Aragonite is a carbonate mineral, one of the two common, naturally occurring crystal forms of calcium carbonate, CaCO3 (the other form is the mineral calcite.) It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with acicular crystals. Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.