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
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
and
vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
The
crystal lattice
In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by
: \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n ...
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
Carinthia (german: Kärnten ; sl, Koroška ) is the southernmost Austrian state, in the Eastern Alps, and is noted for its mountains and lakes. The main language is German. Its regional dialects belong to the Southern Bavarian group. Carint ...
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
The Keuper is a lithostratigraphic unit (a sequence of rock strata) in the subsurface of large parts of west and central Europe. The Keuper consists of dolomite, shales or claystones and evaporites that were deposited during the Middle and Late ...
facies of the
Triassic. This type of aragonite deposit is very common in Spain, and there are also some in France.
[
An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia.
In the US, aragonite in the form of ]stalactite
A stalactite (, ; from the Greek 'stalaktos' ('dripping') via
''stalassein'' ('to drip') is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble an ...
s and "cave flowers" ( anthodite) is known from Carlsbad Caverns and other caves. For a few years in the early 1900s, aragonite was mined at Aragonite, Utah (now a ghost town).
Massive deposits of oolitic aragonite sand
Oolitic aragonite sand is composed of the calcium carbonate mineral, aragonite, with an egg-like shape (" oolitic" from the Ancient Greek word ᾠόν for "egg") and sand grain size. This sand type forms in tropical waters through precipitation, s ...
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
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
.
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 ]ammonite
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...
s forms an iridescent
Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
material called ammolite.
Aragonite also forms naturally in the endocarp of ''Celtis occidentalis
''Celtis occidentalis'', commonly known as the common hackberry, is a large deciduous tree native to North America. It is also known as the nettletree, sugarberry, beaverwood, northern hackberry, and American hackberry. It is a moderately long-l ...
''.
Aragonite also forms in the ocean inorganic precipitates called marine cements (in the sediment) or as free crystals (in the water column).
Inorganic precipitation of aragonite in caves can occur in the form of speleothems. Aragonite is common in serpentinites where magnesium-rich pore solutions apparently inhibit calcite growth and promote 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
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
is essentially unknown. It can also be synthesized by adding a calcium chloride solution to a sodium carbonate
Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...
solution at temperatures above or in water-ethanol mixtures at ambient temperatures.
Physical properties
Aragonite is not the thermodynamically stable phase of calcium carbonate at any pressure below about at any temperature. Aragonite nonetheless frequently forms in near-surface environments at ambient temperatures. The difference in stability between aragonite and calcite, as measured by the Gibbs free energy of formation
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...
, 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
In materials science, Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald, describes the formation of polymorphs. The rule states that usually the less stable polymorph crystallizes first. Unstable polymorphs more closely resemble ...
, 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
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
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
A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...
calcium carbonate.
Aragonite has been successfully tested for the removal of pollutants like zinc, cobalt and lead from contaminated wastewaters.
Claims that magnetic water treatment
Magnetic water treatment (also known as anti-scale magnetic treatment or AMT) is a method of supposedly reducing the effects of hard water by passing it through a magnetic field as a non-chemical alternative to water softening. Magnetic water trea ...
can reduce scaling
Scaling may refer to:
Science and technology
Mathematics and physics
* Scaling (geometry), a linear transformation that enlarges or diminishes objects
* Scale invariance, a feature of objects or laws that do not change if scales of length, energ ...
, by converting calcite to aragonite, have been met with skepticism,[
] but continue to be investigated.
Gallery
Aragonite crystal - Los Molinillos, Ceunca, Spain - 4x3.6x3.5cm 100g.jpg, Aragonite crystal from Los Molinillos, Ceunca, Spain
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
The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from Pembina Valley in Canada to New Mexico.
The Pierre Shale was described by Meek and Hayden in 1862 in th ...
, Late Cretaceous, South Dakota)
Aragonite layers in a blue mussel shell.jpg, Scanning electron microscope image of aragonite layers in the nacre
Nacre ( , ), also known as mother of pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent.
Nacre is f ...
of a blue mussel (''Mytilus edulis'')
Aragonit_-_Fluorescence.gif, Fluorescence of aragonite
See also
* Aragonite sea
An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The chemical conditions of the seawater must be notably high in magnesium content relative to calcium (high Mg/Ca ratio) for ...
* Ikaite, CaCO3·6H2O
* Monohydrocalcite, CaCO3·H2O
* Nacre
Nacre ( , ), also known as mother of pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent.
Nacre is f ...
, otherwise known as "Mother-of-Pearl"
References
External links
The Ochtinska Aragonite Cave in Slovakia
Kosovo Caves Aragonite Formations
{{Authority control
Calcium minerals
Carbonate minerals
Cave minerals
Aragonite group
Orthorhombic minerals
Minerals in space group 62
Luminescent minerals
Evaporite