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ROCKS CARBONATE - siege, item, consisting of more than 50% of one or more carbonate m-fishing; these are limestones, dolomites and transitional differences between them. Siderite, magnesite, and ankerite sediments are limited in distribution. P. to., which are already ores; along with breinerite, witherite, rhodochrosite, strontianite, and oligonite, they form interlayers, lenses, and concretions. Aragonite, which forms the skeletons and shells of many organisms, or precipitates chemically, is not very stable and is usually absent from ancient P. to. P. to. clastic, pyroclastic and chemogenic material, clay and siliceous materials, org. leftovers. Of the autogenous minerals, glauconite, quartz, chalcedony, anhydrite, gypsum, pyrite, alkali feldspars, etc. are found. P. to. refers, as a rule, to rock formations with a rigid connection between the grains, that is, to solid p.; P. to. can be dense, porous and fissured; the last two varieties stand out in porous and fractured carbonate reservoirs. The textures of sieges, in particular, and P. k. (Teodorovich, 1941), can be estimated for sieges, formations as a whole, depending on the layering - lapido textures (layered, micro-, obliquely and non-layered) and for individual interlayers of layered sediments , formations (or non-layered areas as a whole) - stratitextures (random, plane-parallel textures of layering and growth, textures of “flows”, “cone to cone”, etc.). Items to. have the various structures relating to primary and secondary. On P.'s structures to. it is possible to subdivide into the following tr. : 1) structurally homogeneous (from components of the same type); 2) structurally more or less homogeneous (from evenly distributed components of two or more types); 3) structurally heterogeneous (from areas of different outlines of different structures). Let us give a structural classification of limestones only for the first two groups. It is advisable to use the structural-genetic classification, in which the main gr. - genetic, and smaller ones - structural. There are 4 main genetic groups. limestones with the following subgroups. and types (Teodorovich, 1941, 1958, 1964): I. Clearly organogenic or biogenic: A. Biomorphic: a) stereophytrous - firmly growing (reef cores, biostromes, etc.); 6) hemistereophytrous (organogenic-nodular); c) Astereophytroids, which initially accumulated in the form of silt (Foraminifera, Ostracodidae, etc.) ). B. Fragmentary (spicules, etc.). B. Biomorphic-detritus and detritus-biomorphic: 1) stereophytrous; 2) astereophytrous. G. Biodetritus and biosludge. II. Biochemogenic: A. Coprolitic. B. and C. Lumpy and micro-lumpy (often these are waste products of blue-green algae). G. Clotted. D. Microgranular, microlayered (bacterial). III. Chemogenic: A. Clear-grained. B. Microgranular. C. Oolitic, etc. D. Hostereophytrous - cortical, incrustation, etc. IV. Clastic: A. Conglomerate and breccia. B. Sandstone and siltstone. The most detailed and substantiated genetic classification of limestones was proposed by Shvetsov (1934, 1948). Numerous classifications of mineral rock are known, taking into account, in addition to the carbonate part, the amount of clay or clastic material present in them (Noinsky, 1913; Vishnyakov, 1933; Pustovalov, 1940; Teodorovich, 1958; Khvorova, 1958; and others). Folk's classification is widespread abroad (Folk, 1962). For an in-depth facies analysis of carbonate excels, especially limestones, it is necessary to give the most differentiated quantitative characteristics of their compositional features (Marchenko, 1962). Limestones and dolomites are widely distributed in nature, while limestone-dolomite deposits are less developed and widely used in industry (metallurgical, chemical, textile, paper, construction, etc.) and in agriculture (fertilizers). V. I. Marchenko, O. I. Nekrasova, G. I. Teodorovich.

Source: Geological Dictionary

ROCKS CARBONATE - siege, item, consisting of more than 50% of one or more carbonate m-fishing; these are limestones, dolomites and transitional differences between them. Siderite, magnesite, and ankerite sediments are limited in distribution. P. to., which are already ores; along with breinerite, witherite, rhodochrosite, strontianite, and oligonite, they form interlayers, lenses, and concretions. Aragonite, which forms the skeletons and shells of many organisms, or precipitates chemically, is not very stable and is usually absent from ancient P. to. P. to. clastic, pyroclastic and chemogenic material, clay and siliceous materials, org. leftovers. Of the autogenous minerals, glauconite, quartz, chalcedony, anhydrite, gypsum, pyrite, alkali feldspars, etc. are found. P. to. refers, as a rule, to rock formations with a rigid connection between the grains, that is, to solid p.; P. to. can be dense, porous and fissured; the last two varieties stand out in porous and fractured carbonate reservoirs. The textures of sieges, strata, in particular, and strata (Teodorovich, 1941), can be estimated for sieges, formations as a whole, depending on the layering - (layered, micro-, oblique, and non-layered) and for individual interlayers of layered sediments, formations (or non-layered areas as a whole) - stratitextures (random, plane-parallel textures of layering and growth, textures of “flows”, “cone to cone”, etc.). Items to. have the various structures relating to primary and secondary. On P.'s structures to. it is possible to subdivide into the following tr. : 1) structurally homogeneous (from components of the same type); 2) structurally more or less homogeneous (from evenly distributed components of two or more types); 3) structurally heterogeneous (from areas of different outlines of different structures). Let us give a structural classification of limestones only for the first two groups. It is advisable to use the structural-genetic classification, in which the main gr. - genetic, and smaller ones - structural. There are 4 main genetic groups. limestones with the following subgroups. and types (Teodorovich, 1941, 1958, 1964): I. Clearly organogenic or biogenic: A. Biomorphic: a) stereophytrous - firmly growing (reef cores, biostromes, etc.); 6) hemistereophytrous (organogenic-nodular); c) Astereophytroids, which initially accumulated in the form of silt (foraminifera, ostracods, etc.). B. Fragmentary (spicule, etc.) P.). B. Biomorphic-detritus and detritus-biomorphic: 1) stereophytrous; 2) astereophytrous. G. Biodetritus and biosludge. II. Biochemogenic: A. Coprolitic. B. and C. Lumpy and micro-lumpy (often these are waste products of blue-green algae). G. Clotted. D. Microgranular, microlayered (bacterial). III. Chemogenic: A. Clear-grained. B. Microgranular. C. Oolitic, etc. D. Hostereophytrous - cortical, incrustation, etc. IV. Clastic: A. Conglomerate and breccia. B. Sandstone and siltstone. The most detailed and substantiated genetic classification of limestones was proposed by Shvetsov (1934, 1948). Numerous classifications of mineral rock are known, taking into account, in addition to the carbonate part, the amount of clay or clastic material present in them (Noinsky, 1913; Vishnyakov, 1933; Pustovalov, 1940; Teodorovich, 1958; Khvorova, 1958; and others). Folk's classification is widespread abroad (Folk, 1962). For an in-depth facies analysis of carbonate excels, especially limestones, it is necessary to give the most differentiated quantitative characteristics of their compositional features (Marchenko, 1962). Limestones and dolomites are widely distributed in nature, while limestone-dolomite deposits are less developed and widely used in industry (metallurgical, chemical, textile, paper, construction, etc.) and in agriculture (fertilizers). V. I. Marchenko, O. I. Nekrasova, G. I. Teodorovich.

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Carbonate rocks

Carbonate rocks include rocks that are 50% or more composed of carbonate minerals: calcite - CaCO3, aragonite - CaCO3, dolomite - Ca, Mg (CO3) 2, less often siderite - FeCO3 and ankerite Ca (Fe, Mg) 2.

Since calcite and dolomite compose thick layers and strata of limestones and dolomites, and ankerite and siderite occur in sedimentary rocks as inclusions, nodules, in a scattered form, therefore, only calc-magnesian carbonate rocks are more often considered.

According to the mineralogical composition, calcareous-magnesian carbonate rocks are subdivided into limestones and dolomites. These rocks often contain clayey, silty, and sandy admixtures. In addition, there are carbonate rocks of mixed composition.

Limestones

Limestones are called carbonate rocks, which are 50% or more composed of the mineral calcite.

There are 4 structural, and in essence - these are structural-genetic groups of carbonate rocks (M.S. Shvetsov, 1958):

1) organogenic

2) grainy

3) clastic

4) significantly changed

Within the groups, according to the shape, size and ratio of structural elements (shells, crystals, fragments, etc.), types of rocks are distinguished.

Structural-genetic classification of limestones

Group I Organogenic

A. Biomorphic

1. Bioherm (reef)

a) coral

b) bryozoans

c) algal (stromatolitic, oncolithic)

2. Whole shell

a) large-shell (shellfish):

1. brachiopods

2. pelecypods

3. gastropods

4. cephalopods, etc.

b) small-shelled:

1. foraminiferal (fusulin, globigerin, nummulite, etc.)

2. ostracods

3.coccolithic

B. Detrital (organogenic-detrital):

1. brachiopods

2. pelecypods

3. bryozoans

4. crinoids

5. coccolithic

6. polydetrital

Group II Granular (chemogenic):

1. micro-grained, fine-grained, medium-grained, coarse-grained

2. oolitic and pisolithic

Group III Detrital (various size and roundness)

Group IV Changed:

1. recrystallized: coarse, medium, fine, and inequigranular

2. granular: part of lumpy and pseudo-oolitic

3. coprogenous: part of pseudo-oolithic and lumpy

4. substitution

According to the skeletal remains of organisms, organogenic rocks (almost exclusively limestones) are divided into biomorphic - biohermal and whole-shell and detrital.

Biohermal limestones include coral, bryozoan, algal limestones. They are distinguished by a lenticular, even columnar shape of the deposit, uneven layering or its absence, usually by spores. Bioherms are characterized by an abundance of attached organisms forming large clusters. Shells of other organisms are also found here - whole and detritus.

Representatives of organogenic limestones are reef bioherm limestones, which consist of the remains of colonial or accreting organisms. Biogenic limestones compose various bodies or layers. They are the basis of fossil reefs - organogenic structures that have reached sea level, which are breakwaters. Reefs are formed by various organisms.

A characteristic feature of reef limestones is their occurrence in the form of thick and irregularly shaped massifs, rising sharply above the sediments that formed simultaneously with them. Clastic limestones, formed due to the destruction of reefs, adjoin the reefs at angles of 30-50o. The thickness of the reefs is about 1000 m or more.

A feature of biohermal limestones is: 1) their formation due to specific groups of organisms;

2) massive structure;

3) biohermal textures;

4) no admixture of detrital material;

5) an abundance of caverns filled with syngenetic and epigenetic carbonates;

6) inlay structures.

Whole-shell limestones consist of whole shells. In turn, they are divided into shell rocks, consisting of large shells (usually pelecypods, gastropods, brachiopods) and rocks, consisting of small and tiny shells of ostracods, coccolithophorids, foraminifers (fusulins, globigerins, nummulites).

Detrital or detrital (organogenic-detrital) limestones consist of fragments of skeletal remains of organisms, in contrast to clastic limestones, they (that is, fragments of shells) are not rounded. Limestones differ according to the systematic affiliation of organic remains and are either homogeneous in composition - monodetrital (pelecypod, foraminiferal, crinoid, algal), as well as mixed - polydetritic (crinoid - brachiopod, brachiopod - crinoid, etc.).

Detrital limestones are classified according to the size of the fragments and are distinguished:

Coarse detrital (fragments larger than 1 mm)

coarse detritus (1-0.5 mm)

medium detritus (0.5-0.25 mm)

fine detritus (0.25-0.1 mm)

and fine-detrital or sludge (< 0,1 мм)

Granular are a product of a chemical charge occurring in silt waters. They differ in uniformity and density. These include inequigranular limestones, oolitic, pisolite, pseudo-oolitic.

Among the granular limestones are:

1) coarse-grained (grains larger than 0.5 mm)

2) medium-grained (0.1 - 0.5 mm)

3) fine-grained (0.1-0.01 mm)

4) micro-grained (0.01-0.0001 mm)

5) colloidal granular (grains are less than the resolving power of the microscope, i.e. approximately< 0,0001 мм).

This group includes calcareous tuffs, which are continental formations. They are formed on land at the exit of springs as a result of the absorption of CO2 by plants, which causes the precipitation of calcite, most often on the leaves and stems of plants. Therefore, these deposits are porous and have peculiar patterns.

When such limestones (granular) are formed without the participation of plants, they have a microlayered texture, an elongated - granular structure. These types of limestones include stalactites, stalagmites, travertines, this group of rocks is called lime incrustations.

Oolitic limestones, more rarely dolomites, are chemical sediments of warm moving waters, where calcite or dolomite is deposited in thin (up to hundredths of a mm) concentric shells around the embryonic grain, which can be grains of sand, shell fragments, calcareous silt clots. Oolites are oval or spherical in shape, usually up to 2 mm in size; larger ooliths are called pisoliths or beans. In the process of diagenesis, oolites, due to recrystallization or recrystallization, acquire a radial-radiant (spherolitic) structure, i.e. their fine-grained carbonate turns into acicular.

In the case of granulation, the oolites lose their concentric and radially radiant structure and turn into pseudo-oolites - lumps of fine-grained carbonate. They are cemented by coarse-grained calcite with a granoblastic texture.

Clastic limestones. They are composed of fragments of organogenic or granular (chemogenic) limestones of varying degrees of roundness, among them are:

1) conglomerate, breccia (fragments larger than 1 cm)

2) gravel, gruss (debris 10-1 mm)

3) sandstone (fragments 1-0.1 mm)

4) siltstone (fragments<0,1мм)

They are characterized by poor sorting. By genesis, these are syngenetic breeds, i.e. limestones were formed not from terrigenous material, but from calcareous sediment or shells in place, in the wave-surf zone, and this is their difference from clastic rocks.

Clastic limestones are connected by a gradual transition with detritus limestones and, from which, despite the organogenic nature of the fragments, they differ in the roundness of the latter, which indicates a significant washing and processing of limestone fragments or shells by moving water.

Altered limestones include limestones of various groups that have undergone various changes at the stages of diagenesis and metagenesis, as a result of the processes of recrystallization, granulation, replacement, as a result of the vital activity of organisms.

Recrystallization is a process in which larger crystals grow that are more stable in a given environment. This happens, as a rule, with acidification of the medium, an increase in temperature and pressure; in the presence of pores, voids, granular inclusions (sandy-silty material), under conditions that increase the mobility of atoms and heterogeneity of the rock. At the same time, micro-, fine-grained limestones become medium- and coarse-grained, acquire a sugar-like appearance, the primary structures disappear and the rock acquires relict structures, which are poorly defined. If limestones turn into marbles, then the primary structure is not established at all, sometimes polysynthetic twins develop in calcite.

Granulation is the reverse process of recrystallization. During limestone granulation, large crystals and the spherulitic structure of oolites, the skeletal remains of organisms, disintegrate into small, randomly oriented ones. Limestones with an uneven crystal structure are described as pseudo-oolitic, which differ from oolitic in the absence of a concentric structure, or as lumpy or clotted rock.

As a result of the processes of replacement, calcitization, dolomitization, crushing of sandstones, siltstones and other rocks, new rocks are formed, in which relic (primary) structures are locally preserved. In the case of complete processing of the original rock, new structures and textures develop.

Coprogenic limestones are quite widespread and represent clusters (up to 1 mm) of rounded, elongated coprolites, which consist of microgranular calcite. Coprolites pass lime sludge through the intestines, and as a result, lumps of microgranular calcite are formed.

The occurring clotted and lumpy limestones are considered by some scientists to be coprogenous, altered at various stages.

limestone dolomite carbonate rock

Dolomites

Dolomites are rocks composed of more than 50% of the mineral dolomite. As an impurity, the rock contains calcite, less often pyrite, chalcedony, quartz, organic matter, anhydrite, and clay minerals.

Clastic, algal and chemogenic dolomites are widespread. Among clastic dolomites, conglomerates, breccias, rocks with a much smaller grain size, sometimes up to sandy size (1-0.15 mm), are distinguished. They are composed of rounded and angular fragments of dolomite, which are cemented with dolomite or calcite cement. There is an admixture of terrigenous material.

Clastic dolomites have spread among dolomite strata of considerable thickness and are formed as a result of washing of these strata in beach conditions, in shallow water. Less commonly, breccias are of chemical origin. These are weathering breccias on dolomitic rocks.

Dolomites with an organogenic structure contain various organic remains composed of pelitomorphic, fine-grained dolomite and cemented by pelitomorphic or granular dolomite; calcite is often present in the cement. Dolomites of this type are formed during dolomitization of calcareous sediments or during epigenetic replacement of limestones at the stages of catagenesis or metagenesis. Sometimes dolomites contain remains of brachiopods, bryozoans, and corals.

Organogenic - include algal dolomites. They are composed mainly of blue-green and green algae, which concentrate magnesium carbonate in their bodies. The cement in the rock is dolomite, which is usually very small. Bioherm dolomites are characterized by high porosity and cavernousness. Sometimes there are dolomites with redeposited algae. They are distinguished by thin horizontal layering and greater density.

Chemogenic dolomites are composed of pelitomorphic and fine-grained dolomite, organic remains are practically absent, sometimes contain an admixture of clay material in the form of thin layers of hydromicaceous and montmorillonite composition.

Oolitic dolomites are composed of ooliths with a radially radiant and concentric structure, they are cemented by pelitomorphic and granular dolomite, rarely contain remains of marine fauna - crinoids, mollusks.

Carbonate rocks of mixed composition

Mixed carbonate rocks include:

dolomitic limestones (25-50% dolomite), calcareous dolomites (more than 50% dolomite), siliceous limestones and dolomites, carbonaceous limestones, clayey limestones-marls.

Siliceous limestones contain up to 25% silica, silicite - up to 50% (Baykov et al., 1980). The rocks are characterized by high strength, silica segregations are clearly visible in them. With a silica content of more than 50%, the rocks will be called silicites.

Carbonaceous limestones contain up to 50% carbonaceous material and are found among coal seams. Usually the rocks are black, they contain plant imprints, charred plant remains, and this is their difference from other carbonate rocks.

This group of carbonate rocks includes calcareous and dolomitic clays, siltstones, mudstones, and sandstones.

Marls also belong to rocks of mixed composition. These are pelitomorphic, fine-grained, soft, rarely hard rocks of various colors. Composition Calcite (rarely dolomite) and fine clay material, which may be present in significant amounts (up to 50%). The admixture of clay material is distributed fairly evenly over the rock; often thin layers or lenses of clay are found in the strata of marls. Basically, the composition of the clay substance is represented by montmorillonite. The rocks contain glauconite, pyrite, barite, a lot of organic material represented by skeletons of foraminifers, coccolithophorids, etc. Marls form thick strata, they alternate with limestones, dolomites, writing chalk, sometimes with sandy-clayey rocks.

Origin of carbonate rocks

Clastic limestones are formed as a result of the destruction and washing of older limestones and mechanical processing of the skeletons of limestone organisms. Shells and their fragments are subjected to mechanical processing in the zone of surf, unrest, as a result of tidal currents, and to one degree or another roll over. Shells are crushed and silt eaters. This is how the main part of shallow-water carbonate sediments of modern seas is formed. When debris is buried close to sources of drift (without mechanical treatment), breccias are formed. Limestones formed as a result of mechanical processing of shells are called organogenic-detrital.

Bioherm limestones are a product of vital activity of animals and plants. These include bioherms - lifetime accumulations of attached organisms in a growth position, and biocenoses - lifetime accumulations of organisms that live together in a certain area of ​​the bottom of the pool.

Chemogenic limestones are formed during sedimentogenesis and early diagenesis. Chemogenic planting occurs in modern seas and oceans, as well as in land water bodies with an arid climate. The role of the chemogenic CaCO3 charge in the geological past was more significant. As a result of the chemogenic cage, pelitomorphic, oolitic limestones and numerous carbonate nodules are formed in terrigenous rocks. The mechanism of this process is as follows. In the waters of the seas and oceans of low latitudes in the shallow area, as well as in the land water bodies of the arid zone, Ca carbonate is contained in an amount close to saturation, or even saturates the water. CaCO3 monocarbonate is a practically insoluble compound (its solubility is 0.001 g per 100 g of water). With an excess of CO2 in water, it turns into bicarbonate - Ca (HCO3) 2 - a highly soluble compound. In natural waters, there is a moving equilibrium:

CaCO3 + CO2 + H2O = Ca(HCO3)2

When excess CO2 is released into the atmosphere, the equilibrium shifts towards the formation of water-insoluble monocarbonates. The reason for the decrease in the CO2 content can be the warming of the water, the activity of organisms (algae), agitation, which removes excess CO2 and supplies the smallest CaCO3 crystals (seed) during the agitation of the silt.

There are several points of view on the origin of dolomites. Currently, the existence of 3 genetic types of dolomites is considered proven:

1. Primary dolomites - sedimentary, formed as a result of chemogenic cage from the waters of the basin. This type of dolomite is widespread in the Proterozoic and Lower Paleozoic deposits.

2. Dolomites, which were formed during the period of diagenesis under the influence of sea and silt waters on calcareous and calcareous-dolomite sediments.

3. Dolomites formed as a result of metasomatism (during catagenesis, metagenesis and hypergenesis) under the influence of magnesium-enriched waters on limestone rocks), the so-called epigenetic dolomites.

Limestones compose thick strata in the Cambrian of Siberia, the Urals, and Central Asia; in the Silurian of the St. Petersburg region, the Baltic states, the Urals, Central Asia, Ciscaucasia; in the Devonian of the Russian platform, the Urals, Siberia; in the Carboniferous of the Russian Platform. In Triassic deposits, they are found in the Caucasus, Crimea, and Central Asia; in the Jurassic they are developed in the Caucasus, in the Crimea; in Cretaceous sediments are represented by chalk and limestone; in Tertiary deposits are widely distributed in the Caucasus, Transcaucasia.

Dolomites are less common than limestones. They have been studied in the Cambrian of Siberia; in the Silurian - on the Siberian platform and in the Baltics; in the Devonian - Central Asia; Devonian and Carboniferous on the Russian Platform; in Perm - in the east of the Russian platform; Upper Jurassic - in the Pamir-Altai system; in Tertiary deposits - in Tajikistan.

Limestones are one of the important minerals. Their main consumers are the metallurgical and cement industries. They are widely used in construction, chemical, glass and agriculture industries. Large reserves of oil and gas are associated with carbonate reservoirs. Limestones are associated with sheet-like deposits of barite, magnesite, fluorite, calcareous manganese ores, solid and disseminated antimonite ores; sheet-like and vein-like deposits of siderite; sheet-like deposits and lenses of strontium; uranium-vanadium and tyuyamunite ores; layers and deposits of irregular shape of disseminated ores of lead, zinc, antimony, mercury, copper (copper is often mixed with cobalt); irregular deposits of arsenopyrite (Handbook of lithology, 1983). In phosphorite-bearing and bituminous limestones, along with a high content of phosphorus, there are increased amounts of strontium, barium, molybdenum, uranium, etc. Ancient karsts in carbonate rocks in some cases contain bauxites, ores of nickel, cobalt, copper, iron and manganese, precious stones, phosphorites, kaolins, refractory clays, glass sands, ochers. Among the carbonate rocks in the veins and voids there are nodules of Icelandic spar.

The consumer of dolomites and dolomitic limestones is ferrous metallurgy, where these rocks are used as a refractory material, flux and ore for magnesium. In the building materials industry, dolomite is used for the production of magnesia cement, thermal insulation materials, lime, as well as for facing material and building stone, high-strength cement, etc.

In small quantities, dolomite is used in the rubber, leather and paper industries, in abrasive production, as well as in agriculture for liming acidic soils.

It has been established that in the early stage of arid lithogenesis, dolomite formation is accompanied by the precipitation of copper, lead, and zinc (in equal concentrations), while the association of dolomite with halite and sulfates is characteristic of the late stage.

The formation of some epigenetic deposits of uranium, copper, lead, zinc, vanadium, and other metals is often accompanied by very significant dolomitization. Secondary transformations of carbonate rocks also significantly affect the porosity and permeability of rocks hosting large oil and gas deposits.

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Breeds of chemical and biochemical origin

The rocks of this group are formed as a result of various chemical processes, as well as the vital activity of animal and plant organisms both in the aquatic environment and on the land surface. Rocks of chemical and organogenic origin are considered jointly, since very often these two groups are connected by mutual transitions, and their genesis cannot always be established accurately. One rock may contain both chemogenic and organogenic components.

These rocks are usually classified according to their chemical composition and the following most common groups of rocks are distinguished:

1. carbonate rocks, in which rock-forming minerals are represented by carbonate minerals (calcite and dolomite);

2. siliceous rocks, composed of silica minerals (opal, chalcedony and quartz);

3. evaporites (sulphate and halide rocks), consisting of minerals sulfates and halides;

4. phosphate rocks, whose main mineral is apatite;

5.ferruginous rocks, containing carbonates, sulfides or hydroxides of iron;

6.caustobiolites ( carbonaceous fossil fuels ).

Carbonate rocks include sedimentary formations composed of 50 percent or more of carbonate minerals. Most often these minerals are calcite, dolomite, less often aragonite. Depending on the predominance of calcite or dolomite in the composition of sediments, two main groups of carbonate rocks are distinguished - limestones and dolomites, interconnected by transitional (mixed) varieties.

Limestones are the most common carbonate rocks. They are 50 percent or more composed of calcite. By origin, limestones are divided into organogenic (biogenic), biochemogenic, chemogenic and detrital. When diagnosing limestones, one should first of all use a reaction with dilute hydrochloric acid, under the influence of which they boil violently.

For biogenic limestones, the main rock-forming components are the skeletons of invertebrates of varying degrees of preservation and the remains of algae. Their organic origin can often be determined macroscopically: even with the naked eye, shells and shell fragments can be distinguished in their composition.

Limestones, which consist of well-preserved brachiopod shells or mollusk valves, are called shell rock.

calcareous tuffs- large-pore rocks, the formation of which is associated with the deposition of calcite from groundwater and mineral springs.

Clastic limestones consist of carbonate fragments of various shells.

Dolomites- These are carbonate rocks composed of 50 percent or more of the mineral of the same name. By genesis, dolomites are a chemogenic rock.

Dolomites are formed in two ways:

1. by chemical precipitation of CaMg (CO 3) 2 from solutions. They are characterized by layered textures, micro- and fine-grained structures.

2. by replacing CaCO 3 with dolomite; their structures are clear-crystalline, often with "shadow" structures of the primary rock.

Macroscopically distinguishing limestone from dolomite can be very difficult. The diagnostic sign is the reaction with 5% hydrochloric acid: dolomites boil only when ground into powder.

Carbonate rocks in natural conditions often create mixed limestone-dolomite varieties. In addition to carbonate minerals, clayey and detrital material can also participate in the composition of carbonate rocks. If their amount is less than 5%, the breed is classified as pure, a higher content is reflected in the name of the breed.

Mergeli- rocks of intermediate composition in the series clay - limestone. Their main component is calcite (about 50%) or dolomite and clay material. The admixture of clay material in limestone is determined by the "dirty" spot left on the sample after exposure to dilute hydrochloric acid.

The description of limestones and dolomites should be carried out in the following order:

1. Name.

3. Fortress (hardness).

4. Fracture (conchoidal, earthy, stepped, large-crystalline, etc.).

5. The main structural-genetic type (for example, biogenic oolitic, microgranular, etc.).

6. The presence and characteristics of impurities.

7. "Special signs" (pores, caverns, stylolites, etc.).

8. Rock texture (chaotic, layered, platy, etc.).

Siliceous rocks (silicites)

Siliceous rocks are sedimentary formations containing more than 50% silica in the form of biogenic, biochemogenic and chemogenic components. The main minerals of siliceous rocks are opal, chalcedony, cristobalite, and quartz.

According to their genesis, silicites are subdivided into biogenic and chemogenic. The formation of biogenic silicites is due to the development of organic remains that build their skeleton from silica, which they extract from sea water. Chemogenic limestones are represented by colloform silica and microgranular groundmass.

diatomites- accumulations of microscopic skeletons of diatoms, consisting of opal. They are white, microporous (their porosity reaches 95%), soft and very light. These rocks are similar to chalk, but do not react with hydrochloric acid and are lighter than chalk. Diatomites differ from white kaolin clays in their lack of plasticity and lower specific gravity. Their distinguishing feature is the ability to intensively absorb water. Each student can check this by touching the sample with his tongue: the diatomite will immediately “stick” to it.

tripoli very similar to diatomites, but have a colloid-chemical origin. They consist of the smallest spheres-opal grains. The color of the rocks is light, characterized by high porosity.

Opoki differ from tripoli in a darker color - from dark gray to black. In addition, these rocks are more solid and "voiced" (when struck with a hammer), unlike diatomites and tripoli - "deaf" rocks. When splitting, acute-angled fragments with a conchoidal fracture are formed. The flasks are composed of siliceous minerals with an admixture of rare spicules of sponges and radiolarians.

Flint found in sedimentary rocks in the form of concretions and nodules of various shapes. Gray-yellow, red-brown and black colors are characteristic. Often have a concentric-zonal internal structure. Composed of chalcedony contaminated with clay impurities. Formed in rocks due to coagulation of silica gels in voids.

jasper- dark, red, less often greenish, yellowish and blue, banded or spotty rocks, composed of microgranular chalcedony or quartz. They are of volcanic-sedimentary origin.

Clastic limestones are formed as a result of the destruction and washing of older limestones and mechanical processing of the skeletons of limestone organisms. Shells and their fragments are subjected to mechanical processing in the zone of surf, unrest, as a result of tidal currents, and to one degree or another roll over. Shells are crushed and silt eaters. This is how the main part of shallow-water carbonate sediments of modern seas is formed. When debris is buried close to sources of drift (without mechanical treatment), breccias are formed. Limestones formed as a result of mechanical processing of shells are called organogenic-detrital.

Bioherm limestones are a product of vital activity of animals and plants. These include bioherms - lifetime accumulations of attached organisms in a growth position, and biocenoses - lifetime accumulations of organisms that live together in a certain area of ​​the bottom of the pool.

Chemogenic limestones are formed during sedimentogenesis and early diagenesis. Chemogenic planting occurs in modern seas and oceans, as well as in land water bodies with an arid climate. The role of the chemogenic CaCO3 charge in the geological past was more significant. As a result of the chemogenic cage, pelitomorphic, oolitic limestones and numerous carbonate nodules are formed in terrigenous rocks. The mechanism of this process is as follows. In the waters of the seas and oceans of low latitudes in the shallow area, as well as in the land water bodies of the arid zone, Ca carbonate is contained in an amount close to saturation, or even saturates the water. CaCO3 monocarbonate is a practically insoluble compound (its solubility is 0.001 g per 100 g of water). With an excess of CO2 in water, it turns into bicarbonate - Ca (HCO3) 2 - a highly soluble compound. In natural waters, there is a moving equilibrium:

CaCO3 + CO2 + H2O = Ca(HCO3)2

When excess CO2 is released into the atmosphere, the equilibrium shifts towards the formation of water-insoluble monocarbonates. The reason for the decrease in the CO2 content can be the warming of the water, the activity of organisms (algae), agitation, which removes excess CO2 and supplies the smallest CaCO3 crystals (seed) during the agitation of the silt.

There are several points of view on the origin of dolomites. Currently, the existence of 3 genetic types of dolomites is considered proven:

• 1. Primary dolomites - sedimentary, formed as a result of chemogenic cage from the waters of the basin. This type of dolomite is widespread in the Proterozoic and Lower Paleozoic deposits.
• 2. Dolomites, which were formed during the period of diagenesis under the influence of sea and silt waters on calcareous and calcareous-dolomite sediments.
• 3. Dolomites formed as a result of metasomatism (during catagenesis, metagenesis and hypergenesis) under the influence of magnesium-enriched waters on limestone rocks), the so-called epigenetic dolomites.

Limestones compose thick strata in the Cambrian of Siberia, the Urals, and Central Asia; in the Silurian of the St. Petersburg region, the Baltic states, the Urals, Central Asia, Ciscaucasia; in the Devonian of the Russian platform, the Urals, Siberia; in the Carboniferous of the Russian Platform. In Triassic deposits, they are found in the Caucasus, Crimea, and Central Asia; in the Jurassic they are developed in the Caucasus, in the Crimea; in Cretaceous sediments are represented by chalk and limestone; in Tertiary deposits are widely distributed in the Caucasus, Transcaucasia.

Dolomites are less common than limestones. They have been studied in the Cambrian of Siberia; in the Silurian - on the Siberian platform and in the Baltics; in the Devonian - Central Asia; Devonian and Carboniferous on the Russian Platform; in Perm - in the east of the Russian platform; Upper Jurassic - in the Pamir-Altai system; in Tertiary deposits - in Tajikistan.

Limestones are one of the important minerals. Their main consumers are the metallurgical and cement industries. They are widely used in construction, chemical, glass and agriculture industries. Large reserves of oil and gas are associated with carbonate reservoirs. Limestones are associated with sheet-like deposits of barite, magnesite, fluorite, calcareous manganese ores, solid and disseminated antimonite ores; sheet-like and vein-like deposits of siderite; sheet-like deposits and lenses of strontium; uranium-vanadium and tyuyamunite ores; layers and deposits of irregular shape of disseminated ores of lead, zinc, antimony, mercury, copper (copper is often mixed with cobalt); irregular deposits of arsenopyrite (Handbook of lithology, 1983). In phosphorite-bearing and bituminous limestones, along with a high content of phosphorus, there are increased amounts of strontium, barium, molybdenum, uranium, etc. Ancient karsts in carbonate rocks in some cases contain bauxites, ores of nickel, cobalt, copper, iron and manganese, precious stones, phosphorites, kaolins, refractory clays, glass sands, ochers. Among the carbonate rocks in the veins and voids there are nodules of Icelandic spar.

The consumer of dolomites and dolomitic limestones is ferrous metallurgy, where these rocks are used as a refractory material, flux and ore for magnesium. In the building materials industry, dolomite is used for the production of magnesia cement, thermal insulation materials, lime, as well as for facing material and building stone, high-strength cement, etc.

In small quantities, dolomite is used in the rubber, leather and paper industries, in abrasive production, as well as in agriculture for liming acidic soils.

It has been established that in the early stage of arid lithogenesis, dolomite formation is accompanied by the precipitation of copper, lead, and zinc (in equal concentrations), while the association of dolomite with halite and sulfates is characteristic of the late stage.

The formation of some epigenetic deposits of uranium, copper, lead, zinc, vanadium, and other metals is often accompanied by very significant dolomitization. Secondary transformations of carbonate rocks also significantly affect the porosity and permeability of rocks hosting large oil and gas deposits.

On Earth, there are a huge number of different rocks. Some of them have similar characteristics, so they are combined into large groups. For example, one of them is carbonate rocks. Read about their examples and classification in the article.

Origin Classification

Carbonate rocks were formed in different ways. In total there are four ways of formation of this type of rocks.

• from chemical precipitation. Thus, dolomites and marls, limestones and siderite appeared.
• From organogenic sediments rocks such as algal and coral limestones were formed.
• From the wreckage sandstones and conglomerates formed.
• Recrystallized rocks- these are some types of dolomites and marble.

Structure of carbonate rocks

One of the most important parameters by which rocks necessary for production and processing are selected is their structure. The most important aspect of the structure of carbonate rocks is their granularity. This parameter divides breeds into several types:

• Coarse-grained.
• Coarse-grained.
• Medium grained.
• Fine-grained.
• Fine-grained.

Properties

Due to the fact that there are a large number of carbonate-type rocks, each of them has its own properties, for which it is very much appreciated in production and industry. What are the physical and chemical properties of carbonate rocks known to people?

• Good solubility in acids. Limestones dissolve in a cold state, and magnesite and siderite - only when heated. However, the result is similar.
• High frost resistance and good fire resistance- undoubtedly, the most important qualities of many carbonate rocks.

Limestone rocks

Any carbonate rock consists of the minerals calcite, magnesite, siderite, dolomite, as well as various impurities. Due to differences in composition, this large group of rocks is subdivided into three smaller ones. One of them is limestone.

Their main component is calcite, and depending on the impurities, they are divided into sandy, clayey, siliceous and others. They have different textures. The fact is that on the cracks of their layers one can see traces of ripples and raindrops, salt crystals that are soluble, as well as microscopic cracks. Limestones can vary in color. The dominant color is beige, grayish or yellowish, while the impurities are pink, greenish or brownish.

The most common limestone rocks are the following:

• Chalk- very soft rock, which is easily rubbed. It can be broken by hand or ground into powder. It is considered a type of cemented limestone. Chalk is an invaluable raw material used in the production of cement building material.
• calcareous tuffs- porous loose rock. It is fairly easy to develop. Shells have almost the same meaning.

Dolomitic rocks

Dolomitic - these are rocks, the content of the mineral dolomite in which is more than 50%. Often they contain impurities of calcite. Because of this, one can observe some similarities and differences between the two groups of rocks: dolomites proper and limestone.

Dolomites differ from limestone in that they have a more pronounced luster. They are less soluble in acids. Even the remains of organic matter are much less common in them. The color of dolomites is represented by greenish, pinkish, brownish and yellowish hues.

What are the most common dolomite rocks? It will, first of all, cast - a denser stone. In addition, there is a pale pink grinerite, it is widely used in interior design. Teruelite is also a variety of dolomite. This stone is remarkable in that it occurs in nature only in black, while the rest of the rocks of this group are painted in light shades.

Carbonate-argillaceous rocks, or marls

The composition of carbonate rocks of this type includes a lot of clay, namely, almost 20 percent. The breed itself with this name has a mixed composition. Its structure necessarily contains aluminosilicates (clay decomposition products of feldspar), as well as calcium carbonate in any form. Carbonate-argillaceous rocks are a transitional link between limestones and clay. Marls can have a different structure, dense or hard, earthy or loose. Most often they occur in the form of several layers, each of which is characterized by a certain composition.

High-quality carbonate rock of this type is used in the production of crushed stone. Marl, containing gypsum impurities, is of no value, therefore this variety of it is almost never mined. If we compare this type of rock with others, then most of all it is similar to shale and siltstone.

Limestone

Any classification of carbonate rocks contains a group called "limestones". The stone that gave it its name has been widely used in various industries. Limestone is the most popular rock in its group. It has a number of positive qualities, thanks to which it has become widespread.

There is limestone of different colors. It all depends on how much iron oxides are contained in the rock, because it is these compounds that color limestone in many tones. Most often these are brown, yellow and red shades. Limestone is a fairly dense stone, it lies underground in the form of huge layers. Sometimes whole mountains are formed, the fundamental component of which is this rock. You can see the layers described above near rivers with steep banks. Here they are very visible.

Limestone has a number of properties that distinguish it from other rocks. It is very easy to distinguish between them. The easiest way that you can do at home is to put some vinegar on it, just a few drops. After that, hissing sounds will be heard and gas will be released. Other breeds do not have such a reaction to acetic acid.

Usage

Each carbonate rock has found application in some industry. Thus, limestones, along with dolomites and magnesites, are used in metallurgy as fluxes. These are substances that are used in the smelting of metals from ore. With their help, the melting point of ores is reduced, which makes it easier to separate metals from waste rocks.

Such a carbonate rock as chalk is familiar to all teachers and schoolchildren, because with its help they write on the blackboard. In addition, the walls are whitewashed with chalk. It is also used to make dentifrice powder, but this pasta substitute is currently hard to come by.

Limestone is used to produce soda, nitrogenous fertilizers, and calcium carbide. Carbonate rock of any of the presented types, for example, limestone, is used in the construction of residential, industrial premises, as well as roads. It is widely used as a facing material and concrete aggregate. It is also used to obtain with minerals and to saturate the soil with limestone. For example, crushed stone and rubble are created from it. In addition, cement and lime are produced from this rock, which are widely used in many types of industry, for example, in metallurgical and chemical industries.

collectors

There is such as collectors. They have an ability that allows them to hold water, gas, oil, and then give them back during development. Why is this happening? The fact is that a number of rocks have a porous structure and this quality is very much appreciated. It is due to their porosity that they can contain a large amount of oil and gas.

Carbonate rocks are high quality reservoirs. The best in their group are dolomites, limestones, and also chalk. 42 percent of the applied oil reservoirs and 23 percent of the gas reservoirs are carbonate. These rocks take the second place after terrigenous ones.