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Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments. The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal. Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching helictitic forms called ''flos-ferri'' ("flowers of iron") from their association with the ores at the Carinthian iron mines.

Occurrence

The type location for aragonite is Molina de Aragón in the Province of Guadalajara in Castilla-La Mancha, Spain, for which it was named in 1797. Aragonite is found in this locality as cyclic twins inside gypsum and marls of the Keuper facies of the Triassic. This type of aragonite deposit is very common in Spain, and there are also some in France and Morocco. An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia. In the US, aragonite in the form of stalactites and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves. Massive deposits of oolitic aragonite sand are found on the seabed in the Bahamas. Aragonite is the high pressure polymorph of calcium carbonate. As such, it occurs in high pressure metamorphic rocks such as those formed at subduction zones. Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia). Several serpulids have aragonitic tubes. Because the mineral deposition in mollusk shells is strongly biologically controlled, some crystal forms are distinctively different from those of inorganic aragonite. In some mollusks, the entire shell is aragonite; in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite). The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite. Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothems, respectively. Aragonite is not uncommon in serpentinites where high Mg in pore solutions apparently inhibits calcite growth and promotes aragonite precipitation. Aragonite is metastable at the low pressures near the Earth's surface and is thus commonly replaced by calcite in fossils. Aragonite older than the Carboniferous is essentially unknown. It can also be synthesized by adding a calcium chloride solution to a sodium carbonate solution at temperatures above or in water-ethanol mixtures at ambient temperatures.

Physical properties

Aragonite is thermodynamically unstable at standard temperature and pressure, and tends to alter to calcite on scales of 107 to 108 years. The mineral vaterite, also known as μ-CaCO3, is another phase of calcium carbonate that is metastable at ambient conditions typical of Earth's surface, and decomposes even more readily than aragonite.


Uses


In aquaria, aragonite is considered essential for the replication of reef conditions. Aragonite provides the materials necessary for much sea life and also keeps the pH of the water close to its natural level, to prevent the dissolution of biogenic calcium carbonate. Aragonite has been successfully tested for the removal of pollutants like zinc, cobalt and lead from contaminated wastewaters.Köhler, S., Cubillas, et al. (2007) Removal of cadmium from wastewaters by aragonite shells and the influence of other divalent cations. Environmental Science and Technology, 41, 112-118. It is claimed by some water conditioner manufacturers that their technology converts calcite to aragonite in order to reduce limescale.{{citation needed|date=November 2020

Gallery

Aragonite 2 Enguidanos.jpg|Aragonite crystals from Cuenca, Castile-La Mancha, Spain Aragonite - Pantoja, Toledo, Castile-La Mancha, Spain.jpg|Aragonite crystal cluster from Spain Image:BaculitesSuturesAragonite.jpg|Remnant biogenic aragonite (thin, rainbow-colored shell) on the ammonite ''Baculites'' (Pierre Shale, Late Cretaceous, South Dakota) Aragonite layers in a blue mussel shell.jpg|Scanning electron microscope image of aragonite layers in the nacre of a blue mussel (''Mytilus edulis'') Aragonit_-_Fluorescence.gif|Fluorescence of aragonite

See also

* Aragonite sea * Ikaite, CaCO3·6H2O * Monohydrocalcite, CaCO3·H2O * Nacre, otherwise known as "Mother-of-Pearl" * Oolitic aragonite sand

References



External links


The Ochtinska Aragonite Cave in Slovakia Kosovo Caves Aragonite Formations
Category:Calcium minerals Category:Carbonate minerals Category:Cave minerals Category:Aragonite group Category:Orthorhombic minerals Category:Minerals in space group 62 Category:Luminescent minerals Category:Evaporite