Calcite, calcite, calcite spar or double spar, is a very common mineral from the mineral class of "carbonates and nitrates" with the chemical composition Ca[CO3] and thus chemically calcium carbonate.

Calcite crystallizes in the trigonal crystal system and develops various crystal or aggregate forms (habit). Calcite is colorless and transparent. However, it can also appear white due to multiple light refraction caused by lattice structure defects or polycrystalline formation, whereby the transparency decreases accordingly, and can take on a yellow, pink, red, blue, green, brown or black color due to foreign impurities.

With a Mohs hardness of 3, calcite is a medium-hard mineral, which means it can be scratched with a copper coin. It serves as a reference value on the Friedrich Mohs scale, which goes up to 10 (diamond).

Etymology and history

Calcite in the form of limestone was already known in ancient times and was referred to in Ancient Greece as χάλιξ chálix, meaning "small stone" or "gravel", but also lime or limestone. The term calx used in the Roman Empire is regarded as a loan word from the Greek, but only means raw and slaked lime, which was used as mortar. The limestone used as a building material was classified as marble.

In 1845, Wilhelm von Haidinger coined the name calcite (originally calcite) for the mineral, which is still used today. He objected to the lack of a superordinate name for all its forms (limestone, calcite, double spar, moonmilk, etc.). He based this on the superordinate name calcaire from Delamétherie and Beudant, which, however, remained limited to the French language.

The ability of calcite to crystallize in all forms and combinations of the rhombohedral system was of considerable importance for the derivation of the laws of crystallography. The English physician William Pryce had already anticipated the foundations of crystallography in 1778 when he stated in Mineralogia Cornubiensis that all forms of calcite result from simple cleavage from the basic form of the rhombohedron. The French mineralogist

René-Just Haüy (1743-1822) developed the first practical crystallography on this basis. As is so often the case, there is a legend surrounding Haüy's discovery. A large calcite crystal fell from Haüy's table to the floor and shattered into many individual pieces. When he picked up the fragments, Haüy noticed that although they all had a different shape, they all resembled the rhombohedral Icelandic spat. Haüy repeated the process with the different crystal forms of calcite and each time he obtained a rhombohedron. From this observation, he concluded that the crystals were formed from the repetition of the elementary lattice or elementary cell in the three spatial directions.

He recorded his observations in 1781 and 1782 in his book Memoire sur la structure des crystaux. In it, the basic laws of crystallography were formulated for the first time and explained using the example of calcite.

Formation and locations

The balance of the above reaction shifts increasingly to the right as the temperature rises. In warm waters, living organisms can therefore build calcareous shells with less energy input. Scale is formed when calcareous water is heated.

Calcite can appear solid as well as granular, fibrous or in crystals and in the latter case shows the greatest variety of forms of all minerals. As a rock-forming mineral, it is one of the most common minerals in the earth's crust and occurs in magmatic rocks, for example in carbonatites, in metamorphic (marble) or sedimentary rocks such as limestone. It occurs alone or with other minerals in veins, but also forms on the earth's surface. Calcite often forms/forms through biomineralization.

By far the largest calcite deposits can be traced back to marine sediments. The calcite-containing skeletons and shells of countless small marine animals such as mussels, corals and various protists, such as coccolithophores, are deposited on the seabed. These calcareous algae are smaller than 30 micrometers and are classified as nanoplankton. They form tiny calcareous shields, known as coccoliths, which sink to the ocean floor after the algae die off. The chalk cliffs of Dover are made up of such coccoliths. Coral reefs also play a prominent role in calcite formation.

Inorganic, abiogenic formation areas of calcite are shallow, tropical marine platforms located in the intertidal area. Calcite is precipitated there in the form of millimeter-sized spheres (calcooids). Calcite in marble is the result of thermal metamorphosis of calcite sediments.

From a sea depth of 3500 meters, the so-called calcite compensation depth, calcite dissolves completely in water. Therefore, neither calcite-containing sediments nor mussel shells or skeletons are preserved at this depth.

Calcite is deposited in the statolithic membrane of the macular organs of the inner ear. It plays a major role in the perception of acceleration and the direction of sound.

Iceland is particularly well known for its extraordinary calcite finds, where the largest crystals to date have been found alongside the water-clear double spar. At Helgustadir near Reyðarfjörður, the largest crystal measured 7 m × 7 m × 2 m and the heaviest weighed 280 tons. In the "Sterling Bush" cave in Lewis County (New York), a calcite rhombohedron measuring 109 cm × 95 cm × 46 cm and weighing around 500 kg was found.

One of the largest calcites on display in museums, weighing 230 kg, can be found in the Natural History Museum in London.