Aragonite
A crystalline form of calcium carbonate that makes up coral skeletons and mollusk shells, more soluble in seawater than calcite.
A mineral polymorph of calcium carbonate (CaCO₃) commonly found in marine organisms like corals, mollusks, and coralline algae. It forms the structural skeleton of many reef-building corals and is more chemically reactive in seawater than the alternative calcite form, making it sensitive to ocean acidification.
What this means in real life
When ocean water becomes more acidic from carbon dioxide absorption, it dissolves coral skeletons made of this mineral faster than the coral can rebuild them, weakening the reef structure.
What it isn’t
It is not the same as calcite, another calcium carbonate form. While both are CaCO₃, aragonite is less stable in seawater and dissolves more readily under acidic conditions, whereas calcite is more chemically stable.
Commonly misused online
People sometimes use it interchangeably with 'limestone' or 'calcium carbonate' without recognizing that aragonite is a specific, more fragile crystal form that behaves differently in ocean chemistry.