Description
Calcite, also known as limespar, is a mineral composed primarily of calcium carbonate (CaCO3) and is one of the most abundant minerals in the Earth's crust. Calcium carbonate forms when soluble calcium ions react with CO2, with carbon dioxide forming a carbonate ion in the presence of a positive ion. Because calcium carbonate is insoluble, the resulting CaCO3 precipitates according to the reaction: Ca2+(aq) + CO32-(aq) → CaCO3(s). Calcite is common in sedimentary rocks, veins, and deposits in hot springs and caves in karst regions, where it occurs as stalactites. It is the primary mineral in limestone and thus the most abundant 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. It plays a role in the chemical weathering of igneous and metamorphic rocks, leading to the formation of sedimentary rocks. Calcite dissolves under slight pressure in CO2-rich water. As rainwater falls through the air, it absorbs gaseous CO2 and becomes acidic, enabling it to dissolve limestone. This acidic water can transport limestone in underground streams, where, upon contact with atmospheric pressure, CO2 escapes and calcite precipitates, resulting in the formation of stalagmites and stalactites in caves. The transparent Icelandic variety of calcite, known as Iceland spar, was formerly used to make Nicol prisms. It is speculated that Vikings may have navigated with crystals of this mineral, allowing them to determine the sun's position even in cloudy weather. Today, limestone is primarily used in construction, for making lime and cement, while marble serves as a building and ornamental stone.
