Calcium carbonate

Calcium carbonate, CaCO₃, is one of the most common compounds on Earth, making up about 7% of Earth’s crust. It occurs in a wide variety of mineral forms, including limestone, marble, travertine, and chalk. Calcium carbonate also occurs combined with magnesium as the mineral dolomite, CaMg (CO₃)₂. Stalactites and stalagmites in caves are made of calcium carbonate, as are a variety of animal products, notably coral, seashells, eggshells, and pearls.

Calcium carbonate has two major crystalline forms—two different geometric arrangements of the calcium ions and carbonate ions that make up the compound—aragonite and calcite. All calcium carbonate minerals are conglomerations of various-sized crystals of these two forms, packed together in different ways and containing various impurities. The large, transparent crystals known as Iceland spar, however, are pure calcite.

In its pure form, calcium carbonate is a white powder with a specific gravity of 2.71 in the calcite form or 2.93 in the aragonite form. When heated, it decomposes into calcium oxide (CaO) and carbon dioxide gas (CO₂). It also reacts vigorously with acids to release a froth of carbon dioxide bubbles. It is said that Cleopatra, to show her extravagance, dissolved pearls in vinegar (acetic acid).

Every year in the United States alone, tens of millions of tons of limestone are dug, cut, or blasted out of huge deposits in Indiana and elsewhere. It is used mostly for buildings and highways and in the manufacture of steel, where it is used to remove silica (silicon dioxide, SiO₂) and other impurities in the iron ore. Calcium carbonate decomposes to calcium oxide in the heat of the furnace, and the calcium oxide reacts with the silica to form calcium silicates (slag), which float on the molten iron and can be skimmed off.

Calcium carbonate deposits can be formed in sea-water when calcium ions dissolved from other minerals react with dissolved carbon dioxide (carbonic acid, H₂ CO₃). The resulting calcium carbonate is insoluble in water and sinks to the bottom.

Most of the calcium carbonate deposits found today were formed by sea creatures millions of years ago when oceans covered much of what is now land. They manufactured shells and skeletons of calcium carbonate using the calcium ions and carbon dioxide in the oceans, just as clams, oysters, and corals do today. When these animals died, their shells settled on the sea floor. Now, long after the seas have gone, they have been compressed into thick deposits of limestone. England’s White Cliffs of Dover are chalk—a soft, white porous form of limestone made from the shells of microscopic sea creatures called Foraminifera that lived about 136 million years ago. Blackboard “chalk,” in contrast, is mostly gypsum, or calcium sulfate (CaSO₄).

In pearls—which mollusks make when irritated by a foreign body in their flesh—and in seashells, the individual CaCO₃ crystals are invisibly small, even under a microscope. But they are laid down in such a perfect order that the result is smooth, hard, shiny, and sometimes even iridescent, as in the rainbow colors of abalone shells. In many cases, the mollusk makes its shell by laying down alternating layers: calcite, aragonite, calcite, aragonite, and so on. This gives the shell great strength, as in a sheet of plywood in which the grain of the alternating wood layers runs in crossed directions.

Calcium Carbonate

OVERVIEW

Calcium carbonate (KAL-see-um CAR-bun-ate) is one of the most common compounds on Earth, making up about 7 percent of Earth's crust. It occurs in a number of minerals and other natural materials, including aragonite, calcite, chalk, limestone, marble, marl, oyster shells, pearls, and travertine. Stalactites and stalagmites found in caves are made primarily of calcium carbonate. As indicated by the melting points of aragonite and calcite, the compound's physical properties may differ somewhat depending on its crystal form. It typically occurs as an odorless, tasteless white powder or colorless crystals.

KEY FACTS

OTHER NAMES:

Limestone; chalk; aragonite; calcite

FORMULA:

CaCO₃

ELEMENTS:

Calcium, carbon, oxygen

COMPOUND TYPE:

Inorganic salt

STATE:

Solid

MOLECULAR WEIGHT:

100.09 g/mol

MELTING POINT:

Calcite: 1330°C (2430°F); aragonite: decomposes at about 825°C (1520°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Very slightly soluble in water; soluble in dilute acids; insoluble in organic solvents

HOW IT IS MADE

Calcium carbonate is so abundant in nature that demand for the compound can be met by mining natural sources, such as chalk, limestone, and marble quarries. The compound can also be produced in the laboratory by reacting calcium chloride (CaCl₂) with sodium carbonate (Na₂CO₃). The calcium carbonate formed in this reaction precipitates out of (separates from) the solution and can be recovered by filtration.

COMMON USES AND POTENTIAL HAZARDS

One well known use of calcium carbonate is as an antacid, a substance that neutralizes excess stomach acid (hydrochloric acid; HCl) and relieves the symptoms of acid indigestion, heartburn, upset stomach, and sour stomach. Large amounts of the chemical are also used for a variety of industrial uses, including:

• In agriculture, where it is used to maintain proper acidity of soil and supply calcium needed by growing plants;

Interesting Facts

• It is said that Cleopatra showed her extravagance by dissolving pearls (which are made of calcium carbonate) in vinegar and drinking the resulting solution.
• Stalagmites and stalactites form in caves when calcium bicarbonate (CaHCO₃) dissolved in groundwater reaches the top of a cave and gives up carbon dioxide (CO₂) to the air. When calcium bicarbonate loses carbon dioxide, it is converted to calcium carbonate, which precipitates out on the roof of the cave as a stalactite. If the calcium bicarbonate does not lose carbon dioxide until it drips off the roof and falls on the floor of the cave, the calcium carbonate that is formed builds up a stalagmite on the floor of the cave.

• In the paper-making industry, where it is used to make strong products with a very white color, glossy finish, and firm texture for printing and dyeing;
• As a filler in the manufacture of plastics to reduce the cost of production;
• In the construction industry, where it is used in the production of concrete structures, such as paving-stones, tubes, and sewage tanks, in ready-mixed concrete, and in prefabricated elements;
• In the production of paints and other types of coating materials because of its ability to provide weather resistance, protect the coating against corrosion, reduce drying time, and maintain the proper acidity of the coating material;
• In a variety of environmental applications, such as counteracting the increased acidity of lakes and other bodies of water caused by acid precipitation, treating waste gases to remove sulfur and nitrogen oxides that pollute the air; and purifying water and waste water; and
• In the manufacture of lime (calcium oxide; CaO) which, itself, has a host of industrial applications.

Words to Know

PRECIPITATE

A solid material that settles out of a solution, often as the result of a chemical reaction.

FOR FURTHER INFORMATION

"Calcium Carbonate." National Institute for Occupational Safety and Health. http://www.cdc.gov/niosh/npg/npgd0090.html (accessed on September 24, 2005).

"Calcium Carbonate Powder." Reade. http://www.reade.com/Products/Minerals_and_Ores/calcium_carbonate.html (accessed on September 24, 2005).

Tegethoff, F. Wolfgana, with Johannes Rohleder and Evelyn Kroker, eds. Calcium carbonate: From the Cretaceous period into the 21st century. Boston: Birkhäuser Verlag, 2001.

See AlsoCalcium Oxide

Calcium carbonate

Calcium carbonate, CaCO3, is one of the most common compounds on Earth , making up about 7% of Earth's crust. It occurs in a wide variety of mineral forms, including limestone, marble, travertine, and chalk. Calcium carbonate also occurs combined with magnesium as the mineral dolomite, CaMg(CO3)2. Stalactites and stalagmites in caves are made of calcium carbonate. A variety of animal products are also made primarily of calcium carbonate, notably coral, sea shells, egg shells, and pearls.

Calcium carbonate has two major crystalline formstwo different geometric arrangements of the calcium ions and carbonate ions that make up the compound. These two forms are called aragonite and calcite. All calcium carbonate minerals are conglomerations of various-sized crystals of these two forms, packed together in different ways and containing various impurities. The large, transparent crystals known as Iceland spar, however, are pure calcite.

In its pure form, calcium carbonate is a white powder with a specific gravity of 2.71 in the calcite form or 2.93 in the aragonite form. When heated, it decomposes into calcium oxide (CaO) and carbon dioxide gas (CO2). It also reacts vigorously with acids to release a froth of carbon dioxide bubbles. It is said that Cleopatra, to show her extravagance, dissolved pearls in vinegar (acetic acid ).

Every year in the United States alone, tens of millions of tons of limestone are dug, cut, or blasted out of huge deposits in Indiana and elsewhere. It is used mostly for buildings and highways and in the manufacture of steel , where it is used to remove silica (silicon dioxide) and other impurities in the iron ore; the calcium carbonate decomposes to calcium oxide in the heat of the furnace, and the calcium oxide reacts with the silica to form calcium silicates (slag), which float on the molten iron and can be skimmed off.

Deposits of calcium carbonate can be formed in the oceans when calcium ions dissolved from other minerals react with dissolved carbon dioxide (carbonic acid, H2CO3). The resulting calcium carbonate is quite insoluble in water and sinks to the bottom.

However, most of the calcium carbonate deposits that we find today were formed by sea creatures millions of years ago when oceans covered much of what is now land. From the calcium ions and carbon dioxide in the oceans, they manufactured shells and skeletons of calcium carbonate, just as clams, oysters, and corals still do today. When these animals die, their shells settle on the sea floor where, long after the seas have gone, we now find them compressed into thick deposits of limestone. The White Cliffs of Dover in England are chalk, a soft, white porous form of limestone made from the shells of microscopic sea creatures called Foraminifera that lived about 136 million years ago. Blackboard "chalk" isn't made of chalk; it is mostly gypsum, CaSO4.

In pearls—which mollusks make out of their shell-building material when they are irritated by a foreign body in their flesh—and in sea shells, the individual CaCO3 crystals are invisibly small, even under a microscope . But they are laid down in such a perfect order that the result is smooth, hard, shiny, and sometimes even iridescent, as in the rainbow colors of abalone shells. In many cases, the mollusk makes its shell by laying down alternating layers: calcite, aragonite, calcite, aragonite, and so on. This gives the shell great strength, as in a sheet of plywood where the grain of the alternating wood layers runs in crossed directions.

calcium carbonate (CaCO₃) White compound, insoluble in water, that occurs naturally as marble, chalk, limestone and calcite. Crystals are in the hexagonal system and vary in form. Calcium carbonate is used in the manufacture of cement, iron, steel and lime, to neutralize soil acidity and as a constituent of antacids. Properties: r.d. 2.7 (calcite).

cal·ci·um car·bon·ate • n. a white, insoluble solid, CaCO₃, occurring naturally as chalk, limestone, marble, and calcite, and forming mollusk shells and stony corals.