Description
The mineral calcite (also called limespar) consists primarily 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 a positive ion is present in a solution). Calcium carbonate is a poorly soluble salt, causing the resulting CaCO₃ to precipitate; this occurs according to the following reaction: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s). Calcite is a common constituent of sedimentary rocks, veins, and in deposits in hot springs and caverns in karst regions (as stalactites). It is the mineral that forms limestone and is therefore the most common mineral of biological origin. Calcite is the main constituent of limestone, chalk, and marble, and is also an important component of marl, calcareous sandstone, and limestone. Calcite is responsible for 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 capable of dissolving lime. In underground currents, this material can be transported in solution until, upon 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 Icelandspar, was used to make Nicol prisms. Some researchers hypothesize that the Vikings navigated the sea using crystals of this mineral, which allowed them to accurately determine the position of the sun even in cloudy weather. Today, limestone is mainly used in construction as a building element and for the production of lime and cement, while marble is used as a building and ornamental stone.
