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Aragonite is a
carbonate mineral Carbonate minerals are those minerals containing the carbonate ion, CO32−. Carbonate divisions Anhydrous carbonates *Calcite group: trigonal **Calcite CaCO3 **Gaspeite (Ni,Mg,Fe2+)CO3 **Magnesite MgCO3 **Otavite CdCO3 **Rhodochrosite MnCO3 **S ...
, one of the three most common naturally occurring crystal forms of
calcium carbonate Calcium carbonate is a chemical compound with the formula Ca CO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone Limestone is a common type of carbonate rock, carbonate sediment ...

calcium carbonate
, (the other forms being the
mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...

mineral
s
calcite Image:Calcite.png, 235px, Crystal structure of calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on Scr ...

calcite
and vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments. The
crystal lattice A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosco ...
of aragonite differs from that of calcite, resulting in a different crystal shape, an
orthorhombic crystal systemIn crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek words ''crystallon'' "cold drop, frozen dr ...
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
Carinthian
iron mines.


Occurrence

The type location for aragonite is Molina de Aragón in the
Province of Guadalajara Guadalajara (; from Arabic '' wādi al-ħajāra'', "streambed/valley of stones") is a province A province is almost always an administrative division within a country or state. The term derives from the ancient Roman '' provincia'', which w ...
in Castilla-La Mancha,
Spain , * gl, Reino de España, * oc, Reiaume d'Espanha, , , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = , national_anthem = , image_map = , map_caption = , image_ ...
, for which it was named in 1797. Aragonite is found in this locality as cyclic twins inside gypsum and marls of the
Keuper The Keuper is a lithostratigraphy, lithostratigraphic unit (a sequence of rock strata) in the Subsurface (geology), subsurface of large parts of west and central Europe. The Keuper consists of dolomite (rock), dolomite, shales or claystones and evap ...
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, subduction zones. Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia). Several Serpulidae, 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 thermodynamics, thermodynamically unstable at standard temperature and pressure, and tends to alter to
calcite Image:Calcite.png, 235px, Crystal structure of calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on Scr ...

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 Aquarium, 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 (chemistry), dissolution of biogenic
calcium carbonate Calcium carbonate is a chemical compound with the formula Ca CO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone Limestone is a common type of carbonate rock, carbonate sediment ...

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, Ikaite, CaCO3·6H2O * Monohydrocalcite, 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
Calcium minerals Carbonate minerals Cave minerals Aragonite group Orthorhombic minerals Minerals in space group 62 Luminescent minerals Evaporite