Aragonite

Aragonite is a carbonate mineral and one of the three main natural crystal forms of calcium carbonate, alongside calcite and vaterite. It forms through both biological and physical processes, such as precipitation in marine and freshwater settings. Its crystal structure is orthorhombic, typically producing needle-like crystals, and repeated twinning can create pseudo-hexagonal shapes. It may appear columnar or fibrous, and occasionally forms branching, helictitic growths known as flos-ferri, named for their association with iron ores in Carinthia.

The mineral was first described in 1754 by José Torrubia, who noted large quantities of "hexagonal" crystals called "torrecillas" near the Gallo River in Molina de Aragón. It was named by Abraham Gottlob Werner in 1797 after Molina de Aragón, though he mistakenly thought the location was in Aragon. In its type locality, aragonite occurs as cyclic twins within gypsum and marls of the Triassic Keuper facies. Such deposits are common in Spain and also found in France. Notable aragonite caves include the Ochtinská Aragonite Cave in Slovakia and the Cave of El Soplao in Cantabria, Spain, discovered in 1908 within a former lead and zinc mine and opened to the public in 2005. In the United States, aragonite stalactites and cave flowers appear in Carlsbad Caverns and other caves. For a brief period in the early 1900s, aragonite was mined at Aragonite, Utah, now a ghost town. Massive oolitic aragonite sand deposits exist on the seabed in the Bahamas.

Aragonite is the high-pressure form of calcium carbonate, found in metamorphic rocks from subduction zones. It forms naturally in nearly all mollusk shells and in the calcareous skeletons of warm- and cold-water corals. Some serpulid worms have aragonitic tubes. Biological control over shell deposition can produce crystal shapes distinct from inorganic aragonite. In some mollusks, the entire shell is aragonite; in others, it appears only in parts of a bimineralic shell. The nacreous layer of aragonite in certain extinct ammonite fossils yields an iridescent material called ammolite. Aragonite also occurs in the endocarp of the hackberry tree, in the skeletons of some calcareous sponges, and as inorganic marine cements or free crystals in the ocean. In caves, it forms speleothems, and it is common in serpentinites where magnesium-rich fluids inhibit calcite growth. Aragonite is metastable at Earth's surface and typically converts to calcite in fossils over tens of millions of years; aragonite older than the Carboniferous is virtually unknown. It can be synthesized by mixing calcium chloride and sodium carbonate solutions above 60 °C or in water-ethanol mixtures at room temperature.

Aragonite is thermodynamically unstable at pressures below about 3,000 bars and any temperature, yet it frequently forms at ambient conditions. Weak Van der Waals forces influence its crystallographic and elastic properties. The stability difference between aragonite and calcite, measured by Gibbs free energy, is small, and grain size or impurities can affect which forms. Its formation where calcite is stable may follow Ostwald's step rule, with a less stable phase appearing first. Magnesium ions can favor aragonite over calcite. Once formed, aragonite tends to alter to calcite over 10 to 100 million years. Another metastable calcium carbonate phase, vaterite, decomposes even more readily.

In aquariums, aragonite is considered essential for replicating reef conditions, providing materials for marine life and helping maintain natural pH to prevent dissolution of biogenic calcium carbonate. It has also been tested successfully for removing pollutants like zinc, cobalt, and lead from contaminated wastewater.