Carbonate minerals
Carbonate minerals are a significant group of minerals characterized by the presence of the carbonate ion (CO₃)²⁻ in their chemical structure. Comprising around sixty different varieties, the most common carbonate minerals include calcite, aragonite, dolomite, magnesite, and siderite. Calcite, in particular, is abundant, forming key rock types such as limestone and marble, and is crucial in various applications including cement production and agriculture. These minerals can be found in diverse environments, from sedimentary deposits in tropical marine settings to cave formations and dry lakebeds.
Carbonate minerals are notable for their reaction to hydrochloric acid, which causes effervescence—a property often used for identification. They typically exhibit soft textures and characteristic rhombohedral cleavage. Their uses extend beyond construction materials; some serve as essential ores for metals like zinc, copper, and magnesium, while others are crafted into ornamental stones or pigments. Additionally, sodium carbonate and bicarbonate are widely utilized in cleaning products, food preparation, and pharmaceuticals. Overall, carbonate minerals play a vital role in both natural processes and industrial applications.
Carbonate minerals
Where Found
Calcite, a common carbonate mineral, dominates the metamorphicrock marble and the sedimentary rock limestone. It also occurs in cave and hot spring deposits, some dry lake deposits (including oolitic sands of Great Salt Lake, Utah), and modern marine sediment in some tropical areas such as the Great Bahama Bank, Florida, Mexico, the Persian Gulf, and Australia. Shells of many marine invertebrates are made of calcium carbonate (including corals, molluscs such as bivalves and snails, echinoderms such as sand dollars and sea urchins, and planktonic organisms whose microscopic shells accumulate to form chalk). In arid climates calcium carbonate accumulates in soil to form calcrete or caliche (hardpan). Carbonates other than calcium carbonate occur in sedimentary deposits and in association with ore veins.
Primary Uses
Calcite has been used in building (cement, structural and ornamental stone), as a flux in smelting various types of metal ores, in agriculture, in the chemical industry for the manufacture of various products, for polishing, and as a filler in paint and rubber. Other carbonate minerals are used as ores of various metals, in manufacturing, as ornamental stone, or in jewelry.
Technical Definition
Carbonate minerals contain the carbonate anion, (CO3)-2, in their chemical formula. There are approximately sixty carbonate minerals, but many are rare. Among the more common carbonates are calcite and aragonite (both CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3), and siderite (FeCO3). Carbonate minerals effervesce in hydrochloric acid, but with some carbonates, the acid must be hot or the mineral must be powdered to obtain the reaction. Most carbonates are soft, and rhombohedral cleavage is common.
Description, Distribution, and Forms
Carbonate minerals may be divided into three groups, each of which has a similar crystal structure: the calcite group, the dolomite group, and the aragonite group. Some carbonate minerals are “polymorphs” of one another, with identical chemical formulas but different crystal structures. An example is CaCO3, which exists in nature as three different crystal structures: calcite (hexagonal system), aragonite (orthorhombic system), and vaterite (hexagonal, also called μ-calcite).
The calcite group belongs to the hexagonal crystal system, hexagonal-scalenohedral class. This group includes calcite (CaCO3), magnesite (MgCO3), siderite (FeCO3), rhodochrosite (MnCO3), and smithsonite (ZnCO3). The dolomite group belongs to the hexagonal crystal system, rhombohedral class. This group includes dolomite (CaMg(CO3)2) and ankerite (CaFe(CO3)2). The aragonite group belongs to the orthorhombic crystal system, rhombic-dipyramidal class. This group includes aragonite (CaCO3), witherite (BaCO3), strontianite (SrCO3), and cerussite (PbCO3).
The basic copper carbonates, malachite (Cu2CO3(OH)2) and azurite (Cu3(CO3)2(OH)2), belong to the monoclinic crystal system, prismatic class. Other monoclinic carbonates are trona (NaHCO3·Na2CO3·2H2O), hydromagnesite (Mg5(CO3)4(OH)2·4H2O), and artinite (Mg2(CO3)(OH)2·3H2O).
The most abundant carbonate mineral is calcite (CaCO3), which comprises limestone, chalk, travertine, tufa (sedimentary rocks), and marble (metamorphic rock). Most limestone forms in warm, shallow seas, far from sources of land-derived sediment. Chalk is made of the shells of microscopic floating organisms which once lived in the sea. Spring deposits are travertine or tufa, and cave deposits (stalactites and stalagmites) are travertine. These deposits form from the evaporation of groundwater carrying dissolved calcium carbonate. Marble is limestone which has been changed by heat and pressure. Malachite and azurite are associated with the oxidized portions of copper deposits and with copper veins through limestone deposits.
Sodium carbonate minerals are present in association with dry salt lake deposits in some parts of the world. These include trona, natron (Na2CO3·10H2O), thermonatrite (NaCO3·H2O), nahcolite (NaHCO3), gaylussite (CaCO3·Na2CO3·5H2O), pirssonite (CaCO3·Na2CO3·2H2O), and shortite (2CaCO3· Na2CO3).
History
Calcite, because of its abundance, has a rich history. Because calcite can preserve fossil records, its presence helps date cultural artifacts. Chalk has been used for writing for thousands of years.
Obtaining Carbonate Minerals
The most important use of calcite is in the production of cements and lime. When limestone is heated to about 900° Celsius, it loses CO2 and is converted to quicklime or lime (CaO). Mixed with sand, quicklime forms mortar. When mixed with water, it hardens or “sets,” swelling and releasing heat. The most widely produced cement is portland cement (used in concrete), which is generally made from limestone and silica- and alumina-bearing material such as clay or shale. The raw materials are ground together, and the mixture is heated in a kiln until it fuses into a “clinker,” which is then crushed to a powder.
Uses of Carbonate Minerals
Lime (CaO) is also used in agriculture to neutralize acid in soils, in the manufacture of paper, glass, and whitewash, and in tanning leather. It is used in refining sugar, as a water softener, and as a flux for smelting various types of ores. Fine-grained limestone has been used in lithography (printing). Blocks of cut limestone and marble are used as building stone and ornamental stone and may be polished. Crushed limestone is used as aggregate in concrete and as road metal. Dolomite has uses similar to those of calcite.
Several carbonates are metal ores: dolomite and magnesite (ores of magnesium),rhodochrosite (manganese), siderite (iron), smithsonite (zinc), strontianite (strontium), witherite (barium), cerrusite (lead), malachite and azurite (copper), and trona (sodium).
Magnesite is used in the manufacture of refractory materials capable of withstanding high temperatures, for special types of cements, and in the paper, rubber, and pharmaceutical industries. Strontianite is also used in the manufacture of fireworks, producing a purplish-red flame. Malachite (green) and azurite (blue) are used as pigments. Sodium carbonate and sodium bicarbonate are important in the manufacture of washing soda (or sal soda) and are used as cleaning agents and water softeners. They are used in the manufacture of glass, ceramics, paper, soap, and sodium-containing compounds (such as sodium hydroxide) as well as in petroleum refining. Sodium bicarbonate, also known as baking soda, is an important part of baking powder, is a source of carbon dioxide in fire extinguishers and is used medicinally to neutralize excess stomach acid. Several carbonates are used as ornamental stone and in jewelry, including malachite, azurite, aragonite (alabaster), rhodochrosite, and smithsonite.
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