Aragonite (variety 'Flos Ferri')

Hardness: 3.5-4 on Mohs scale. Color: Typically white, colorless, gray, yellow, green, blue, pink, or brownish. Luster: Vitreous to resinous. Crystal Structure: Orthorhombic. Cleavage: Poor in one direction. Specific Gravity: 2.9-3.0. The specimen shows a 'flos ferri' habit, which is a coral-like or intricate branching form; often fibrous or columnar aggregates.

Aragonite forms in various low-temperature environments, often associated with sedimentary processes. It can be found in hot springs, caves as speleothems (like stalactites and stalagmites), and in mollusk shells. The 'flos ferri' variety specifically forms in iron ore deposits (hence 'flower of iron') or in caves where water containing calcium carbonate seeps through. It is polymorphous with calcite, meaning it has the same chemical composition but a different crystal structure, and forms under different pressure and temperature conditions, usually lower temperatures.

While aragonite is not as widely used as calcite, it is valued by mineral collectors, especially aesthetic specimens like 'flos ferri'. It has some industrial uses, for example, in aquaculture to raise pH and provide calcium for marine life, and in some biomaterials research due to its presence in skeletal structures. High-quality specimens can be polished for decorative items.

Aragonite is readily distinguished from calcite by its higher specific gravity and the lack of perfect rhombohedral cleavage. It will effervesce in dilute hydrochloric acid, like calcite. The 'flos ferri' habit from iron mines (particularly in Styria, Austria) is famous for its intricate beauty. Aragonite is less stable than calcite at surface conditions and will slowly transform into calcite over geological time, especially at higher temperatures.

In the field, aragonite can often be identified by its characteristic crystal habits (acicular, prismatic, or coral-like 'flos ferri'). It also reacts with dilute acid, similar to calcite. Distinguished from calcite by its higher specific gravity, lack of characteristic rhombohedral cleavage, and when available, its crystal habit. Commonly found in cave systems (as speleothems), hot springs (tufa), and as veins in serpentinite. Notable localities for 'flos ferri' include the Styrian iron ore mines in Austria, and various other caves and mineral deposits worldwide.