Description
Aragonite, a calcium carbonate with the chemical formula CaCO3, is an unstable polymorph of calcite, making it difficult to distinguish because they often share similar properties. However, a color test can provide clarity: aragonite turns black with a Feigl's 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 foliated habit of calcite. Powder diffraction can clearly demonstrate the difference, as aragonite has an orthorhombic structure, unlike the trigonal structure of calcite. Aragonite forms at low temperatures near the Earth's surface and is found in geysers, hot springs, caves, as oolites in shallow seas, in shells, and pearls. Well-known European sites include Aragon in Spain, Limousis in France, Girgenti in Sicily, Bilin in Bohemia and Eisenertz in Austria.
