Aragonite, CaCO₃, is classified as a Aragonite Group Carbonate. A calcium carbonate mineral with a formula unit identical to — and thus a polymorph of — calcite, aragonite has an orthorhombic crystal system (Calcite forms trigonal crystals) and a pseudocubic crystal habit. Aragonite’s structure consists of triangular carbonate ion groups (CO₃), with a carbon at the center of the triangle and three oxygens, one at each corner.

Distribution

Aragonite is the main component of many organic substances, such as pearl and coral. It is found in some eggshells (e.g., turtles), in the shells of bivalves and monovalves, and in other biological carbonates. The iridescent surface of pearl and mother-of-pearl is a layer of Aragonite secreted by mollusks and related invertebrates. Aragonite is less stable and less common than calcite. Aragonite can be found as hot spring deposits when water, releasing calcium upon reaching the air, forms mounds and thick crusts around the spring (“travertine”). Gem-quality crystals of aragonite can be found in Germany and Austria. Other sources include Czechoslovakia, Sicily, Greece, Spain, and Japan. Its only real use is as specimens for collectors.

Importance in Soils

Calcite is a common mineral in dry regions, but aragonite is less stable and less common, occurring mainly as shell fragments. Aragonite may be present in sedimentary deposits. Soils are leached of carbonates, including aragonite, in humid climates.

Dal Negro, A., and L. Ungaretti. 1971. Refinement of the crystal structure of aragonite. American Mineralogist 56:768-772.