Sculpture of mother and child, made of Mexican fire calcite.

The mineral calcite (also known as calcium carbonate) mainly consists of the salt calcium carbonate (CaCO3) and is one of the most common minerals in the Earth's crust. Calcium carbonate is formed when soluble calcium ions come into contact with CO₂ (carbon dioxide forms a carbonate ion when there is a positive ion in a solution). Calcium carbonate is a poorly soluble salt, causing the formed CaCO3 to precipitate; this occurs according to the following reaction: Ca2(aq) + CO2(aq) → CaCO2(s) Calcite is a common component of sedimentary rocks, veins, and deposits in hot springs and caves in karst areas (as stalactites). It is the mineral that forms limestone and thus the most common mineral of biological origin. Calcite is the main component of limestone, chalk rock, and marble, and it is also an important component of marl, calcareous sandstone, and limestone shales. Calcite causes chemical weathering in igneous and metamorphic rocks, leading to the formation of sedimentary rocks. Calcite is soluble in CO₂-containing water under light pressure. During its fall through the air, rainwater absorbs gaseous carbon dioxide, making it acidic. This acidic rainwater is capable of dissolving limestone. In underground streams, this material can be transported in solution until, upon contact with atmospheric pressure, CO₂ escapes and calcite precipitates. This is how stalagmites and stalactites are formed in caves. In the past, the transparent Icelandic form of calcite, known as Iceland spar, was used to manufacture Nicol prisms. Some researchers hypothesize that the Vikings navigated at sea using crystals of this mineral, which also allowed for accurate determination of the sun's position even in cloudy weather. Nowadays, limestone is primarily used in construction as a building element and for the production of lime and cement, while marble is used as a building and decorative stone.