New find Mint Green Calcite (tumbled stones) from Durango in Mexico. 30-50mm

Calcite, also known as calcite, is a mineral composed mainly of calcium carbonate (CaCO3) and is one of the most common 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 poorly soluble, the CaCO3 formed 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 areas, where it occurs as stalactite. It is the primary mineral in limestone and therefore the most common mineral of biological origin. Calcite is the main component of limestone, chalk and marble, and is also an important component of marl, calcareous sandstone and lime shale. It plays a role in the chemical weathering of igneous and metamorphic rocks, leading to the formation of sedimentary rocks. Calcite dissolves under light pressure in CO2-rich water. When rainwater falls through the air, it absorbs gaseous CO2 and becomes acidic, making it able to dissolve limescale. This acidic water can transport lime 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 Icelandic spar, was formerly used to make Nicol prisms. It is speculated that Vikings may have navigated using crystals of this mineral, which allowed them to determine the position of the sun even in cloudy weather. Nowadays, limestone is mainly used in construction, for making lime and cement, while marble serves as a building and decorative stone.