Calcite is the most common carbonate mineral. It has a formula unit composition of CaCO₃. Its crystal system is trigonal and its habit varies more than any other mineral. Common habits are tabular and prismatic. Calcite has perfect rhombohedral cleavage.

Distribution

Distribution and uses

Calcite is widely distributed and a major mineral of calcareous sedimentary (limestone) and metamorphic (marble) rocks. Some magnesium substitution for calcium is permissible, creating magnesian calcites. Limestone is used in cement, gravel, and building stone, and in agriculture as ‘aglime’ for raising soil pH. Calcite is also a biomineral formed by many species, including the eggshells of reptiles (except turtles), dinosaurs, and birds.

Calcite is present in about one-third of the soils of the world, primarily in arid and semi-arid regions, where the source was either limestone parent rock or calcareous air-borne dust, although in some climates the soil calcite is the result of coral and shells. Calcareous soils typically have a pH above 7.5.

Markgraf, SA, and RJ Reeder. 1985. High-temperature structure refinements of calcite and magnesite. American Mineralogist. 70:590-600.