Aragonite is a naturally occurring mineral composed of calcium carbonate, sharing the same chemical formula (CaCO3) as the more common mineral calcite. The difference lies in its crystal structure, which is orthorhombic rather than the trigonal structure of calcite, giving it unique physical properties and crystal habits. This distinct internal arrangement causes aragonite crystals to often form elongated, needle-like shapes or complex, branching aggregates. The mineral is also gaining industrial interest for applications in water purification and as a safe, pure source of calcium carbonate.
Geological and Hydrothermal Settings
Aragonite is considered the high-pressure polymorph of calcium carbonate, meaning it forms under conditions of elevated pressure and relatively low temperature compared to calcite. This specialized environment is found deep within the Earth in subduction zones, where tectonic plates slide beneath one another, subjecting rock to immense forces. Aragonite is a characteristic mineral in metamorphic rocks formed under these high-pressure, low-temperature conditions.
The mineral is also commonly found in hydrothermal systems where superheated, mineral-rich water interacts with rock layers, often precipitating aragonite as sinter—a porous deposit that forms as the water cools. In Spain’s Iberian Basin, aragonite crystals have been linked to the upward flow of hydrothermal fluids through faults, indicating the mineral’s role as a marker for ancient tectonic activity.
Within the more accessible environment of limestone caves, aragonite forms a variety of delicate structures known as speleothems. When the chemistry of the dripping water favors aragonite growth over calcite, it can create intricate, branching formations. One notable variety is flos ferri, or “flowers of iron,” which are fragile, coral-like or helical clusters of aragonite crystals often found in abandoned mines or cave fissures.
Biogenic Sources and Marine Environments
The majority of new aragonite formed on Earth is produced by living organisms in shallow marine environments. Marine invertebrates, such as warm- and cold-water corals, use aragonite to construct their rigid skeletons, which form the structural framework of coral reefs. Similarly, many mollusks, including clams, snails, and oysters, secrete layers of aragonite to build their shells, with the iridescent nacre of pearls and mother-of-pearl being a classic example.
The chemical conditions of the ocean currently favor the precipitation of aragonite over calcite, a state often characterized as an “aragonite sea.” This condition is largely controlled by a high magnesium-to-calcium ratio in the seawater, as the magnesium ion inhibits the formation of calcite. This high ratio encourages the biomineralization of aragonite in many calcifying organisms.
Non-biogenic aragonite precipitation also occurs readily in warm, shallow tropical waters, contributing to vast sedimentary deposits. A prime example is the formation of ooids, small, spherical grains that grow as calcium carbonate precipitates around a nucleus in agitated water. Massive deposits of oolitic aragonite sand cover the seabed in places like the Bahamas, created by wave action and water chemistry. These deposits are also a source of marine cementation, where aragonite precipitates within the sediment pores, binding the grains together.
Notable Global Deposits
Aragonite is distributed globally, but several locations are renowned for the purity, quantity, or unique crystal habit of their deposits. The mineral was first named after the village of Molina de Aragón in Spain, where it is found as distinct cyclic twins within gypsum and marls. Spain remains a classic source, with other localities like La Pesquera also producing fine specimens.
Central Europe contains famous aragonite caves, such as the Ochtinská Aragonite Cave in Slovakia, which features rare helictitic and branching formations. World-class pseudohexagonal crystals have been collected from Podrečany in Slovakia and from the Agrigento area of Sicily, Italy, often associated with sulfur deposits. Layered aragonite from hot spring deposits can also be found in Karlovy Vary in the Czech Republic.
In North America, spectacular cave formations are found in the United States, including the needle-like anthodites and stalactitic aragonite of Carlsbad Caverns in New Mexico. The Bisbee mining district in Arizona is another site known for its extensive network of caves containing aragonite in various habits, including large clusters of the flos ferri variety. The Bahamas are globally significant for their massive, commercially important deposits of oolitic aragonite sand.