Aragonite is a polymorph of the mineral calcite, both having the chemical composition CaCO3. Its structure differs from calcite and leads to a different crystal shape, an orthorhombic system with acicular crystals. By repeated twinning pseudo-hexagonal forms result. It may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri (flowers of iron) from their association with the ores at the Carthinian iron mines. The type location for aragonite is Molina de Aragón (Guadalajara, Spain), 25 km outside Aragon.
It exists naturally in the shells of chiton and other mollusks. The fossil shells of some extinct ammonites form a mineral called ammolite, which is primarily aragonite with impurities that make it iridescent and valuable as a gemstone.
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