Aragonite

Hardness: 3.5-4 on Mohs scale; Color: Usually white, colorless, gray, yellow, green, or violet; Luster: Vitreous to resinous; Crystal structure: Orthorhombic, often forming prismatic, acicular (needle-like), fibrous, or stalactitic habits. Can also form pseudo-hexagonal twinned crystals. Perfect cleavage on one plane (pinacoidal); Specific Gravity: 2.95.

Aragonite forms in a variety of low-temperature environments, often biologically mediated. It is the metastable polymorph of calcium carbonate at surface pressures and temperatures, eventually transforming to calcite over geological time. It can form in marine and freshwater environments, hot springs (as tufa), caves (as stalactites and stalagmites), and in the shells of marine organisms. Geological age varies greatly depending on the specific formation.

While less common than calcite, aragonite is used in soil conditioners, as an ornamental stone, and sometimes in jewelry (especially iridescent forms like 'pearl' aragonite). It is also scientifically important for paleoclimate studies.

Aragonite is a polymorph of calcite, meaning it has the same chemical composition (CaCO3) but a different crystal structure. This difference makes aragonite less stable at surface conditions and it gradually converts to calcite. Organisms like corals, mollusks, and some algae precipitate aragonite for their shells and skeletons.

Identify in the field by its crystal habit (often acicular or pseudo-hexagonal), its moderate hardness (scratches easily with a knife, but not with a fingernail), and its effervescence in dilute hydrochloric acid (like calcite). It can be distinguished from calcite which usually has rhombohedral cleavage. Commonly found in hot spring deposits (e.g., Carlsbad, New Mexico; various locations in Europe), caves, and certain marine sediments. For collectors, well-formed crystal clusters are prized.