Calcite is one of the most common and widely distributed minerals on Earth, composed of calcium carbonate (CaCO₃). It is known for its remarkable variety of crystal forms and colors, as well as its important role in both geological and biological processes. Calcite is a significant component of sedimentary rocks like limestone and marble and is also a major contributor to the formation of stalactites and stalagmites in caves.
The name "calcite" is derived from the Latin word "calx," meaning lime, which refers to limestone. This connection is fitting, as limestone is primarily composed of calcite. The term was first used in the mid-19th century, though the mineral itself has been known and utilized for thousands of years.
Calcite is the primary mineral in limestone, a sedimentary rock formed from the accumulation of marine organism shells, coral, and other calcium-rich materials. Over time, the calcite in these materials recrystallizes, forming solid rock. In caves, calcite precipitates from calcium-rich water dripping from the ceiling, forming speleothems like stalactites and stalagmites. This occurs as carbon dioxide escapes from the water, reducing its ability to hold dissolved calcium carbonate, which then crystallizes as calcite. Calcite is also a major component of marble, a metamorphic rock formed from the recrystallization of limestone under high pressure and temperature. In hydrothermal environments, calcite can precipitate from hot, mineral-rich water circulating through fractures in rocks. These veins of calcite can be associated with other minerals such as quartz and sulfides.
