Aragonite

Aragonite is a carbonate mineral, one of the two common polymorphs of calcium carbonate, CaCO3, the other being calcite. It crystallizes in the orthorhombic system and typically forms slender prismatic or acicular crystals, as well as fibrous or granular aggregates. It is a major component of some shells and skeletal frameworks in marine organisms.

Natural aragonite occurs widely in marine environments. It forms the shells of many mollusks, the skeletons of corals, and some echinoderms, and can be found as microscopic crystals in sediments and in hydrothermal deposits. In seawater, the magnesium-to-calcium ratio influences its precipitation; historically, high ratios have favored aragonite, contributing to what is described as an aragonite sea in geological terms.

Aragonite is less stable than calcite at surface conditions and is more soluble in water, so it gradually converts to calcite over geological timescales. It also dissolves more readily in acid than calcite, a property exploited in paleoclimate studies and various industrial applications.

The mineral is named after Aragon, Spain. In addition to its biological and geological significance, aragonite is mined as a source of calcium carbonate for industrial uses and serves as a valuable material in studies of past environments, where its isotopic composition in shells and carbonate rocks helps reconstruct ancient temperatures and seawater chemistry.