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.