Aragonite shouldn't exist at Earth's surface, yet it builds almost every seashell
Chemically, aragonite is just calcium carbonate, the same stuff as chalky calcite. Physically it is the version that ought to form only under crushing pressure. Yet corals and nearly all molluscs build their skeletons from it in ordinary seawater, and it slowly turns into calcite over tens of millions of years.
Calcium carbonate crystallises in three common natural forms: calcite, aragonite and vaterite. Aragonite packs its atoms differently from calcite, giving needle-like crystals in the orthorhombic system, and repeated twinning makes shapes that look hexagonal. The name comes from Molina de Aragón in Spain, where José Torrubia described clusters of hexagonal-looking crystals in 1754; Abraham Gottlob Werner named the mineral in 1797, wrongly believing the town lay in Aragon.
It is the high-pressure form, found in metamorphic rocks from subduction zones, and below roughly 3,000 bars it is thermodynamically unstable at any temperature. Still, it forms readily near the surface. One explanation is Ostwald's step rule, the tendency for a less stable phase to appear first, and magnesium ions in water also seem to block calcite and favour aragonite. The energy difference between the two is small. Given 10 to 100 million years, aragonite tends to convert to calcite, which is why fossils often lose it and aragonite older than the Carboniferous is essentially unknown.
Life is its biggest producer. Almost all mollusc shells contain it, either entirely or alongside calcite, as do the skeletons of reef-building corals and some sponges and tube worms. The shimmering nacre in some extinct ammonite shells survives as the gemstone ammolite. Biological aragonite outperforms the mineral kind: in one experiment, adding amino acids at the levels found in marine shells made synthetic aragonite significantly harder, with glycine giving the biggest boost for the amount absorbed.
Inorganic aragonite decorates caves. Slovakia's Ochtinská Aragonite Cave and Spain's El Soplao, discovered in 1908 inside a lead and zinc mine and opened to visitors in 2005, are famous for it, and branching growths called flos-ferri, flowers of iron, were named at Carinthian iron mines. Reef aquarium keepers rely on it to hold water chemistry close to natural levels.
Source: Aragonite
