Carbonate rocks carbonate rocks widely. Origin, formation conditions of carbonate rocks Carbonate clayey or clayey carbonate

Carbonate rocks are sedimentary or metamorphic rocks of limestone, dolomite and carbonate-argillaceous composition. All varieties of carbonate rocks - limestone, chalk, shell limestone, calcareous tuff, marl limestone, marl, with the exception of marble - are used in the production of cement.

All these rocks, along with calcium carbonate CaCO 3, may contain impurities of clay substances, dolomite, quartz, and gypsum. The content of clay substances in calcareous rocks is not limited; impurities of dolomite and gypsum in large quantities are harmful.

The quality of carbonate rocks as a raw material for the production of cement depends on their physical properties and structure: rocks with an amorphous structure interact more easily with other components of the raw mixture during firing than rocks with a crystalline structure.

Limestones- one of the main types of lime raw materials. Dense limestones, widespread, often have a fine-grained structure.

The density of limestones is 2700-2760 kg/m 3 ; compressive strength up to 250-300 MPa; humidity ranges from 1 to 6%. The most suitable for the production of cement are marl and porous limestones with low compressive strength and not containing silicon inclusions.

Chalk- sedimentary soft, easily rubbed rock, which is a kind of weakly cemented smearing limestone. Chalk is easily crushed when water is added and is a good raw material for cement production.

Marl- sedimentary rock, which is a mixture of the smallest particles of CaCO 3 and clay with an admixture of dolomite, fine quartz sand, feldspar, etc. Marl is a transitional rock from limestone (50-80%) to clay rocks (20-50%). If in marls the ratio between CaCO 3 and clayey rock approaches that required for the production of cement and the values ​​of silicate and alumina modules are within acceptable limits, then marls are called natural or cement. The structure of marls is different: dense and hard or earthy-loose. Marls occur mostly in the form of layers that differ from one another in composition. The density of marls ranges from 200 to 2500 kg/m3; humidity depending on the content of clay impurities 3-20%.

For the production of cement, various types of carbonate rocks can be used, such as: limestone, chalk, calcareous tuff, shell limestone, marl limestone, marl, etc.

In all these rocks, along with calcium carbonate, mainly in the form of calcite, preferably finely dispersed, there may be impurities of clay substances, dolomite, quartz, gypsum, and a number of others. Clay in the production of cement is always added to limestone, so the admixture of clay substances in it is desirable. Impurities of dolomite and gypsum in large quantities are harmful. The content of MgO and SO 3 in calcareous rocks should be limited. Quartz grains are not a harmful impurity, but they impede the production process.

The quality of carbonate rocks also depends on their structure: rocks with an amorphous structure interact more easily with other components of the raw mixture during firing than rocks with a crystalline structure.

dense limestones, often having a fine-grained structure, are widespread and are one of the main types of lime raw materials. There are siliceous limestones impregnated with silicic acid. They are characterized by particularly high hardness. The presence of individual siliceous inclusions in limestone makes it difficult to use, since these inclusions must be separated manually or at concentrating plants by flotation.

Enrichment of cement raw materials by flotation is used only at some foreign cement plants that have substandard raw materials. Such enrichment may be useful only in those areas where there is no purer raw material suitable for the production of cement.

Chalk is a soft, easily rubbed rock, consisting of particles with a highly developed surface. It is easily crushed when water is added and is a good raw material for cement production.

calcareous tuffs- highly porous, sometimes loose carbonate rock. Tuffs are relatively easy to mine and are also good limestone raw materials. Approximately the same properties are possessed by shell limestones.

The volumetric weight of dense limestones is 2000-2700 kg / m 3, and chalk - 1600-2000 kg / m3. The moisture content of limestone ranges from 1-6%, and chalk 15-30%.

Marly and porous limestones with low compressive strength (100-200 kg/cm 2 ) that do not contain siliceous inclusions are most suitable for cement production. Compared to hard and dense varieties, such limestones are more easily crushed and more quickly react with other components of the raw mixture during firing.

Marl is a sedimentary rock, which is a natural homogeneous mixture of calcite and clay substance with an admixture of dolomite, fine quartz sand, feldspar, etc. There are calcareous marls, clay marls, etc. If in marls the ratio between calcium carbonate and clay substance approaches that required for the production of cement and the values ​​of the silicate and alumina modules are within acceptable limits, then they are called natural or cement. They are fired in the form of pieces (without any additives) in shaft furnaces, which eliminates the preliminary preparation of the raw mixture and reduces the cost of the finished product. However, such marls are very rare.

Marls have a different structure. Some of them are dense and hard, others are earthy. They lie mostly in the form of layers differing from each other in composition. The volumetric weight of marls usually ranges from 2000-2500 kg/m3; their humidity, depending on the content of clay impurities, is 3-20%.

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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. Autogenous minerals include glauconite, quartz, chalcedony, anhydrite, gypsum, pyrite, alkali feldspars, and others. 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 interbeds, 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. Autogenous minerals include glauconite, quartz, chalcedony, anhydrite, gypsum, pyrite, alkali feldspars, and others. 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.

The most widespread carbonate rocks are limestones and dolomites. Sideritolites, magnesitolites, rhodochrositolites are much less common. The specificity of carbonate rocks lies in a wide variety of structural types, which is explained by the variety of settings and methods of their formation.

Limestones are rocks composed primarily of calcite. In the presence of an admixture of sandy-silty material, clay, silica, dolomite, glauconite, bitumen, etc., limestones are called sandy, silty, clayey, siliceous, dolomitic, glauconite bituminous, etc., respectively. The structure of carbonate rock is determined by the type of structural grains ( components), cement content and pore space. The structural grains in limestones can be clastic grains (carbonate and non-carbonate composition, lithoclasts and crystal clasts), biomorphic grains (whole shell and whole skeletal, detrital, etc.), various spheroaggregates (oolites, pisolites, spherulites, lumps, etc.) and crystals minerals. Limestones are characterized by great textural diversity. On the surface of the layers, signs of ripples, syngenetic cracks, traces of raindrops, traces of soluble salt crystals, traces of vital activity of organisms can be observed. Limestones are painted in light tones of beige, yellowish and gray with greenish, pinkish or brownish hues. Organic remains are often found in limestones.

Dolomites are rocks consisting of more than 50% of the mineral dolomite. Dolomites often contain an admixture of calcite, due to which all transitions between limestones and dolomites are observed. The admixture of clay substance causes a continuous series: dolomite - marl. Dolomite is scratched with a steel needle and differs from limestone in that it is less soluble in acids and has a stronger luster. Dolomite can be reliably determined only by chemical analysis (it does not react with hydrochloric acid, unlike limestone). Organic remains are found in them much less frequently than in limestones. Dolomites often contain very characteristic authigenic mineral impurities (sulphates, celestite, fluorite, ferrous and oxide compounds of iron, silica, organic matter), as well as allothigenic, represented by clay matter, rarely sandy and silty material, usually of non-carbonate composition. Dolomites are beige, gray with greenish, pinkish, brown or yellowish hues.

Marls are rocks of mixed clay-carbonate composition. Marls usually contain 25–50% insoluble sediment. Depending on the composition of rock-forming carbonate minerals, marls are divided into calcite and dolomite, and on the composition of the insoluble residue - into siliceous marls and marls proper. They usually have a pelitomorphic or fine-grained structure. When the carbonate content is over 75–80%, the rock is called argillaceous limestone; if the clay content in the rock exceeds 75%, the rock is called calcareous or dolomite clay. Marls are pelitomorphic, in most cases soft, staining, with an earthy fracture of the rock. They easily absorb water and often disintegrate when weathered into a loose mass (“rubbers”, “cracks”). Usually they are painted in light shades of gray with greenish, pinkish or yellowish tints. But there are also brightly colored varieties of red, purple and brown (in red-colored strata). Thin layering is not typical for marls. Marls contain clastic grains of quartz, feldspars, and accessory minerals as impurities. Of the authigenic minerals in marls, glauconite is noted, often in the form of spherical and kidney-shaped grains, barite, zeolites (mainly mordenite), pyrite and marcasite in the form of the smallest spherical grains. Siliceous concretions in marls are usually represented by chalcedony, quartz, or opal. Iron oxides and hydroxides (mainly products of the oxidation of iron sulfides) color marl in yellowish and reddish hues. The marls contain remains of mollusks, ostracods, foraminifers, and other shells; the remains of the smallest calcareous algae, rhabdolites and coccoliths, are widespread. In dolomite marls, definable organic remains are very rare. Sometimes, in the bulk of marls, organic matter and carbonaceous particles are observed, either evenly distributed in the rock, or forming clusters in the form of separate spots. Limestones and dolomites differ from marls in greater density, the ability to disintegrate into platy units, and often have a crystalline-grained structure that is not characteristic of marls.

Marls differ from clays in the absence of plasticity, which is so characteristic of clayey rocks. The external signs of marls, by which they are determined in the field, vary significantly depending on the composition and amount of clay components. Minerals that are not able to swell from the presence of moisture (kaolinite, hydromica, fine opal), being in the form of an impurity in carbonate rock, do not affect its appearance. A slight admixture of minerals of the montmorillonite group gives the marl the appearance of a rukhlyak, which is determined by the ability of montmorillonite to easily disintegrate into a loose mass during temporary moistening and subsequent drying. Silica in marls is represented by the smallest rounded opal bodies, amorphous silica precipitated from water along with calcium carbonate. The size of these particles usually does not exceed 0.01 mm. The study of the insoluble residue of marls gives an idea of ​​the composition of clay minerals and other impurities that make up the rock.

interstices of clay minerals with potassium ions. The presence in the rock of minerals (feldspars) hydrolyzed during catagenesis, which are sources of potassium, ensures relatively rapid dehydration of clays; its absence delays this process until the metagenesis stage, when most of the potassium feldspars are destroyed. Thus, the phase of oil formation can be delayed in time with a lack of potassium in the rock.

test questions

1. What are the classifications of clay rocks according to physical properties,

mineral composition, method of formation?

2. How we represent the crystal lattice structure of clay miners

3. What are the features of the structure of clay minerals of the main groups of kaolinite, illite, smectites, vermiculite, chlorites?

4. What are mixed-layer minerals?

5. What conditions exist in nature for the formation of various clay minerals?

6. What is the essence of the change in clay minerals and rocks at various stages of lithogenesis?

7. What is the role of clays in oil and gas formation?

Chapter 7. CARBONATE ROCKS

Carbonate rocks make up 15--20% of the volume of all sedimentary formations, contain the largest deposits of oil and gas and are distributed within platform and geosynclinal areas, occur in complexes of a wide age range - from Precambrian to the present.

Carbonate rocks and sediments include formations composed of 50% or more carbonate minerals. Among the latter, compounds of the calcite and dolomite groups are most common, the crystals of which have a trigonal system. Minerals of the aragonite group belonging to orthorhombic carbonates are noted less frequently in the rock-forming components.

In addition to CaCO3, the calcite group includes magnesite (MgCO3), rhodo chrosite (MnCO3), smithsonite (ZnCO3). The second group includes dolomite (CaMg2 ), ankerite (CaFe2 ) and kutnahorite (СаМn[СO3 ]2 ). Aragonite is similar in composition to calcite, but this group also includes strontianite (SrCO3) and cerussite (PbCO3). Minerals of these groups are capable of isomorphism and form solid solutions with wide miscibility.

Calcite is a stable form of CaCO3 over a wide range of temperatures and pressures. Calcite can be pure CaCO3 or contain Fe, Mg, Mn metals. Most often calcium is replaced by magnesium. Calcite with a MgCO3 content of more than 5% belongs to high magnesian.

Aragonite is an unstable form of CaCO3 and is known mainly in modern sediments of biogenic origin. Over time, it can spontaneously transform into calcite. Aragonite lattices only in some cases attach the Mg2 + ion up to 0.001%. The amount of strontium added to them can reach a concentration of about 1%.

Dolomite composes rocks of the same name, which are usually of chemogenic origin. The crystal lattice of dolomite is highly ordered and is formed by the substitution of Ca atoms in calcite through one per Mg atom. In dolomite, Mg2+ is often replaced by Fe2+ with the formation of a continuous series of solid solutions up to ankerite. Iron-rich dolomite is usually found with iron ores. Rock-forming dolomite is characterized by low iron content.

The minerals described above react differently with hydrochloric acid. Calcite and aragonite readily dissolve in cold 2-10% HCl. Dolomite reacts with acid either in powder form or after heating with HCl. Recognition of carbonate minerals is facilitated by etching and staining in samples and thin sections, as well as the study of powders of these components by chemical, thermal and X-ray diffraction analysis. In recent years, the microstructures of carbonate minerals have been studied using scanning electron microscopy. To solve the questions about the origin of carbonates, isotopic studies are carried out and the ratios 1 8 O / 1 6 O and 1 3 C / 1 2 C are determined.

Depending on the predominance of CaCO3 or CaMg(CO3 )2 in the composition of the deposits, two main groups of carbonate rocks are distinguished - limestones and dolomites, connected by mixed differences between themselves and with other sedimentary formations. For example, rocks containing commensurate amounts of carbonate and clay material are called marls.

§ 1. LIMESTONES

Limestones are carbonate rocks, consisting of 50% or more of calcite and (or) aragonite. Limestones that do not contain impurities are white in color. The appearance of gray and black colors is associated with an admixture of clay and organic matter. Greenish tones are found in the presence of glauconite

or chlorite. Iron oxides give limestones their reddish tint.

The structures of limestones are determined by the way they are formed and are determined by the presence of a number of components.

Limestone classification. The genetic classifications of limestones proposed in our country by M.S. Shvetsov, N.M. Strakhov, G.I. Teodorovich, I.V. the sequence of their formation.

Sedimentogenic (primary) features make it possible to distinguish biogenic (organogenic), biochemogenic, chemogenic and detrital limestones. This takes into account that the rock-forming component (biogenic, chemogenic, etc.) is at least 50% of the main mineral mass of the carbonate rock:

biogenic (organogenic) - biomorphic (autochthonous reef and allochthonous shell), organogenic-nodular (stromatolitic, oncolitic), biodetritus (organogenic-detrital);

biochemogenic - fine and micro-lumpy, clotted, pellet, coprolite;

chemogenic - oolitic, microgranular; detrital - pebble, gravel, sandstone.

Biogenic (organogenic) limestones are one of the most common types of carbonate rocks. The organogenic components in them are represented by shells of brachiopods, various types of mollusks, remains of crinoids, calcareous algae, corals, and other organisms. Most of the biogenic limestones are formed from undisplaced (autochthonous) or, to varying degrees, displaced (allochthonous) benthos remains. Limestones also arise from the skeletons of planktonic organisms. Finally, organic residues can be treated in an aqueous medium, rounded and their fragments sorted by size during sedimentation. The mineral composition of rock-forming organic remains is represented by aragonite, low- and high-magnesian calcite. Organogenic components can be cemented by granular carbonates of various sizes. Among the biogenic limestones, there are biomorphic and biodetritus types related by transitional varieties.

Biomorphic limestones composed mainly of whole skeletons of organisms and have three main varieties - reef, organogenic-nodular, and shell or shell

Rice. 35. Algal bioherm limestone (according to I. A. Shchekotova).

A - orthonella, B - bevocastrin. Increased twenty.

isometric or elliptical in terms of the body; reef peaks in the form of towers with steep slopes; atolls surrounding lagoons; barrier and coastal reefs. The type of limestone under consideration is immediately formed solid and forms the core of the reef. According to the composition of reef-forming rocks, for example, coral, brachiopod-bryozoan, crinoid-bryozoan, and algal limestones are distinguished (Fig. 35). The space between the remains of marine organisms, intergrown and often encrusted with crystalline calcite, is often filled with their own fragments of various sizes, and sometimes even with terrigenous and ash material. As a result, patterned textures appear in the form of alternating massive, lenticular, spotty cavernous and reticulate areas (Fig. 36).

Organogenic limestones are formed at the expense of algal nodules, stromatolites and oncoliths. The nodules are represented during sedimentation as loose, semisolid, nonstratified, mono- or polycentrically overgrown formations. Stromatolites are layered formations that have arisen due to the precipitated carbonate substance in the rhizomes of algae or due to the replacement of their fibers. Oncolites are concentric-layered, formed in place of balls of benthic algae, formations, the core of which can be zoogenic remains.

Shell limestones are formed at the bottom of the pool in the form of a loose sediment. They may represent accumulations of large shells - pelecypods, brachiopods, etc., small shells - ostracods, pterogods, foraminifers (Fig. 37). When shells are formed by large attached forms, rudists, the resulting formations are structurally reminiscent of reefs. Usually shell limestones

Rice. 36. Sieve reef limestone

Rice. 37. Foraminiferal limestone (magn. 96, nicoli//)

occur in clear layers with a thickness of several tens of centimeters to several hundred meters.

A biomorphic formation is also chalk, composed largely of the skeletal remains of calcareous plankton algae - coccolithophorides. Initially, during life, the algae cell is surrounded by tiny rounded disks, consisting of carbonate crystals and called coccoliths. The diameter of the discs is fractions of a millimeter (Fig. 38). In addition, lumps of pellets are also present in the chalk.

Biodetritus (organogenic clastic) limestones are bedded accumulations of crushed shells (Fig. 39).

Rice. 38. Cocco-cast chalk (SEM. magnification 8000)

Rice. 39. Limestone organogenic-detrital (magn. 100, nicol +)

These rocks, as a rule, are polydetritus, for example, crinoid-brachypod-bryozoans. Detritus can be rounded to varying degrees and correspond in size to sandy and silty grains. In the composition of biodetritus limestones, there are fragments of the sculpture of marine organisms, connected during life by soft tissues - spicules of sponges, segments of crinoids. The textures of biodetritus limestones are massive and layered, similar to the textures of sandy-silty rocks.

Biochemogenic limestones are formed due to the products of vital activity of organisms. So, as a result of the accumulation of waste products of blue-green algae, accumulating CaCO3 and perforating the skeletal remains of organisms,

fine-, micro-lumpy and clotted limestones.

As a result of the vital activity of molluscs, worms and crustaceans, passing through themselves aragonite silts enriched with organic matter, accumulations of fecal products occur -

Rice. 40. Limestone pelitomorphic (SEM, magnification 1500)

Rice. 41. Oolitic limestone (magnification 64, nicol +)

coprolites. This also includes pellets - lumps of organic and mineral material thrown out by planktonic organisms and settling to the bottom.

Chemogenic limestones are represented by microgranular and oolitic varieties. Micrograined (pelitomorphic) limestones

are characterized by carbonate grains 0.001-0.005 mm in size (Fig. 40). Oolitic limestones consist largely of rounded calcite formations of a radially concentric structure. The centers of ooliths are terrigenous grains, or fragments of fauna, carbonate clots. The diameter of ooliths varies within 0.1-1.5 mm. They are cemented by granular calcite of chemogenic origin (Fig. 41).

Table 7. The main types of carbonate rocks (according to R. Falk, with simplification)

Clastic limestones arise due to the destruction of all of the above types of carbonate rocks. According to the structure, the accumulations formed in this way belong to pebbles, sands, and silts, having textures characteristic of clastic rocks.

AT As a result of post-sedimentary recrystallization, limestones acquire secondary structures: clear-grained (grain diameter more than 0.1 mm), crustified, uneven recrystallization (porphyroblastic and false clastic). The primary, sedimentogenic appearance of limestones is very much obscured by secondary processes. In this case, it is necessary to single out a group of limestones of unclear origin with cryptogenic structures.

AT The practical work of lithologists for carbonate rocks also uses the terminology and classification developed in oil

companies in the USA. R. Folk's well-known classification is based on the relationship between allochemic and orthochemic structural components (Table 7).

Allochemogenic components, or allochems, consist of separate aggregates of carbonate composition of various origins that have undergone some transportation. The following four types of allochems have the main rock-forming significance: 1) intraclasts - fragments of lithified redeposited carbonate formations to varying degrees, subsynchronous to sedimentation, 2) oolites, 3) skeletal remains (whole and fragmentary), 4) various clots (pellets, coprolites) .

The orthochemes include micrit and sparite. Micrit is a microgranular mass, which is the lithified equivalent of lime sludge deposited by chemogenic or biochemogenic means. When

The absence of micritic components in limestones may indicate a calm gyrodynamic setting of sedimentation. The term sparite in the American literature usually means an aggregate of clear-crystalline calcite (dolomite), the grain size of which is more than 0.004 mm, usually 0.01-0.02 mm. Such a crystalline mass usually acts as a cement that holds allochems together and can be formed due to the recrystallization of micrite.

R. Folk singled out the main types of rocks on the basis of the ratio of allochems, micrite and sparite in them.

Allochemogenic breeds contain more than 10% of allochems, which are bonded with micrite or sparite. The composition of allochems is reflected in the first part of the name of the rock, and the cement is characterized in the second part. For example, intrasparite consists of at least 10% intraclasts cemented with clear crystalline calcite-sparite. In the case of the presence of only micritic cement, such a rock is called intramicrite. In the same way, oomicrites and oosparites (cemented oolites), biomicrites and biosparites (skeletal remains cemented with micro or clear-crystalline calcite) are distinguished.

Orthochemogenic rocks contain no more than 10% allochemogenic impurities. Pure differences are identified as micrites. Dismicrites contain single inclusions, or their number does not exceed 1%.

In addition to the mentioned types of rocks, R. Folk also identified autochthonous reef rocks (biolithites). According to the classification of R. Dunham, such limestones are called boundstone. This means that the primary biogenic rock-forming components were held together during burial, which is evidenced by the accretion of skeletal remains, lamination that is not subject to gravity, the presence of cavities lined with sediment and overlain by organic remains, the size of which exceeds intergranular pores.

The identification of other types of carbonate rocks by R. Dunham is also carried out taking into account the primary, to varying degrees, porous framework. The author believes that some calcareous particles can create a framework with very high primary porosity. This porous sediment can then be infiltrated by carbonate silt components by infiltration from the overburden. R. Dunham based his classification on quantitatively and structurally significant differences between fine material and grains. As a result, on this basis, several classes of rocks were identified. The class of rocks in which fine silt components serve as a support includes two types: 1) mudstone, in which fine carbonate silt is the basis, and particles larger than 0.02 mm (grains) are scattered in the groundmass and make up no more than 10%, 2 ) wackstone contains more than 10% grains dispersed in the sludge so that they cannot form

frame. A sign of another class of rocks is a framework of mutually supported grains. These include packstone, which contains some pelitic carbonate material in the intergranular spaces, and grainstone, which does not contain silt in the intergranular spaces. R. Dunham notes that deposits containing almost no fine silt are formed not only with high water mobility, but also when the detritus accumulation rate is higher than the silt sedimentation rate, as well as when silt is washed out from previously deposited sediments. In all the limestone types listed above, the primary structures are recognizable. Recrystallized varieties are considered in the group of altered carbonate rocks.

Origin and distribution of calcium carbonates in sedimentation basins. The formation and distribution of calcareous sediments depends on the climate, water composition, and the nature of the sedimentation basin. Carbonate accumulation occurs in arid and warm humid areas. Calcium carbonates in both areas are formed in lakes, inland seas and bays, in marginal seas and oceans. In the arid zone, in addition, carbonates are formed on the land surface.

The accumulation of calcium carbonates in lakes is observed in the transition region from the humid zone to the arid zone. Here, in lakes with hard water, calcareous sediments accumulate directly after the sandy sediments among underwater thickets. As one moves to the deeper parts of the basin, the lime content in sediments decreases. Lime sediments are distributed in a similar way in desalinated inland seas of the Baltic type.

In the marginal seas of normal salinity, located in a temperate climate, carbonate accumulation practically does not occur, but in a warmer climate, CaCO3 accumulations are developed not only in the coastal zone, but also in the pelagic zone.

Carbonate sediments in the inner Black Sea basin and the Mediterranean Sea, which has many morphological features of the ocean, are predominantly confined to the deep waters. In the oceans, modern sediments enriched in calcium carbonate gravitate towards parts far from the coast at low latitudes. The confinement of high carbonate sediments to the parts of the seas and oceans more distant from the coast is explained by the decrease in the diluting effect of terrigenous material in these parts, which clearly shows the relationship between carbonate sediments and coastal morphology. The more mountainous the catchment area, the more vigorously denudation occurs on it, the more rivers there are, the farther carbonate sediments retreat from the coast. With a flat coast, a small number of rivers, carbonate deposits approach the coast.

The main role in the accumulation of carbonates belongs to marine

pools. Surface sea water at a temperature of +25 "C is oversaturated with the main carbonate minerals - aragonite, calcite and dolomite. However, the chemical precipitation of carbonates, even in the seas of the tropics and subtropics, is insignificant compared to its biogenic precipitation.

Among recent sediments, purely biogenic carbonates are formed on sea shelves in a temperate humid climate. Biogenic pelagic carbonates (coccolithic and foraminiferal silts) are widespread in the oceans, where they are associated with mid-ocean ridge systems and upwelling regions. Biogenic carbonate accumulation on the shelves of the subtropical and tropical zones is accompanied by the deposition of chemogenic CaCO3. These zones are characterized by the formation of giant carbonate bodies identified as carbonate platforms, the steep shelf slopes of which are bordered by organogenic structures.

The chemical precipitation of CaCO3 from sea waters currently occurs in the form of aragonite and is determined by the reaction

calcium nat calcium ion acid

The HCO3 anion is formed as a result of the following reactions: CO2 +

H2 O H 2 C O 3 , H 2 C O 3 H + + H C O 3 - , H + + CO3 2 HCO 3 - .

Precipitation of CaCO3 is facilitated by a decrease in the content of CO2 in solution, i.e., a decrease in its partial pressure and an increase in the pH of water. This is favored by an increase in water temperature and organic photosynthesis.

At a depth of 3.5-5 km, calcium carbonates are rarely found in silts, which is associated with their dissolution in cold deep waters under conditions of an increased partial pressure of carbon dioxide CO2 and low pH.

The depth at which, according to the data of microscopic studies, the dissolution of calcareous shells is sharply accelerated is called the lysocline (this level differs somewhat for the skeletons of different organisms). The depth at which CaCO3 disappears from sediments is called the carbonate compensation level. At this level, the depth of which varies from 3.8 to 5.2 km in different parts of the World Ocean, the rate of influx of carbonate material is equal to the rate of its dissolution.

In addition to marine sedimentation accumulations of calcium carbonates, their formation in continental conditions of hot regions with alternating dry and rainy seasons is possible. Here, in the near-surface layers of clastic rocks containing CaCO3, illuvial concretion-cementation

calcareous formations - kaliche. Caliche is formed as a result of the dissolution of calcium carbonate by groundwater and its subsequent release from evaporating saturated solutions. Newly formed caliche is white, dusty and brittle. When the surface of the caliche is exposed as a result of the destruction of the soil layer covering it, the pulverized calcite recrystallizes and forms a dense limestone with a brecciated texture.

Among other continental carbonate formations, the nari-carbonate crust stands out, consisting of unreplaced fragments of primary rock surrounded by a fine network of calcareous veinlets. Biochemogenic continental deposits - travertines and their highly porous varieties - calcareous tuffs, fall out of the waters of springs, rivers, lakes and are often associated with karst cavities in the form of collomorphic sinter formations.

Postsedimentary transformations of calcareous deposits. Features of the diagenesis of carbonate sediments are due to the mineral and chemical instability of their components, in relation to which water is the active agent of transformations. As a result, carbonate sediments can undergo strong changes already at the early stages of transformation. The essence of the process of diagenesis of this sedimentary material is closely related cementation and lithification. In this case, sealing plays a less significant role. Experimental compaction of various carbonate sediments indicates that their thickness decreases rapidly, from 30 to 5%. As M.R. Lider showed, the nature of carbonate lithification is in a certain way related to the environment of sedimentation and has its own characteristics in a number of settings from the beach and reefs to the ocean depths.

Above the tide level and the groundwater table, beach rock is formed in the supratidal zone, i.e., cemented carbonate beach sands. Intense lithification of carbonate sediments in the tidal (littoral) zone is carried out under the influence of their periodic drying, mixing of sea water with fresh water, under the influence of bacterial-algal activity and biochemical processes. Cementation of detrital carbonates is caused by precipitation of acicular aragonite in the form of crusts and magnesian-calcite micrite from pore solutions in intergranular spaces. Thus, the rocks are close in appearance to beach rock.

Lithification of layered carbonate sediments of the sublittoral zone occurs under conditions of constant contact with sea waters. Bottom lithification of this type is currently observed in the Persian Gulf at depths from 1 to 60 m, where layers of cemented detrital limestones with a sandy-silty texture, also called calcarenites, form layers 5-10 cm thick. Cementing material in these formations

represented by radial-fibrous or equally crystalline high-magnesian calcite; particle sizes - 1 - 7 microns. The carbonate sediment cemented near the bottom surface is essentially a "stone bottom" - hard ground. The hardground is underlain by less consolidated sediments. As the porosity of the deposits increases, the continuity of their cementation disappears, leading to patchy development of the hardground.

Diagenetic changes in organogenic structures are manifested in the change of cement generations, accompanied by the dissolution and transformation of carbonates. As a result of these processes, the organogenic framework turns into dense reef limestone, partially recrystallized and dolomitic.

Underwater near-surface lithification of deep-sea sediments has been found in many regions of the World Ocean. For example, lithified globigerine oozes in the Mediterranean Sea and the Atlantic Ocean are noted at depths from 200 to 3500 m, where they are represented by recrystallized micrites containing high magnesian calcite and dolomite in their composition. Sometimes they are colored by Fe and Mg oxides and include concretions of these oxides.

Concluding the characterization of the stage of diagenesis, it should be noted that in the cores of modern marine carbonate sediments from different depths, superficially lithified and nonlithified varieties often alternate. Based on this, an assumption arose that surface lithification does not depend on depth and is determined primarily by a slowdown or temporary suspension of sedimentation. Lithification, however, depending on the level of subsidence of sediments, begins at a depth of about 0.8 km, which was established from a number of wells that uncovered pelagic carbonate deposits in the Pacific Ocean. From this depth, as pelagic calcareous oozes turn into limestone, the shells partially dissolve and decompose, turning into a micritic mass, which includes calcite crystals of various sizes. Thus, the nature of diagenetic changes is closely related to the sedimentation environment and the structural and mineralogical features of calcareous deposits.

Catagenetic changes in carbonate rocks manifest themselves in changes in the morphology and size of their components, redistribution of carbonate matter within a reservoir or sequence. In the case of the input-removal of substances, leaching develops, the replacement of some minerals by others, and the filling of voids with carbonate and other minerals.

Limestones in the catagenesis zone very often recrystallize; there is an increase in the size of CaCO3 grains. Morphological

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.