Aragonite crystals “sputniks” small (15-25mm) from the Rif Mountains of Morocco

Aragonite, a calcium carbonate with the chemical formula CaCO3, is an unstable polymorph of calcite, which makes the distinction difficult because they often share the same properties. However, a color test can provide a solution: aragonite turns black with a Feigl solution, while calcite remains colorless. Aragonite is also slightly harder than calcite, which can be demonstrated with a scratch test. Another distinction is the crystal shape; Aragonite usually forms needle-shaped crystals (acicular), as opposed to the leaf-shaped habit of calcite. Powder diffraction can clearly show the difference, as aragonite has an orthorhombic structure, unlike the trigonal structure of calcite. Aragonite is formed at low temperatures near the Earth's surface and occurs in geysers, hot springs, caves, as oolites in shallow seas, in shells and pearls. Well-known European sites include Aragón in Spain, Limousis in France, Girgenti in Sicily, Bilin in Bohemia and Eisenertz in Austria.