Aragonite

Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (Template:Chem2), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

File:AragoniteFromCrystalMaker.png

Aragonite crystal structure

The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal.<ref>Template:Cite journal</ref> 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.<ref name=Sinkankas>Template:Cite book</ref>

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.<ref name="Cairncross2015">Template:Cite book</ref> Aragonite is found in this locality as cyclic twins inside gypsum and marls of the Keuper facies of the Triassic.<ref>Template:Cite book</ref> This type of aragonite deposit is very common in Spain, and there are also some in France.<ref name=Sinkankas/>

An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia.<ref>Template:Cite journal</ref>

In the US, aragonite in the form of stalactites and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves.<ref name="Gonzalez-Lohmann-1988">Template:Cite book</ref> For a few years in the early 1900s, aragonite was mined at Aragonite, Utah (now a ghost town).<ref name="Balaz2009">Template:Cite book</ref>

Massive deposits of oolitic aragonite sand are found on the seabed in the Bahamas.<ref>Template:Cite journal</ref>

File:P-T Diagram for CaCO3.svg

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.<ref name=Nesse>Template:Cite book</ref>

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.<ref name=Boggs>Template:Cite book</ref> Because the mineral deposition in mollusk shells is strongly biologically controlled,<ref name=BelcherEtAl>Template:Cite journal</ref> some crystal forms are distinctively different from those of inorganic aragonite.<ref>Template:Cite journal</ref> In some mollusks, the entire shell is aragonite;<ref>Template:Cite journal</ref> in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite).<ref name=BelcherEtAl/> The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite.<ref>Template:Cite journal</ref>

Aragonite also forms naturally in the endocarp of Celtis occidentalis.<ref>Template:Cite journalTemplate:Dead link</ref>

The skeleton of some calcareous sponges is made of aragonite.<ref name="Hooper_2002">Template:Cite book</ref><ref name="Bergquist_2001">Template:Cite book</ref>

Aragonite also forms in the ocean inorganic precipitates called marine cements (in the sediment) or as free crystals (in the water column).<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> Inorganic precipitation of aragonite in caves can occur in the form of speleothems.Template:Sfn Aragonite is common in serpentinites where magnesium-rich pore solutions apparently inhibit calcite growth and promote aragonite precipitation.<ref>Template:Cite journal</ref>

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.<ref>Template:Cite journal</ref>

Aragonite can be synthesized by adding a calcium chloride solution to a sodium carbonate solution at temperatures above Template:Convert or in water-ethanol mixtures at ambient temperatures.<ref>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. {{#invoke:doi|main}}.</ref>

Physical properties

Aragonite is a thermodynamically unstable phase of calcium carbonate at any pressure below about Template:Convert at any temperature.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> The presence of magnesium ions may inhibit calcite formation in favor of aragonite.<ref>Template:Cite journal</ref> Once formed, aragonite tends to alter to calcite on scales of 10⁷ to 10⁸ years.<ref name=BMM>Template:Cite book</ref>

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.<ref name="Ni_and_Ratner_2008 ">Template:Cite journal</ref><ref>Template:Cite journal</ref>

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.<ref>Orr, J. C., et al. (2005) Anthropogenic ocean acidification over the 21st century and its impact on calcifying organisms. Nature 437: 681–686</ref>

Aragonite has been successfully tested for the removal of pollutants like zinc, cobalt and lead from contaminated wastewaters.<ref>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. {{#invoke:doi|main}}</ref>

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

• 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
• List of minerals
• Monohydrocalcite, CaCO₃·H₂O
• Nacre, otherwise known as "Mother-of-Pearl"

External links

• The Ochtinska Aragonite Cave in Slovakia
• Kosovo Caves Aragonite Formations