Description
Aragonite is a calcium carbonate with the chemical formula CaCO3. It is an unstable polymorph of calcite, making it difficult to distinguish from calcite, as most properties are similar. A color test can help: with a Feigl's solution, aragonite turns black, while calcite remains colorless. Aragonite is also slightly harder than calcite, which can be determined by a scratch test. A third method is the habit: aragonite usually grows in needle-shaped crystals (acicular), while calcite has a foliated habit. Powder diffraction can clarify the difference, as aragonite has an orthorhombic structure and calcite a trigonal one. Aragonite forms at low temperatures near the Earth's surface and occurs in geysers, hot springs, caves, as oolites in shallow seas, in shells, and in pearls. Localities in Europe include Aragon (Spain), Limousis (France), Girgenti (Sicily), Bilin (Bohemia) and Eisenertz (Austria).
