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Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biophysics processes, including precipitation from marine ecosystem 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 crystal twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching helictite forms called flos-ferri ("flowers of iron") from their association with the at the Carinthian iron mines. (1964). 9780442276249, Van Nostrand. ISBN 9780442276249
An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia.
In the US, aragonite in the form of and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves. (1988). 9781461237488, Springer-Verlag. ISBN 9781461237488 For a few years in the early 1900s, aragonite was mined at Aragonite, Utah (now a ghost town).
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 such as those formed at subduction. (2025). 9780195106916, Oxford University Press. ISBN 9780195106916
Aragonite forms naturally in almost all mollusk shells, and as the Calcareous of warm- and cold-water (Scleractinia). Several Serpulidae have aragonitic tubes.
Aragonite also forms naturally in the endocarp of Celtis occidentalis.
The skeleton of some calcareous sponges is made of aragonite.
Aragonite also forms in the ocean inorganic precipitates called marine cements (in the sediment) or as free crystals (in the water column).
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.
Aragonite can be synthesized by adding a calcium chloride solution to a sodium carbonate solution at temperatures above or in water-ethanol mixtures at ambient temperatures.Sand, K.K., Rodriguez-Blanco, J.D., Makovicky, E., Benning, L.G. and Stipp, S. (2012) Crystallization of CaCO3 in water-ethanol mixtures: spherulitic growth, polymorph stabilization and morphology change. Crystal Growth & Design, 12, 842–853. .
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.
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.
(2025). 9780131547285, Pearson Prentice Hall. ISBN 9780131547285 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 forms an iridescent material called ammolite.
(2025). 9780306472602, Kluwer Academic/Plenum. ISBN 9780306472602
(1990). 9781444314175, Blackwell Scientific Publications. ISBN 9781444314175
Inorganic precipitation of aragonite in caves can occur in the form of . Aragonite is common in serpentinites where magnesium-rich pore solutions apparently inhibit calcite growth and promote aragonite precipitation.
Physical properties
Aragonite is a thermodynamics unstable phase of calcium carbonate at any pressure below about at any temperature. Aragonite nonetheless frequently forms in near-surface environments at ambient temperatures. The weak Van der Waals forces inside aragonite give an important contribution to both the crystallographic and elastic properties of this mineral. The difference in stability between aragonite and calcite, as measured by the Gibbs free energy of formation, is small, and effects of grain size and impurities can be important. The formation of aragonite at temperatures and pressures where calcite should be the stable polymorph may be an example of Ostwald's step rule, where a less stable phase is the first to form. The presence of magnesium ions may inhibit calcite formation in favor of aragonite. Once formed, aragonite tends to alter to calcite on scales of 107 to 108 years. (1980). 9780136427100, Prentice-Hall. ISBN 9780136427100
Uses
In Aquarium, 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.Orr, J. C., et al. (2005)
Anthropogenic ocean acidification over the 21st century and its impact on calcifying organisms. Nature 437: 681–686
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