substitution is observed in feldspars - plagioclases, when calcium (Ca2+) replaces sodium (Na+) with the formation of a continuous series of solid solutions. The excess positive charge resulting from the replacement of the Na+ ion by the Ca2+ ion is compensated by the simultaneous replacement of silicon (Si4+) with aluminum (Al3+) in adjacent areas of the structure.
PHYSICAL PROPERTIES OF MINERALS
Although the main characteristics of minerals (chemical composition and internal crystal structure) are established on the basis of chemical analyzes and X-ray diffraction, they are indirectly reflected in properties that are easily observed or measured. To diagnose most minerals, it is enough to determine their luster, color, cleavage, hardness, and density. Luster is a qualitative characteristic of light reflected by a mineral. Some opaque minerals reflect light strongly and have a metallic luster. This is common in ore minerals such as galena (lead mineral), chalcopyrite and bornite (copper minerals), argentite and acanthite (silver minerals). Most minerals absorb or transmit a significant portion of the light falling on them and have a non-metallic luster. Some minerals have a luster that transitions from metallic to non-metallic, which is called semi-metallic. Minerals with a non-metallic luster are usually light-colored, some of them are transparent. Quartz, gypsum and light mica are often transparent. Other minerals (for example, milky white quartz) that transmit light, but through which objects cannot be clearly distinguished, are called translucent. Minerals containing metals differ from others in light transmission. If light passes through a mineral, at least in the thinnest edges of the grains, then it is, as a rule, non-metallic; if the light does not pass through, then it is ore. There are, however, exceptions: for example, light-colored sphalerite (zinc mineral) or cinnabar (mercury mineral) are often transparent or translucent. Minerals differ in the qualitative characteristics of their non-metallic luster. The clay has a dull, earthy sheen. Quartz on the edges of crystals or on fracture surfaces is glassy, ​​talc, which is divided into thin leaves along the cleavage planes, is mother-of-pearl. Bright, sparkling, like a diamond, shine is called diamond. When light falls on a mineral with a non-metallic luster, it is partially reflected from the surface of the mineral and partially refracted at this boundary. Each substance is characterized by a certain refractive index. Since this indicator can be measured with high accuracy, it is a very useful diagnostic feature of minerals. The nature of the luster depends on the refractive index, and both of them depend on the chemical composition and crystal structure of the mineral. In general, transparent minerals containing atoms heavy metals, are characterized by high luster and high refractive index. This group includes such common minerals as anglesite (lead sulfate), cassiterite (tin oxide) and titanite or sphene (calcium titanium silicate). Minerals composed of relatively light elements can also have high luster and a high refractive index if their atoms are tightly packed and held together by strong chemical bonds. A striking example is diamond, which consists of only one light element, carbon. To a lesser extent, this is true for the mineral corundum (Al2O3), the transparent colored varieties of which - ruby ​​and sapphires - are precious stones. Although corundum is composed of light atoms of aluminum and oxygen, they are so tightly bound together that the mineral has a fairly strong luster and a relatively high refractive index. Some glosses (oily, waxy, matte, silky, etc.) depend on the state of the surface of the mineral or on the structure of the mineral aggregate; a resinous luster is characteristic of many amorphous substances (including minerals containing the radioactive elements uranium or thorium). Color - simple and convenient diagnostic sign. Examples include brass-yellow pyrite (FeS2), lead-gray galena (PbS) and silvery-white arsenopyrite (FeAsS2). In other ore minerals with a metallic or semi-metallic luster, the characteristic color may be masked by the play of light in a thin surface film (tarnish). This is common to most copper minerals, especially bornite, which is called "peacock ore" because of its iridescent blue-green tarnish that quickly develops when freshly fractured. However, other copper minerals are painted in familiar colors: malachite - green, azurite - blue. Some non-metallic minerals are unmistakably recognizable by the color determined by the main chemical element (yellow - sulfur and black - dark gray - graphite, etc.). Many non-metallic minerals consist of elements that do not provide them with a specific color, but they have colored varieties, the color of which is due to the presence of impurities of chemical elements in small quantities that are not comparable with the intensity of the color they cause. Such elements are called chromophores; their ions are characterized by selective absorption of light. For example, a deep purple amethyst owes its color to an insignificant admixture of iron in quartz, and a thick green color emerald is associated with the small chromium content of beryl. The color of normally colorless minerals can appear due to defects in the crystal structure (caused by unfilled atomic positions in the lattice or the occurrence of foreign ions), which can cause selective absorption of certain wavelengths in the white light spectrum. Then the minerals are painted in additional colors. Rubies, sapphires and alexandrites owe their color to precisely these light effects. Colorless minerals can be colored by mechanical inclusions. Thus, thin scattered dissemination of hematite gives quartz a red color, chlorite - green. Milky quartz is clouded with gas-liquid inclusions. Although mineral color is one of the most easily determined properties in mineral diagnostics, it must be used with caution as it depends on many factors. Despite the variability in the color of many minerals, the color of the mineral powder is very constant, and therefore is an important diagnostic feature. Typically, the color of a mineral powder is determined by the line (the so-called “line color”) that the mineral leaves when passed over an unglazed porcelain plate (biscuit). For example, the mineral fluorite comes in different colors, but its streak is always white.
Cleavage. Characteristic property minerals is their behavior when splitting. For example, quartz and tourmaline, whose fracture surface resembles a glass chip, have a conchoidal fracture. In other minerals, the fracture may be described as rough, jagged, or splintered. For many minerals, the characteristic is not fracture, but cleavage. This means that they cleave along smooth planes directly related to their crystal structure. The bonding forces between the planes of the crystal lattice can vary depending on the crystallographic direction. If in some directions they are much larger than in others, then the mineral will split across the very weak connection. Since cleavage is always parallel to the atomic planes, it can be designated by indicating crystallographic directions. For example, halite (NaCl) has cube cleavage, i.e. three mutually perpendicular directions of possible split. Cleavage is also characterized by the ease of manifestation and the quality of the resulting cleavage surface. Mica has very perfect cleavage in one direction, i.e. easily splits into very thin leaves with a smooth shiny surface. Topaz has perfect cleavage in one direction. Minerals can have two, three, four or six cleavage directions, along which they split equally easily, or several cleavage directions varying degrees. Some minerals have no cleavage at all. Since cleavage, as a manifestation of the internal structure of minerals, is their constant property, it serves as an important diagnostic feature. Hardness is the resistance that a mineral exhibits when scratched. Hardness depends on the crystal structure: the more tightly the atoms in the structure of a mineral are connected to each other, the more difficult it is to scratch it. Talc and graphite are soft plate-like minerals, built from layers of atoms bonded together by very weak forces. They are greasy to the touch: when rubbed against the skin of the hand, individual thin layers slip off. The hardest mineral is diamond, in which the carbon atoms are so tightly bonded that it can only be scratched by another diamond. At the beginning of the 19th century. Austrian mineralogist F. Moos arranged 10 minerals in increasing order of their hardness. Since then, they have been used as standards for the relative hardness of minerals, the so-called. Mohs scale (Table 1). Table 1.
MOH HARDNESS SCALE

Mineral Relative Hardness
Talc ______1 Gypsum _______2 Calcite ____3 Fluorite ____4 Apatite _____5 Orthoclase ___6 ​​Quartz ______7 Topaz ______8 Corundum _____9 Diamond _____10

To determine the hardness of a mineral, it is necessary to identify the hardest mineral that it can scratch. The hardness of the mineral being examined will be greater than the hardness of the mineral it scratched, but less than the hardness of the next mineral on the Mohs scale. Bonding forces can vary depending on the crystallographic direction, and since hardness is a rough estimate of these forces, it can vary in different directions. This difference is usually small, with the exception of kyanite, which has a hardness of 5 in the direction parallel to the length of the crystal and 7 in the transverse direction. In mineralogical practice, the measurement of absolute hardness values ​​(the so-called microhardness) using a sclerometer device, which is expressed in kg/mm2, is also used.
Density. The mass of atoms of chemical elements varies from hydrogen (the lightest) to uranium (the heaviest). All other things being equal, the mass of a substance consisting of heavy atoms is greater than that of a substance consisting of light atoms. For example, two carbonates - aragonite and cerussite - have a similar internal structure, but aragonite contains light calcium atoms, and cerussite contains heavy lead atoms. As a result, the mass of cerussite exceeds the mass of aragonite of the same volume. The mass per unit volume of a mineral also depends on the atomic packing density. Calcite, like aragonite, is calcium carbonate, but in calcite the atoms are less densely packed, so it has less mass per unit volume than aragonite. Relative mass, or density, depends on the chemical composition and internal structure. Density is the ratio of the mass of a substance to the mass of the same volume of water at 4 ° C. So, if the mass of a mineral is 4 g, and the mass of the same volume of water is 1 g, then the density of the mineral is 4. In mineralogy, it is customary to express density in g/ cm3. Density is an important diagnostic feature of minerals and is not difficult to measure. First, the sample is weighed in air environment and then in the water. Since a sample immersed in water is subject to an upward buoyant force, its weight there is less than in air. The weight loss is equal to the weight of water displaced. Thus, density is determined by the ratio of the mass of a sample in air to its weight loss in water.
CLASSIFICATION OF MINERALS
Although chemical composition has served as the basis for the classification of minerals since the mid-19th century, mineralogists have not always adhered to consensus about what should be the order of arrangement of minerals in it. According to one method of constructing a classification, minerals were grouped according to the same main metal or cation. In this case, iron minerals fell into one group, lead minerals into another, zinc minerals into a third, etc. However, as science developed, it became clear that minerals containing the same nonmetal (anion or anionic group) have similar properties and are much more similar to each other than minerals with a common metal. In addition, minerals with a common anion occur in the same geological setting and are of similar origin. As a result, in modern taxonomy (see Table 2), minerals are grouped into classes based on a common anion or anionic group. The only exception is native elements, which occur in nature by themselves, without forming compounds with other elements.

Table 2.
CLASSIFICATION OF MINERALS

Chemical classes are divided into subclasses (by chemistry and structural motif), which, in turn, are divided into families and groups (by structural type). Individual mineral species within a group may form rows, and one mineral species may have several varieties. By now approx. 4000 minerals are recognized as independent mineral species. New minerals are added to this list as they are discovered and long-known, but discredited, as methods of mineralogical research are improved, they are excluded.
ORIGIN AND CONDITIONS OF FINDING MINERALS
Mineralogy is not limited to determining the properties of minerals; it also studies the origin, conditions of occurrence and natural associations of minerals. Since the origin of the Earth approximately 4.6 billion years ago, many minerals have been destroyed by mechanical crushing, chemical transformation or melting. But the elements that made up these minerals were preserved, regrouped and formed new minerals. Thus, the minerals that exist today are the products of processes that have evolved over the course of geological history Earth. Most of The earth's crust is composed of igneous rocks, which in some places are covered by a relatively thin cover of sedimentary and metamorphic rocks. Therefore, the composition of the earth's crust, in principle, corresponds to the average composition of the igneous rock. Eight elements (see Table 3) make up 99% of the mass of the earth’s crust and, accordingly, 99% of the mass of the minerals composing it.

Table 3.
MAIN ELEMENTS INCLUDED IN THE EARTH'S CRUST

In terms of elemental composition, the earth's crust is a frame structure consisting of oxygen ions associated with smaller ions of silicon and aluminum. Thus, the main minerals are silicates, which account for approx. 35% of all known minerals and approx. 40% - the most common. The most important of them are feldspars (a family of aluminosilicates containing potassium, sodium and calcium, and less commonly barium). Other common rock-forming silicates are quartz (however, it is more often classified as oxides), micas, amphiboles, pyroxenes and olivine.
Igneous rocks. Igneous, or igneous, rocks are formed when molten magma cools and crystallizes. Percentage different minerals and therefore the type of rock formed depend on the ratio of elements contained in the magma at the time of its solidification. Each type of igneous rock usually consists of a limited set of minerals called major rocks. In addition to them, minor and accessory minerals may be present in smaller quantities. For example, the main minerals in granite may be potassium feldspar (30%), sodium calcium feldspar (30%), quartz (30%), micas and hornblende (10%). Zircon, sphene, apatite, magnetite and ilmenite may be present as accessory minerals. Igneous rocks are usually classified based on the type and amount of each feldspar they contain. However, some rocks lack feldspar. Igneous rocks are further classified by their structure, which reflects the conditions under which the rock solidified. Slowly crystallizing deep within the Earth, magma gives rise to intrusive plutonic rocks with a coarse- to medium-grained structure. If magma erupts to the surface as lava, it cools quickly and produces fine-grained volcanic (effusive, or extrusive) rocks. Sometimes some volcanic rocks (for example, obsidian) cool so quickly that they do not have time to crystallize; similar rocks have a glassy appearance (volcanic glasses).
Sedimentary rocks. When bedrock is weathered or eroded, clastic or dissolved material becomes incorporated into the sediment. As a result of chemical weathering of minerals, which occurs at the boundary of the lithosphere and atmosphere, new minerals are formed, for example, clay minerals from feldspar. Some elements are released when minerals (such as calcite) dissolve in surface waters. However, other minerals, such as quartz, even mechanically crushed, remain resistant to chemical weathering. Mechanically and chemically stable minerals with a sufficiently high density released during weathering form on earth's surface placer deposits. From placers, most often alluvial (river), gold, platinum, diamonds, other precious stones, tin stone (cassiterite), and minerals of other metals are mined. Under certain climatic conditions, thick weathering crusts are formed, often enriched with ore minerals. Weathering crusts are associated with industrial deposits of bauxite (aluminum ores), accumulations of hematite (iron ores), hydrous nickel silicates, niobium minerals and others. rare metals. The bulk of weathering products is carried through a system of watercourses into lakes and seas, at the bottom of which it forms a layered sedimentary layer. Shales are composed primarily of clay minerals, while sandstone is composed primarily of cemented quartz grains. Dissolved material may be removed from the water by living organisms or precipitated through chemical reactions and evaporation. Calcium carbonate is absorbed from sea ​​water molluscs that build their hard shells from it. Most limestones are formed by the accumulation of shells and skeletons of marine organisms, although some calcium carbonate is precipitated chemically. Evaporite deposits are formed as a result of the evaporation of sea water. Evaporites are a large group of minerals, which include halite (table salt), gypsum and anhydrite (calcium sulfates), sylvite (potassium chloride); they all have important practical use. These minerals are also deposited during evaporation from the surface of salt lakes, but in this case, an increase in the concentration of rare elements can lead to additional precipitation of some other minerals. It is in this environment that borates are formed.
Metamorphic rocks. Regional metamorphism. Igneous and sedimentary rocks, buried at great depths, under the influence of temperature and pressure, undergo transformations called metamorphic, during which the original properties of rocks change, and the original minerals recrystallize or are completely transformed. As a result, minerals are usually arranged along parallel planes, giving the rocks a schistose appearance. Thin schistose metamorphic rocks are called shales. They are often enriched in plate silicate minerals (mica, chlorite or talc). Coarser schistose metamorphic rocks are gneisses; they contain alternating bands of quartz, feldspar and dark-colored minerals. When schists and gneisses contain some typically metamorphic mineral, this is reflected in the name of the rock, for example, sillimanite or staurolite schist, kyanite or garnet gneiss.
Contact metamorphism. When magma rises to the upper layers of the earth's crust, changes usually occur in the rocks into which it has intruded, the so-called. contact metamorphism. These changes are manifested in the recrystallization of the original or the formation of new minerals. The extent of metamorphism depends on both the type of magma and the type of rock it pervades. Clayey rocks and rocks similar in chemical composition are transformed into contact hornfels (biotite, cordierite, garnet, etc.). The most intense changes occur when granitic magma intrudes into limestones: thermal effects cause their recrystallization and the formation of marble; as a result chemical interaction with limestones, solutions separated from the magma are formed large group minerals (calcium and magnesium silicates: wollastonite, grossular and andradite garnets, vesuvianite, or idocrase, epidote, tremolite and diopside). In some cases, contact metamorphism introduces ore minerals, making the rocks valuable sources of copper, lead, zinc and tungsten.
Metasomatosis. As a result of regional and contact metamorphism, there is no significant change in the chemical composition of the original rocks, but only their mineral composition and appearance. When solutions introduce some elements and remove others, a significant change in the chemical composition of the rocks occurs. Such newly formed rocks are called metosomatic. For example, the interaction of limestones with solutions released by granitic magma during crystallization leads to the formation around granite massifs of zones of contact-metasomatic ores - scarps, which often host mineralization.
ORE DEPOSITS AND PEGMATITE
The chemical composition of coarse-grained granite can differ significantly from the composition of the original magma. The study of rocks showed that minerals are released from magma in a certain sequence. Iron- and magnesium-rich minerals such as olivine and pyroxenes, as well as accessory minerals, crystallize first. Due to their higher density than the surrounding melt, they settle downward as a result of the process of magmatic segregation. It is believed that dunites are formed in this way - rocks consisting almost entirely of olivine. Similar origins are attributed to some large accumulations of magnetite, ilmenite and chromite, which are the iron, titanium and chromium series respectively. However, the composition of the melt remaining after minerals are removed by magmatic segregation is not completely identical to the composition of the rock formed from it. During the crystallization of the melt, the concentration of water and other volatile components (for example, fluorine and boron compounds) increases in it, and along with them many other elements whose atoms are too large or too small to enter the crystalline structures of rock-forming minerals. Aqueous fluids released from crystallizing magma can rise through cracks to the Earth's surface, into an area of ​​lower temperatures and pressures. This causes the deposition of minerals in cracks and the formation of vein deposits. Some veins are composed mainly of non-metallic minerals (quartz, calcite, barite and fluorite). Other veins contain minerals of metals such as gold, silver, copper, lead, zinc, tin and mercury; accordingly, they may represent valuable ore deposits. Since such deposits are formed with the participation of heated aqueous solutions, they are called hydrothermal. It should be said that the largest hydrothermal deposits are not vein, but metasomatic; they are sheet-like or other shaped deposits formed by replacing rocks (most often limestone) with ore-bearing solutions. The minerals that make up such deposits are said to be of hydrothermal-metasomatic origin. Pegmatites are genetically related to crystallizing granitic magma. A mass of highly mobile fluid, still rich in the elements that make up the rock-forming minerals, can be ejected from the magma chamber into the host rock, where it crystallizes to form bodies of a coarse-grained structure, composed mainly of rock-forming minerals - quartz, feldspar and mica. Such rock bodies, called pegmatites, are highly variable in size. The maximum length of most pegmatite bodies is several hundred meters, but the largest of them reach a length of 3 km, and for small ones it is measured in the first meters. Pegmatites contain large crystals of individual minerals, including the world's largest feldspars several meters long, mica - up to 3 m in diameter, quartz - weighing up to 5 tons. Rare elements are concentrated in some pegmatite-forming fluids (often in the form of large crystals) , for example, beryllium - in beryl and chrysoberyl, lithium - in spodumene, petalitite, amblygonite and lepidolite, cesium - in polucite, boron - in tourmaline, fluorine - in apatite and topaz. Most of these minerals are of jewelry varieties. The industrial importance of pegmatites is partly due to the fact that they are a source of precious stones, but mainly - high-grade potassium feldspar and mica, as well as ores of lithium, cesium and tantalum, and partly beryllium.