Aragonite
Encyclopedia
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms
of calcium carbonate
, Ca
C
O
3 (the other form being the mineral
calcite
). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with acicular crystals. Repeated twinning
results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching stalactitic
forms called flos-ferri ("flowers of iron") from their association with the ore
s at the Carinthian
iron mines.
for aragonite is Molina de Aragón
(Guadalajara
, Spain
), 25 km from Aragon
for which it was named in 1797. An aragonite cave, the Ochtinská Aragonite Cave
, is situated in Slovakia
. In the USA, aragonite in the form of stalactite
s 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 forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water coral
s (Scleractinia
). 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). Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothem
s, respectively. The nacreous layer of the aragonite fossil
shells of some extinct ammonite
s forms an iridescent material called ammolite
. Ammolite is primarily aragonite with impurities that make it iridescent and valuable as a gemstone
.
Aragonite is metastable and is thus commonly replaced by calcite in fossils. Aragonite older than the Carboniferous
is essentially unknown.
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.
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
, Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
3 (the other form being the mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with acicular crystals. Repeated twinning
Crystal twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....
results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching stalactitic
Stalactite
A stalactite , "to drip", and meaning "that which drips") is a type of speleothem that hangs from the ceiling of limestone caves. It is a type of dripstone...
forms called flos-ferri ("flowers of iron") from their association with the ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
s at the Carinthian
Duchy of Carinthia
The Duchy of Carinthia was a duchy located in southern Austria and parts of northern Slovenia. It was separated from the Duchy of Bavaria in 976, then the first newly created Imperial State beside the original German stem duchies....
iron mines.
Occurrence
The type locationType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
for aragonite is Molina de Aragón
Molina de Aragón
Molina de Aragón is a municipality located in the province of Guadalajara, Castile-La Mancha, Spain. According to the 2009 census , the municipality has a population of 3,671 inhabitants...
(Guadalajara
Guadalajara (province)
Guadalajara is a province of central/north-central Spain, in the northern part of the autonomous community of Castile-La Mancha. It is bordered by the provinces of Cuenca, Madrid, Segovia, Soria, Zaragoza, and Teruel...
, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
), 25 km from Aragon
Aragon
Aragon is a modern autonomous community in Spain, coextensive with the medieval Kingdom of Aragon. Located in northeastern Spain, the Aragonese autonomous community comprises three provinces : Huesca, Zaragoza, and Teruel. Its capital is Zaragoza...
for which it was named in 1797. An aragonite cave, the Ochtinská Aragonite Cave
Ochtinská Aragonite Cave
Ochtinská Aragonite Cave is a unique aragonite cave situated in southern Slovakia, near Rožňava. Although only 300 m long, it is famous for its rare aragonite filling. There are only three aragonite caves discovered in the world so far. In the so-called Milky Way Hall, the main attraction of the...
, is situated in Slovakia
Slovakia
The Slovak Republic is a landlocked state in Central Europe. It has a population of over five million and an area of about . Slovakia is bordered by the Czech Republic and Austria to the west, Poland to the north, Ukraine to the east and Hungary to the south...
. In the USA, aragonite in the form of stalactite
Stalactite
A stalactite , "to drip", and meaning "that which drips") is a type of speleothem that hangs from the ceiling of limestone caves. It is a type of dripstone...
s and "cave flowers" (anthodite
Anthodite
Anthodites are speleothems composed of long needle-like crystals situated in clusters which radiate outward from a common base. The "needles" may be quill-like or feathery...
) is known from Carlsbad Caverns and other caves. Massive deposits of oolitic aragonite sand
Oolitic aragonite sand
Oolitic aragonite sand forms in tropical waters. Most of the topography of the Bahama Banks is composed of calcium carbonate oolitic aragonite sand material....
are found on the seabed in the Bahamas.
Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s (Scleractinia
Scleractinia
Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...
). 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). Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothem
Speleothem
A speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...
s, respectively. The nacreous layer of the aragonite fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
shells of some extinct ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s forms an iridescent material called ammolite
Ammolite
Ammolite is a rare and valuable opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral that makes up nacreous pearls...
. Ammolite is primarily aragonite with impurities that make it iridescent and valuable as a gemstone
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
.
Aragonite is metastable and is thus commonly replaced by calcite in fossils. Aragonite older than the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
is essentially unknown.
Physical properties
Aragonite is thermodynamicallyThermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
unstable at standard temperature and pressure, and tends to alter to calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
on scales of 107 to 108 years. The mineral vaterite
Vaterite
Vaterite is a mineral, a polymorph of calcium carbonate. It was named after the German mineralogist Heinrich Vater. It is also known as mu-calcium carbonate and has a JCPDS number of 13-192. Vaterite, like aragonite, is a metastable phase of calcium carbonate at ambient conditions at the surface...
, 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.
See also
- Aragonite seaAragonite seaAn aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic carbonate precipitates. Therefore, the chemical conditions of the seawater must be notably high in magnesium content for an aragonite sea to form...
- Ikaite, CaCO3·6H2OIkaiteIkaite is the mineral name for the hexahydrate of calcium carbonate, CaCO3·6H2O. It is only found in a metastable state, and decomposes rapidly once removed from near-freezing water. It is more commonly known through its pseudomorphs.-Distribution:...
- Monohydrocalcite, CaCO3·H2OMonohydrocalciteMonohydrocalcite is a mineral that is a hydrous form of calcium carbonate, CaCO3·H2O. It was formerly also known by the name hydrocalcite, which is now discredited by the IMA. It is a trigonal mineral which is white when pure...
- NacreNacreNacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....
, otherwise known as "Mother-of-Pearl" - Oolitic aragonite sandOolitic aragonite sandOolitic aragonite sand forms in tropical waters. Most of the topography of the Bahama Banks is composed of calcium carbonate oolitic aragonite sand material....