Calcium carbonate or lime is an ionic compound that occurs in nature in various forms. Mountains, coral reefs and stalactite caves are largely made of limestone - a substance formed through a series of chemical reactions over the course of the earth's history. Marble and chalk are both special forms of calcium carbonate.

Calcium carbonate is a chemical compound of calcium, carbon and oxygen with the chemical formula CaCO3. As a calcium salt of carbonic acid, it belongs to the carbonate group of substances. It is a colorless, crystalline solid whose crystal structure consists of the ions Ca2+ and CO32- in a ratio of 1:1.

The most important sources of this substance are shells and limestone (CaCO3). There are also other minerals that contain this chemical compound, such as chalk, marble and many others.

Calcium carbonate is one of the most widespread compounds on earth, especially in the form of sedimentary rock. It occurs mainly in the form of the mineral calcite (calcite, biclase), which is one of the most common minerals in the earth's crust. Not only is it the predominant mineral in massive limestone, but in combination with quartz, barite and fluorite it is also the source rock of many ore veins. It can even be the sole component of veins whose thickness ranges from a few centimetres to several dozen meters. Other modifications of calcium carbonate are the minerals aragonite and vaterite. The name aragonite is derived from the most important occurrence of the mineral in Aragon. It occurs more frequently near seawater. The reason for this is the magnesium contained in the seawater, which favors the formation of aragonite over calcite. Alongside calcite, aragonite is the most common biomineral. It is the inorganic component of mother-of-pearl in shells, is frequently found in pearls and can also be found in mussels.

Ikaite is the hydrate of calcium carbonate and was first discovered in nature in 1963 in the form of columns up to 20 meters high at the Ikkafjord in Greenland. The place where it was found also gave the mineral its name. The mineral is unstable at atmospheric pressure above 0 °C and transforms into calcite above this temperature. Its formation is favored by magnesium (which is abundant in seawater) and other additives.

Ikaite occurs as prismatic crystals, but also as fibrous masses or stalactites. It has a density of 3.39 g/cm3 and a hardness of 2.5 on the Mohs scale. It is transparent to translucent with a white color and glassy luster.

There are also two other pseudopolymorphic minerals, monohydrocalcite and ikaite, which are hydrates of calcium carbonate. The unstable monohydrocalcite is the monohydrate of calcium carbonate. The first natural occurrence of monohydrocalcite was described in 1959 and 1964 as impure sediment in Lake Issyk-Kul in Kyrgyzstan. In living organisms, it is found in the bladder stones of guinea pigs and in the ear stones of some vertebrates.

Calcium carbonate is a mineral that occurs in many places on earth. It is the main component of sedimentary limestone, metamorphic marble and other sedimentary rocks such as oolite or stromatolite. In living nature, it is found in the exoskeleton of crustaceans, corals, mussels, snails and protozoa. In some of these it is also present in one of the various known amorphous calcium carbonate phases (ACC).

The first limestones of any significant size were formed by stromatolites more than two billion years ago.

Calcium carbonate is widespread in the soil. The most important calcium minerals in our soils are calcite and dolomite [CaMg(CO3)2]. They are released into the soil when carbonate rocks or marl weather.

Contrary to popular belief, the bones and teeth of vertebrates do not contain calcium carbonate, but rather the calcium-containing substances hydroxyapatite (in bones) and also fluorapatite in teeth. However, calcium carbonate is found in plants, for example. The leaf hairs of the red dogwood are coated with calcium carbonate.