Mineral Review

Which mineral belongs to the class of carbonates?

Spacecraft RBONATS Carbonates – salts of carbonic acid – H2С03. Carbonates make up up to 1,7% of the weight of the earth’s crust. Their shine is non-metallic. All minerals belonging to this class are of medium hardness, earthy varieties are soft. An extremely characteristic feature of the entire class of carbonates is the reaction with dilute hydrochloric acid (10% solution), which releases CO2. Some members of this class react with hydrochloric acid in the cold, while others react only with heated hydrochloric acid. The most characteristic shape for carbonate crystals is a rhombohedron (trigonal system). Minerals belonging to this class are light. All carbonates give the devil. Their density is low. Carbonates are divided into anhydrous and hydrous. The color of anhydrous carbonates is not constant, but the line is constant. The color and features of water carbonates are constant, unique to a specific mineral. Carbonates are components of many mineral associations formed in the surface part of the earth’s crust. Carbonates are deposited on the bottom of seas, lakes, lagoons, are formed during the weathering of sulfides and silicates, are released from hot and cold groundwater, and are part of the shells of many invertebrate animals. Carbonates are predominantly nonmetallic and partially ore minerals. Common rocks are composed of carbonates – limestone, dolomite, marble. Calcite (lime spar) – CaCO 3 Physical properties. Glass luster, pearlescent; earthy and dense calcite matte. Hardness 3, earthy varieties are soft. Colorless, white, less often yellow, green, blue, indigo, violet, dark brown, black. The line is white. Hardness 2,5-3. Crystalline calcite exhibits perfect cleavage in three directions along the faces of the rhombohedron. Upon impact, granular differences split in certain directions and produce fragments in the form of rhombohedrons. Solid granular, dense, sintered, porous, earthy, foliated, banded, radial; also crystals, druzy. Calcite crystals come in various shapes. Trigonal syngony. Sometimes it gives false forms for other minerals. Features. Calcite has a non-metallic luster, medium hardness or soft, and boils violently when exposed to dilute hydrochloric acid or vinegar. Calcite can be confused with dolomite and magnesite. The difference is that dolomite reacts with dilute hydrochloric acid only in powder form, magnesite reacts with heated hydrochloric acid. Anhydrite, similar to it, does not react with dilute hydrochloric acid. Chemical properties. It boils violently when exposed to dilute hydrochloric acid. It boils when exposed to vinegar. Varieties. Transparent, birefringent calcite (doubles the image viewed through it) is called Iceland spar, very fine-grained calcite – lithographic stone, foliated calcite – paper spar. A type of calcite is also pearl (pearl). Another type of calcite is marble onyx. Aragonite – CaCO3. Physical properties. The chemical composition is the same as calcite. Rhombic syngony. The appearance of the crystals is prismatic, often pseudohexagonal, needle-shaped. The crystal structure of aragonite is denser than that of calcite, resulting in a difference in density. The aggregates are fibrous, shell-like, dense, oolitic. The color is white, gray, pale yellow, sometimes light green, purple and gray. The line is white, light gray. The luster is glassy, ​​greasy at the break. Hardness 3,5-4. Density 2,95-3,0 (greater than calcite). Features. Easily dissolves in HC l. It differs from similar calcite in its greater density and hardness. Physical properties. Glassy, ​​pearlescent luster. Hardness is average. Color white, yellow, gray, greenish, black. The line is white. Crystalline dolomite has perfect cleavage in three directions along the faces of the rhombohedron. Solid granular marble-like or dense masses. The crystals have the shape of rhombohedrons. Trigonal syngony. Features. Dolomite is characterized by a non-metallic luster, medium hardness and boiling of dolomite powder under the action of dilute hydrochloric acid. Dolomite is similar to calcite. It differs in that calcite reacts violently with dilute hydrochloric acid. Chemical properties. The powder boils when exposed to dilute hydrochloric acid. Магнезит (magnesium spar) – MgCO 3 Physical properties. The luster of granular varieties is glassy, ​​while dense varieties are matte. Hardness is average. The color of granular varieties is grayish-white, yellowish, while that of dense varieties is white, cream, yellowish, brown, gray. The line is white. Crystalline magnesite exhibits perfect cleavage in three directions along the faces of the rhombohedron. The fracture in granular varieties is grainy, while in dense varieties it is uneven. Marble-like masses composed of elongated grains (unlike calcite and dolomite), and porcelain-like dense formations, rarely crystals, in the form of rhombohedrons. Trigonal syngony. Features. Magnesite is characterized by a non-metallic luster, medium hardness and boiling of magnesite powder under the action of heated hydrochloric acid. This is how magnesite differs from similar minerals – calcite, dolomite. It differs from siderite in color. Siderite (iron spar) – FeC 03. Physical properties. The luster is glassy, ​​or matte siderite. Hardness is average. Color yellowish-gray, yellowish-brown, brown. The line is white, sometimes brownish. Crystalline varieties exhibit perfect cleavage in three directions along the faces of the rhombohedron. Solid granular, marble-like, dense, sintered, earthy, spherical, radial-radiate structure inside (spherosiderites), also crystals in the form of rhombohedrons or druses. Trigonal syngony. Features. Siderite is characterized by a non-metallic luster, medium hardness, yellow, brown color, white line and boiling when exposed to heated hydrochloric acid. Siderite is similar to coarse-grained yellowish or brownish marble. Chemical properties. It boils when exposed to heated hydrochloric acid. A drop of hydrochloric acid placed on the surface of siderite due to the formation of FeCl 3 turns yellow. Physical properties. The luster was vitreous, opaque or matte malachite. Hardness is average, earthy varieties are soft. The color is bright green, grass green. Malachite is given its beauty by its bright green color, intricate design, often creating mysterious pictures, bizarre patterns, concentric, banded and radial-radiant structure. The streak is pale green. Sintered, radial, concentrically shell-like, dense earthy; rarely needle-shaped crystals. Monoclinic system. Features. The constant signs for malachite are green color and boiling under the action of dilute hydrochloric acid. The satellite is azurite (blue, boils when exposed to hydrochloric acid). Chemical properties. It boils when exposed to dilute hydrochloric acid. Variety. Copper greens – earthy, soft malachite. Origin. Malachite is formed as a result of chemical weathering of copper-containing minerals (chalcopyrite, native copper, etc.) under the influence of carbon dioxide, water and oxygen. Copper-containing sulfides are converted into sulfates, and then under the influence of carbon dioxide solutions into malachite. Natural carbonates, class of minerals – salts of carbonic acid (H2CO3). Over 200 mineral species are known. The basis of the structure is the planar triangular anion [CO3] 2– in combination with one or two main cations, with additional anions OH–, F–, Cl–, etc.) or without them.

Common natural carbonates

The most widespread natural carbonates are calcium (calcite, aragonite), magnesium (magnesite), iron (siderite), sodium (trona); Less common carbonates are barium (witherite), strontium (strontianite), manganese (rhodochrosite), lead (cerussite), zinc (smithsonite), copper (malachite, azurite), rare earth elements (bastnäsite), uranyl (UO2) 2+ (roetzerfordite), etc. Numerous isomorphic series are characteristic (for example, siderite – magnesite); the phenomenon of polymorphism is inherent (for example, calcium carbonate has three structural modifications, including calcite and aragonite). Calcite. Joplin, Missouri (USA). BRE Archive. Calcite. Joplin, Missouri (USA). BRE Archive.

Structure. Characteristic forms of occurrence and properties

Most natural carbonates crystallize in orthorhombic and monoclinic, less often in trigonal or hexagonal systems. The most common carbonates have the structure of calcite (magnesite, siderite, dolomite, rhodochrosite, smithsonite) or aragonite (strontianite, witherite, cerussite). The characteristic forms of occurrence are granular or dense aggregates; radial-radiant, needle-shaped, sintered, kidney-shaped discharges are also found. Many natural carbonates form well-cut, large-sized crystals. Minerals of this class are mostly colorless or pale in color.
Birefringence in Iceland spar. Spine puncture. Size 3,5 cm. Lower Tunguska basin (Krasnoyarsk Territory, Russia). Collection of V. I. Stepanov of the A. E. Fersman Mineralogical Museum of the Russian Academy of Sciences (Moscow). Photo: Natalya Pekova. Birefringence in Iceland spar. Spine puncture. Size 3,5 cm. Lower Tunguska basin (Krasnoyarsk Territory, Russia). Collection of V. I. Stepanov of the A. E. Fersman Mineralogical Museum of the Russian Academy of Sciences (Moscow). Photo: Natalya Pekova. Minerals containing chromophore ions are intensely colored. Copper carbonates are green and blue (malachite, azurite), iron carbonates are brown (siderite), manganese are pink (rhodochrosite). Mechanical inclusions (bitumen, hematite, chlorite) color carbonates black, red, and green, respectively. Hardness on the Mohs scale from 1 (trona) to 5 (smithsonite). The cleavage is mostly perfect; its surfaces have a characteristic glassy sheen. Natural carbonates dissolve easily in hydrochloric acid, releasing CO2; They are characterized by high birefringence.

Origin. Practical significance

Natural carbonates are widespread in the earth’s crust. Carbonates of calcium, magnesium, sodium (less commonly iron and manganese) are deposited in sea and lake basins by chemogenic or biogenic means, forming strata of limestone, dolomite, and natural soda deposits. Metamorphism leads to the recrystallization of sedimentary carbonate minerals that make up limestones and dolomites, and the formation of marbles. Natural carbonates are typical minerals of medium- and low-temperature hydrothermal veins. Some carbonates crystallize from magmatic melts to form carbonatite lavas or carbonatite bodies in ring-shaped alkaline-ultramafic intrusive massifs. Some natural carbonates are of a metasomatic nature, formed during the secondary carbonatization of wall rocks, which accompanies a number of ore processes. Carbonate minerals are found in weathering crusts that form over silicate rocks in arid climates. Carbonates of lead, zinc, copper, uranyl, and less commonly cobalt and nickel are characteristic minerals of oxidation zones of ore deposits. Natural carbonates are used as raw materials for the cement and chemical industries, as construction and finishing materials, and ornamental stones. They are components of ores of some metals – copper, lead, zinc, strontium, rare earth elements, etc. Pekova Natalya Anatolyevna Published May 31, 2022 at 14:15 pm (GMT+3). Last updated September 11, 2023 at 16:28 (GMT+3). Contact the editors

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