Apple-Candy Calcite from Durango/Mexico.

The mineral calcite (also called calcite) consists mainly 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 CaCO3 formed to precipitate; this occurs according to the following reaction: Ca2+(aq) + CO32-(aq) -→ CaCO3(s) Calcite is a common component of sedimentary rocks, of veins and in deposits in hot springs and caverns in karst areas (as stalactite). It is the mineral that forms limestone and therefore 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 lime shales. Calcite causes chemical weathering in igneous and metamorphic rocks, creating sedimentary rocks. Calcite is soluble in CO2-containing water under slight pressure. As it falls into the air, rainwater absorbs the gaseous carbon dioxide, making it acidic. This acidic rainwater is able to dissolve lime. In subsurface flows this can transport material in solution until, after contact with atmospheric pressure, CO2 escapes and the calcite precipitates. This is how stalagmites and stalactites are formed in caves. Historically, the transparent Icelandic form of calcite, called Icelandic spar, was used to make Nicol prisms. Some researchers assume that the Vikings navigated at sea using crystals of this mineral, which allows the position of the sun to be accurately determined even in cloudy weather. Nowadays, limestone is mainly used in construction as a building element, for the production of lime and cement, while marble is used as a building and decorative stone.