Aragonite

Aragonite is the orthorhombic polymorph of calcium carbonate (CaCO3) that corals, pteropods, and many mollusks use to build skeletons and shells. It is more soluble than calcite, the other common biogenic CaCO3 polymorph, which makes aragonite-building organisms more vulnerable to ocean acidification. Whether aragonite dissolves or precipitates depends on the aragonite saturation state, omega-aragonite, the ratio of the in-situ carbonate-ion concentration to its value at equilibrium. Scleractinian reef corals secrete aragonite skeletons; cold-water pteropods build aragonite shells that dissolve visibly under undersaturation. As dissolved CO2 lowers carbonate-ion concentration, omega falls, raising the energetic cost of calcification.