Description
The mineral aragonite is a calcium carbonate with the chemical formula CaCO3. It is an unstable polymorph of calcite and is therefore difficult to distinguish from calcite, as most properties are similar. A color test can be used to distinguish it: with a Feigl's solution, aragonite turns black, while calcite remains colorless. Furthermore, aragonite is slightly harder than calcite, and a scratch test can be used to determine which mineral it is. A third method of differentiation is the habit: aragonite usually grows in needle-shaped crystals (acicular), while calcite has a foliated habit. Powder diffraction allows for a clear distinction because aragonite has an orthorhombic structure, not a trigonal one like calcite. Aragonite is always formed at low temperatures near the Earth's surface. It is found 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).
