Carboxylic acids are compounds that contain a carboxyl group:
Carboxylic acids are distinguished:
- monobasic carboxylic acids;
- dibasic (dicarboxylic) acids (2 groups UNS).
Depending on their structure, carboxylic acids are distinguished:
- aliphatic;
- alicyclic;
- aromatic.
Examples of carboxylic acids.
Preparation of carboxylic acids.
1. Oxidation of primary alcohols with potassium permanganate and potassium dichromate:
2. Hybrolysis of halogen-substituted hydrocarbons containing 3 halogen atoms per carbon atom:
3. Preparation of carboxylic acids from cyanides:
When heated, the nitrile hydrolyzes to form ammonium acetate:
When acidified, acid precipitates:
4. Use of Grignard reagents:
5. Hydrolysis of esters:
6. Hydrolysis of acid anhydrides:
7. Specific methods for obtaining carboxylic acids:
Formic acid is produced by heating carbon(II) monoxide with powdered sodium hydroxide under pressure:
Acetic acid is produced by the catalytic oxidation of butane with atmospheric oxygen:
Benzoic acid is obtained by oxidation of monosubstituted homologues with a solution of potassium permanganate:
Canniciaro's reaction. Benzaldehyde is treated with 40-60% sodium hydroxide solution at room temperature.
Chemical properties of carboxylic acids.
In an aqueous solution, carboxylic acids dissociate:
The equilibrium is shifted strongly to the left, because carboxylic acids are weak.
Substituents affect acidity due to an inductive effect. Such substituents pull electron density towards themselves and a negative inductive effect (-I) occurs on them. The withdrawal of electron density leads to an increase in the acidity of the acid. Electron-donating substituents create a positive inductive charge.
1. Formation of salts. Reaction with basic oxides, salts of weak acids and active metals:
Carboxylic acids are weak, because mineral acids displace them from the corresponding salts:
2. Formation of functional derivatives of carboxylic acids:
3. Esters when heating an acid with an alcohol in the presence of sulfuric acid - an esterification reaction:
4. Formation of amides, nitriles:
3. The properties of acids are determined by the presence of a hydrocarbon radical. If the reaction occurs in the presence of red phosphorus, it forms the following product:
4. Addition reaction.
8. Decarboxylation. The reaction is carried out by fusing an alkali with an alkali metal salt of a carboxylic acid:
9. Dibasic acid is easily eliminated CO 2 when heated:
Additional materials on the topic: Carboxylic acids.
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Carbonates are a large group of minerals that are widespread. Carbonate class minerals include salts carbonic acid, most often these are calcium, magnesium, sodium, copper salts. In total, about 100 minerals are known in this class. Some of them are very widespread in nature, such as calcite and dolomite.
Structurally, all carbonates belong to the same basic type - the 2- anions are isolated radicals in the shape of flat triangles.
Most carbonates are anhydrous simple compounds, mainly Ca, Mg and Fe with a 2- complex anion. Less common are complex carbonates containing additional anions (OH) - , F - and Cl - . Among the most common anhydrous carbonates, carbonates of the trigonal and orthorhombic systems are distinguished. Carbonates usually have a light color: white, pink, gray, etc., with the exception of copper carbonates, which are green or blue in color. The hardness of carbonates is about 3-4.5; the density is low, with the exception of Zn, Pb and Ba carbonates.
An important diagnostic feature is the effect on acid carbonates (HCl and HNO 3), from which they boil to varying degrees with the release of carbon dioxide. By origin, carbonates are sedimentary (biochemical or chemical sediments) or sedimentary-metamorphic minerals; Surface hydrothermal carbonates characteristic of the oxidation zone and sometimes low-temperature hydrothermal carbonates are also distinguished.
Main carbonate minerals
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singonia |
Hardness |
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Calcite |
Calcite CaCO3 | ||
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Rhodochrosite MnSOz | |||
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Magnesite MgCO3 | |||
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Siderite ReSOz | |||
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Smithsonite ZnCO3 | |||
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Dolomite |
Dolomite CaMg(CO3)2 | ||
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Aragonite |
Aragonite CaCO3 | ||
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Witherite VaCOz | |||
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Strontianite SrCO3 | |||
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Cerussite PbCO3 | |||
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Malachite |
Malachite Cu2(CO3)(OH)2 | ||
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Azurite Cu3(CO3)2(OH)2 | |||
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Rare earth carbonates |
Bastnesit Ce(C03)R | ||
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Parisite Ca (Ce, La) 2 × 3 F 2 | |||
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Sodium Na 2 CO 3 10H 2 O | |||
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Nahkolit NaHCO3 | |||
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Nierereita |
Nierereite Na2Ca(CO3)2 |
Many of the widespread carbonates, especially calcite, magnesite, siderite, and dolomite, have similar crystal morphology, similar physical properties, occur in the same aggregates, and often have a variable chemical composition. Therefore, it can be difficult, and sometimes impossible, to distinguish them by external signs, hardness, cleavage. A simple technique for diagnosing carbonates by the nature of their reaction with hydrochloric acid has long been used. To do this, apply a drop of diluted (1:10) acid to the carbonate grain. Calcite reacts actively, and a drop of solution boils from the released CO2 bubbles, dolomite reacts weakly, only in powder, and magnesite - when heated.
The following laboratory tests provide more reliable results: accurate determination of their refractive indices; carrying out microchemical reactions on polished rock plates with reagents that color different minerals in different colors; thermal analysis (determining the decomposition temperature of a mineral, each carbonate has its own temperature); X-ray studies.
Carbonate deposits
The most common carbonate is calcite. Transparent calcite is called Iceland spar, opaque calcite. Calcite forms rocks such as limestone and chalk. The overwhelming amount of calcite was formed due to its biogenic accumulation. At the same time, calcite of hydrothermal origin is also known. In soils, calcite accumulates as a result of the reaction of calcium released during weathering with carbon dioxide soil air; The soils of arid regions are especially often rich in calcite. Calcite and dolomite form marble. Siderite is a typical mineral of swamp ores; Its endogenous origin is rarely noted. Malachite is a beautiful ornamental stone; like the mineral azurite Cu3(CO3)2(OH)2, which is similar in composition and properties, it is formed on the Earth’s surface as a result of the oxidation of copper sulfides.
Application of carbonates
Calcium, magnesium, barium, etc. carbonates are used in construction, in the chemical industry, optics, etc. Soda (Na2CO3 and NaHCO3) is widely used in technology, industry and everyday life: in the production of glass, soap, paper, as a detergent, for refilling fire extinguishers, in the confectionery business. Acid carbonates play an important physiological role, being buffer substances that regulate the constancy of the blood reaction.
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The interaction of carbonates and bicarbonates of alkali metals or ammonium with uranyl salts leads to the formation of complex ions such as: [ UO2 (CO3) 3 ] 4 -, [ UO2 (CO3) 2 (H2O) 2 ] 2 - etc. The most important in uranium production technology are carbonate complex salts of sodium and ammonium.
The interaction of barium carbonate with niobium pentoxide during non-isothermal heating is accompanied by the appearance on the DTG curve of maximum rates of carbon dioxide release at 670 - 690 and 960 - 980 C. Under isothermal conditions, the reaction proceeds at a noticeable rate at temperatures above 650 C.
The processes of interaction of barium carbonate with oxides of the vanadium subgroup are satisfactorily described by the indicated equations within the range of up to 70 - 80% of the conversion of the initial components into the final reaction product.
When potassium carbonate reacts with acids, salts of these acids are formed with the release of carbon dioxide.
When zirconium carbonate interacts with ammonium carbonate, (NH4) 32OH (CO3) 3 - 2H2P is formed - a colorless crystalline substance, soluble in water and insoluble in ethanol.
When potassium carbonate reacts with hydrogen chloride, potassium chloride, carbon dioxide and water are formed. Determine the amount of potassium chloride and the volume of carbon dioxide (at standard conditions) that are formed from 24 82 g of hydrogen chloride.
Since the interaction of carbonates with acids causes the binding of hydrogen ions, carbonates, like bases, can be used to neutralize acids. Thus, crushed limestone CaCO3 is used for liming soils when they are too acidic.
| Kinetics of interaction of mixtures of MeCO3 and MoO3 at a temperature of 375 C. 1 - MgCO3 Mo03MgMoO4. |
The reaction of alkaline earth metal carbonates with molybdenum trioxide occurs at a noticeable rate at temperatures above 300 C.
Rubidium chloride RbCl is obtained by reacting carbonates with hydrochloric acid; it is highly soluble in water.
Chemical foam is formed when sodium carbonate or bicarbonate reacts with an acid in the presence of a foaming agent. Foaming powder consists of dry salts (aluminum sulfate, sodium bicarbonate) and licorice extract or other foaming agent. When reacting with water, aluminum sulfate (or other sulfate salts), sodium bicarbonate and foaming agent dissolve and immediately react to form carbon dioxide.
Chemical foam is formed when carbonate or bicarbonate reacts with an acid in the presence of a foaming agent. In practice, such foam is produced in portable ejector devices (foam generators) from foam powder and water. Foam powder consists of dry salts (aluminum sulfate, sodium bicarbonate) and licorice extract or other foaming agent. When reacting with water, aluminum sulfate (or other sulfate salts), sodium bicarbonate and foaming agent dissolve and immediately react to form carbon dioxide.
Chemical foam is formed when carbonate or bicarbonate reacts with an acid in the presence of a foaming agent. Such foam is produced in portable ejector devices (foam generators) from foam powder and water. Foam powder consists of dry salts (aluminum sulfate, sodium bicarbonate) and licorice extract or other foaming agent. When reacting with water, aluminum sulfate (or other sulfate salts), sodium bicarbonate and foaming agent dissolve and react to form carbon dioxide.
Many natural substances are actively used by humans in industry, pharmaceuticals and cosmetology. At correct use they can bring us enormous benefits, but even when we systematically encounter such elements in medicines, food and cosmetics, we most often are not aware of the diversity of their qualities. Just such substances include calcium carbonate, the use and properties of which we will now discuss in a little more detail.
Applications of calcium carbonate
Calcium carbonate for the most part It is extracted by humans from various types of minerals, after which it is actively used in industry. So, after purification from foreign impurities, this substance is actively used in the creation of paper, food, plastics, paints and rubber. There was a place for it in the development household chemicals, as well as in construction.
Calcium carbonate is quite actively used in the production of personal care products (for example, it is added to toothpaste), as well as in the medical industry. In food processing, it usually plays the role of an anti-caking agent and also a separating agent in various dairy products.
Properties of calcium carbonate
Calcium carbonate is a white powder or crystals. It has neither smell nor taste. This substance is practically insoluble in water, but is quite soluble in dilute hydrochloric or nitric acid, and the dissolution process is accompanied by the active release of carbon dioxide. The substance “calcium carbonate” is the source of forty percent calcium.
Medicinal properties
Calcium carbonate is able to neutralize hydrochloric acid, helping to significantly reduce the acidity of digestive juice. The medicine has a fairly rapid effect, but after the buffering effect ceases, a slight increase in the production of gastric juice is observed.
Consumption of calcium carbonate helps reduce osteoclast activity and slow down bone resorption. This substance does a good job of optimizing electrolyte balance.
Among other things, calcium carbonate directly supplies the human body with calcium, which takes an active part in the processes of blood clotting, as well as in the formation of bone tissue. Calcium is also needed for excellent heart function and for the complete transmission of nerve impulses.
Application in medicine
Active substance Calcium carbonate can be used to treat patients with excessive acidity of gastric juice, as well as for diseases of the digestive system that occur against the background of such a disorder. Such ailments include exacerbation chronic form gastritis, acute type of gastritis or duodenitis, symptomatic ulcerative lesions of various etiologies. Also on this list is an ulcer in the acute stage, reflux esophagitis, erosive lesions of the mucous membranes, heartburn (after excessive intake of nicotine, coffee, medications and diet disorders).
Also, the use of calcium carbonate may be advisable in the correction of osteoporosis, caries and rickets in children, in the treatment of tetany and osteomalacia. It is recommended to take it when a person’s need for calcium increases, which is observed with breastfeeding, at the stage of active growth, during pregnancy and other similar conditions.
Calcium carbonate is sometimes used as an adjuvant therapy for allergic reactions and hypocalcemia.
Additional Information
Calcium carbonate dosage. Application
Calcium carbonate is administered orally, without regard to meal times, twice or thrice a day in an amount of 250-1000 mg.
It is worth considering that when consuming high doses of this drug for a long time, it is extremely important to systematically monitor the level of calcium in the patient’s blood, as well as monitor kidney function indicators. If calcium carbonate tablets are produced in the form of tablets intended for the prevention and correction of caries, osteoporosis, and rickets, they should not be used as an antacid composition.
Contraindications for calcium carbonate
The use of calcium carbonate is strictly not recommended if the patient has hypersensitivity to this element, as well as with hypercalcemia (vitamin D overdose, hyperparathyroidism and bone metastases). This medication is contraindicated in nephrourolithiasis, multiple myeloma, chronic renal failure, phenylketonuria and sarcoidosis.
Side effects calcium carbonate
In some cases, the use of calcium carbonate may provoke allergic reactions, sometimes such treatment causes the appearance of dyspeptic symptoms, represented by flatulence, epigastric pain, nausea, diarrhea or constipation. If you consume more than two grams of calcium per day, the patient is likely to develop hypercalcemia. In addition, some patients with this treatment face the problem of a secondary increase in gastric secretion.
Please note that exceeding the recommended dosage may result in an overdose of calcium carbonate. This condition requires gastric lavage and administration activated carbon. In addition, symptomatic correction can be carried out, and, if necessary, measures are taken to maintain vital functions.
Thus, the active substance calcium carbonate, the properties of which we have only examined, has sufficient wide range application and can bring enormous benefits to humans.
Ekaterina, www.site
P.S. The text uses some forms characteristic of oral speech.
Material for students 9"Palaeontology and calcium carbonate"
Calcium carbonate
Calcium carbonate(calcium carbonate) - inorganic chemical compound, a salt of carbonic acid and calcium.
Chemical formula- CaCO 3 .
Calcium carbonate in nature
Calcium carbonate is the basis of most natural minerals calcium (chalk, marble, limestone, shell rock, calcite, Iceland spar). In its pure form the substance white or colorless crystals. Calcium compounds - limestone, marble, gypsum (as well as lime - a product of calcination of limestone) have been used in construction for several thousand years ago. Until the end of the 18th century, chemists considered lime simple substance. In 1789, A. Lavoisier suggested that lime, magnesia, barite, alumina and silica are complex substances.
In the natural migration of calcium, a significant role is played by “carbonate equilibrium”, associated with the reversible reaction of the interaction of calcium carbonate with water and carbon dioxide with the formation of soluble bicarbonate:
(equilibrium shifts to the left or right depending on the concentration of carbon dioxide).
Calcium compounds are found in almost all animals and plant tissues. Significant amount Calcium is part of living organisms. The shells and shells of many invertebrates, eggshells, etc. are made of calcium carbonate CaCO 3. In living tissues of humans and animals there is 1.4-2% Ca (by mass fraction); in a human body weighing 70 kg, the calcium content is about 1.7 kg (mainly in the intercellular substance of bone tissue).
Chemical properties of calcium carbonate
When heated, calcium carbonate decomposes into the corresponding oxide and carbon dioxide.
Calcium carbonate reacts with water containing dissolved carbon dioxide, forming solutions of bicarbonates:
When heated and even when trying to separate bicarbonate from solution by removing water at room temperature, it decomposes by the reverse reaction:
Ca 2 + + 2HCO 3 - = CaCO 3 + CO 2 + H 2 O.
Calcium carbonate reacts with acids to release carbon dioxide
Calcium carbonate is insoluble in water and ethanol.
Calcite, lime spar - a mineral, one of the natural forms calcium carbonate. Extremely widespread on the surface of the Earth, a rock-forming mineral. Calcite is composed of limestones, chalk rocks, marls, and carbonatites. Calcite is the most common biomineral: it is part of the shells and endoskeleton of most invertebrates, as well as the integumentary structures of some unicellular organisms.
The name was proposed by Heidinger in 1845 and originates from the name chemical element, from lat. calx (genus calcis) - lime.
In its pure form, calcite is white or colorless, transparent (Iceland spar) or translucent, depending on the degree of perfection of the crystal structure. Impurities give it different colors.
Calcite belongs to the trigonal system. Crystals are very diverse, but most often rhombohedral (sharp, basic and obtuse rhombohedrons). Calcite composes the marble rock and is the main integral part limestones. Often forms pseudomorphs on organic remains, replacing the shells of ancient mollusks and corals (“fossils”).
Limestone
Limestone is a sedimentary rock of organic origin, consisting mainly of calcite crystals of various sizes and formed with the participation of living organisms in sea basins.
Limestone, consisting mainly of shells of marine animals and their fragments, is called shell rock. During metamorphism, limestone recrystallizes and forms marble.
The name of a variety of limestone reflects the presence in it of remains of rock-forming organisms, area of distribution, structure (for example, oolitic limestones), impurities (ferruginous), nature of occurrence (limestone), geological age (Triassic).
Entire mountain ranges in the Alps are made of limestone, and limestone is widespread in other places. Limestone has no shine, it is usually light gray in color, but can be white or dark, almost black, bluish, yellowish or pink, depending on the composition of impurities.
Marble
Marble (ancient Greek μάρμαρος - “white or shiny stone”) is a metamorphic rock consisting only of calcite, as well as organic compounds. Marbles are formed by metamorphism at moderate temperatures and pressures from predominantly carbonate sedimentary rocks. Under these conditions, very small grains of calcium and magnesium carbonate in sedimentary rocks experience "blastosis" - crystal enlargement.
Explored in the world great amount marble deposits. The most famous are Carrara in Italy, Parian and Pendelikon in Greece. In Russia, these are Kibik-Kordonskoye in the Krasnoyarsk Territory, Burovshchina in Transbaikalia, Ufaleyskoye in the Urals, Ruskealskoye and Belogorskoye in Karelia. The color of marble also depends on impurities.
Paleontology
Paleontology(from ancient Greek παλαιοντολογία) - the science of organisms that existed in past geological periods and were preserved in the form of fossil remains, as well as traces of their vital activity.
Paleontologists study not only the remains of animals and plants themselves, but also their fossilized traces, discarded shells and other evidence of their existence. Paleontology also uses methods of paleoecology and paleoclimatology to reproduce the living environment of organisms, compare modern environment habitats of organisms, assumptions of habitats of extinct ones, etc.
Fossil remains or fossils have been used by humans since the Paleolithic. This is evidenced by the finds of necklaces made from fragments of extinct corals and sea urchins, used in burial rituals, and other archaeological finds. Various fossils are mentioned in legends, myths and fairy tales. So, belemnites are called “ damn fingers"and in oriental tales they are considered as fingernails of genies, foraminifera shells - nummulitids in the legends of the battles of Alexander the Great are described as petrified coins.
First scientific written documents about fossil organisms belong to ancient Greek naturalists and philosophers. The successes of natural science of the ancient Greeks were summarized in the works of Aristotle, who lived in 384–322. before new era, - a great thinker of his time, who created the basis for the classification of animals, the rudiments comparative anatomy and embryology. He considered fossils to be the remains of marine animals. Many centuries later in the XV-XVI centuries. This view of fossils was supported by Leonardo da Vinci (1452–1519), although at that time there were other points of view, in particular that fossils are objects created by God after the flood.
In the XVII–XVIII centuries. intensive research begins in various industries natural sciences. This led not only to the accumulation of enormous factual material, but also to the emergence of various ideas and hypotheses. Great importance The development of paleontology was influenced by the works of the Swedish scientist Carl Linnaeus (1707–1778), the founder of classification and systematics. He divided all of nature into three kingdoms: minerals, plants and animals. Brilliant scientists worked simultaneously with Linnaeus: in France, Georges Buffon (1707–1788) and in Russia, Mikhail Lomonosov (1711–1765).
Buffon, considering the origin and development of life, the history of the animal and flora, emphasized the uniform structure of animals, spoke about the presence of intermediate forms between different groups animals and believed that the history of the development of the Earth goes back up to 75,000 years.
M. Lomonosov in his book “On the Layers of the Earth” explained the origin of sedimentary rocks their formation in sea basins. Fossil mollusks found in these rocks owe their origin to seas that existed in past geological eras. Lomonosov imagined the succession of different periods of life on Earth as a sequential alternation of the advance and retreat of the seas, explaining these phenomena by slow fluctuations of the land. The area of distribution of living beings on Earth forms a special shell called the biosphere. The biosphere arose with the appearance of living beings on Earth: it occupies the entire land surface, all bodies of water on the Earth (oceans, seas, lakes, rivers), penetrates into the atmosphere - most organisms rise into the air more than 50 - 70 m, and spores of bacteria and fungi are carried to altitudes of up to 22 km. Life penetrates into the lithosphere, where it is concentrated mainly in the surface of layers at a depth of 6-8 m, but some bacteria are found in layers at a depth of 2-3 km.
In the 90s of the 18th century and early XIX Century surveyor and mining engineer William Smith made extensive use of fossils to establish connections between rock strata in different places. He established the principle of faunal succession, according to which each layer of sedimentary rock contains a certain type of fossil that follows each other in a predictable order, even in strata separated by great distances.
A new stage in the development of paleontology begins with the appearance in 1859 of the most complete theory of evolution at that time, Charles Darwin, which had a decisive influence on everything further development natural sciences. Modern evolutionary paleontology was founded by Vladimir Kovalevsky. It was thanks to Kovalevsky’s research and his findings that Darwinism acquired a paleontological basis.
The conditions of existence on earth are very diverse and are determined by factors of both inorganic and organic order. Inorganic factors include: temperature, humidity, water salinity, pool depth, pressure. Organic factors include those relationships that organisms enter into with each other. These relationships are primarily expressed food connections. Each species has its own range, occupying different parts earth's surface. All organisms on earth live in communities called biocenoses. The organisms that make up the biocenosis react differently to fluctuations in one or another environmental factor - salinity, temperature, pressure. Some can exist with wide fluctuations in one of the environmental factors, and then the prefix “every” is added; others cannot tolerate even a slight change in this factor and then the prefix “steno” is added. If it is depth – eurybate, stenobate; salinity – euryhaline, stenohaline; temperature – eurythermic, stenothermic.
Ammonites – an extinct subclass of cephalopods that existed from the Devonian to the Cretaceous. Ammonites got their name in honor of the ancient Egyptian deity Amun with spiral horns. Most ammonites belong to the ecological group nekton, that is, organisms freely floating in the water column. Some heteromorphic forms were representatives of the benthic (bottom) community. The best swimmers among ammonites were forms with a clearly defined keel. Many paleontologists believe that the complex lobed line is an adaptation to wide vertical distribution in the water column (eurybacy), since the complex lobed line has large area, better strengthens the shell. Ammonites are an extremely important group of marine fossils for stratigraphy. This group is important for determining the relative geological age of sedimentary rocks and for subdividing the deposits of the Jurassic and Cretaceous systems.
Nautiluses- a genus of cephalopod mollusks. This is the only one modern gender subclass of nautiloids and the only ones among modern cephalopods that have an external chamber shell. This subclass appeared in the Cambrian, and during the Paleozoic was very diverse. The spiral shell with a diameter of 15-23 cm is divided into 35-39 chambers, connected in series by a long siphon. The mollusk lives in the front, largest chamber. The shell is used as a float and ballast. By pumping biogas into the shell chambers or pumping it out of them, the nautilus is able to float to the surface of the water or sink into its thickness.
Belemnites- representatives of the order of extinct invertebrate animals of the class of cephalopods, belong to intrashells cephalopods, since all parts of their shell were located inside the body. Belemnites lived from the Carboniferous to Cretaceous period, most widely distributed from the Triassic, became extinct at the end of the Mesozoic. The best preserved fossil is the belemnite rostrum, a strong conical formation located at the posterior end of the body.
Brachiopods- a type of marine invertebrate animal. Known from the Early Cambrian; reached its greatest flourishing in the Devonian. At the turn of the early and late Paleozoic, some of the orders became extinct; in the Carboniferous and Permian periods the orders of productids and spiriferids dominated. After the Permian-Triassic extinction, 4 orders survived to this day. Brachiopods, due to the wealth of remains and their good preservation, are valuable index fossils for establishing the geological age of the strata containing them and the physical and geographical situation that once existed in a given area.
Sea urchins- class of echinoderms. In fossil form they are known from the Ordovician. The body of sea urchins is usually almost spherical, ranging in size from 2-3 to 30 cm; covered with rows of limestone plates. The plates, as a rule, are connected motionlessly and form a dense shell (shell), which does not allow the hedgehog to change shape.
sea lilies- one of the classes of echinoderms. Fossil crinoids are known from the Lower Ordovician. They reached their greatest prosperity in the Middle Paleozoic, when there were up to 11 subclasses and over 5000 species, but by the end Permian period most of them died out. Fossilized remains sea lilies are one of the most common fossils. Some limestone strata dating from the Paleozoic and Mesozoic are composed almost entirely of them. Fossil segments of crinoid stems that resemble gears are called trochites.
Bivalve or elasmobranch mollusks - a class of marine and freshwater sedentary mollusks, the body of which is flattened laterally and enclosed in a shell of two valves. Findings of ancient fossils bivalves date back to the beginning Cambrian period, their age is more than 500 million years. Total number There are approximately 9,200 living species (according to other sources, more than 20 thousand). Bivalves are a class of invertebrates that are exclusively aquatic and are found in fresh and salty waters Worldwide. Most are benthic organisms and live by burrowing into bottom sediments or attaching themselves to underwater objects. The shell valves of bivalves are often symmetrical. The shell valves are connected by a ligament - a ligament consisting of a thickened stratum corneum of the shell. The shell wall consists of three layers: the outer conchiolin layer (periostracum), the inner calcareous layer (ostracum), and the lower nacreous layer (hypostracum). The mineral component of the shell may be exclusively calcite, as in oysters, or calcite and aragonite. Sometimes aragonite also forms a pearlescent layer. In other mollusks, layers of aragonite and calcite alternate.
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