aragoniiitti

Aragoniiitti, known in English as aragonite, is a carbonate mineral and one of the two common polymorphs of calcium carbonate (CaCO3), the other being calcite. It crystallizes in the orthorhombic system and tends to form slender, prismatic, tabular, or needlelike crystals, often in fibrous or radiating aggregates. The mineral is typically colorless or white, but impurities can produce a range of colors.

Occurrence and formation. Aragonite occurs in both biogenic and abiogenic environments. It is a major component of many marine shells, skeletons, and nacre in mollusks and corals, and can precipitate from seawater in certain chemical conditions. It also forms in cave environments as aragonite speleothems and can be found in hydrothermal veins and in some high-pressure sedimentary settings.

Stability and transformation. At standard Earth surface conditions, aragonite is metastable relative to calcite and can convert to calcite over geological timescales through diagenesis. The mineral can be preserved in certain sedimentary records when conditions inhibit transformation or when rapidly buried.

Properties and significance. Aragonite has a hardness around 3.5–4 on the Mohs scale and a specific gravity near 2.93. It is optically distinct from calcite due to its orthorhombic structure and birefringence. The mineral is important in paleoclimatology and paleoceanography because its isotopic composition in shells and carbonate structures records environmental conditions at the time of formation. It is named after the Aragon region in Spain.