Blue Aragonite “Beautiful little rough pieces” from Pakistan.

The mineral aragonite is a calcium carbonate with chemical formula CaCO3. It is an unstable polymorph of calcite and is therefore difficult to distinguish from calcite because most of its properties are similar. A distinction can be made by means of a color test: with a Feigl solution, aragonite becomes black, while calcite remains colorless. In addition, aragonite is slightly harder than calcite and it is possible to determine which mineral it is via a scratch test. A third distinguishing method is habitus: aragonite usually grows in needle-shaped crystals (acicular), while calcite has a leaf-shaped habit. With the aid of powder diffraction the difference can be determined unambiguously because aragonite has an orthorhombic structure and not a trigonal one such as calcite. Aragonite is always formed at low temperatures near the earth's surface. Aragonite is found in geysers, hot springs, caves, as oolites in shallow seas, in shells and in pearls. Locations in Europe are Aragón (Spain), Limousis (France), Girgenti (Sicily), Bilin (Bohemia) and Eisenertz (Austria).