Aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring polymorphs of calcium carbonate, CaCO3. The other polymorph is the mineral calcite. 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 ores at the Carthinian iron mines. The type location for aragonite is Molina de Aragón (Guadalajara, Spain), 25 km outside Aragon. An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia. In the USA, stalactitic aragonite is known from Carlsbad Caverns. Massive deposits of oolitic aragonite 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 corals (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 speleothems, respectively. The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite. Ammolite is primarily aragonite with impurities that make it iridescent and valuable as a gemstone.

Aragonite is thermodynamically unstable at standard temperature and pressure, and tends to alter to calcite on scales of 107 to 108 years. The young age of the California blueschists has been famously demonstrated by the finding therein of aragonite not yet reverted to calcite. 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.

ee also

* list of minerals
* Monohydrocalcite, CaCO3·H2O
* Ikaite, CaCO3·6H2O
* Nacre, otherwise known as "Mother-of-Pearl"
* Pearl
* Ocean acidification
* Ochtinská Aragonite Cave

References

* [http://webmineral.com/data/Aragonite.shtml Webmineral data]
* [http://rruff.geo.arizona.edu/doclib/hom/aragonite.pdf Mineral Data Publishing]
* [http://www.mindat.org/min-307.html Mindat]

External links

* [http://www.ssj.sk/jaskyne/spristupnene/ochtinska-aragonitova/ The Ochtinska aragonite cave]


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  • aragonite — [ aragɔnit ] n. f. • 1834; arragonite 1802; de Aragon, province d Espagne ♦ Minér. Variété cristalline de carbonate de calcium qui peut former des stalactites. ● aragonite nom féminin Variété de carbonate de calcium CaCO3, orthorhombique, qui… …   Encyclopédie Universelle

  • Aragonite — A*rag o*nite, n. [From Aragon, in Spain.] (Min.) A mineral identical in composition with calcite or carbonate of lime, but differing from it in its crystalline form and some of its physical characters. [1913 Webster] || …   The Collaborative International Dictionary of English

  • aragonite — s. f. [Mineralogia] Carbonato de cálcio cristalizável.   ♦ Grafia no Brasil: aragonita …   Dicionário da Língua Portuguesa

  • aragonite — [ə rag′ə nīt΄, ar′ə gənīt΄] n. [after ARAGON2, in Spain] a semihard, orthorhombic mineral resembling calcite, made up of calcium carbonate, CaCO3: it is found in sedimentary and metamorphic rocks and in the skeletons of marine organisms …   English World dictionary

  • Aragonite — Pour les articles homonymes, voir CaCO3. Aragonite[1] Catégorie V : carbonates et nitrates[2] …   Wikipédia en Français

  • aragonite — /euh rag euh nuyt , ar euh geuh /, n. a mineral, orthorhombic calcium carbonate, CaCO3, chemically identical with calcite but differing in crystallization and in having a higher specific gravity and less marked cleavage. [1795 1805; named after… …   Universalium

  • aragonite —    1. A relatively rare form of calcium carbonate (CaCO3), chemically identical to the more common calcite but of orthorhombic crystal form. Its pure form is metastable in the cave environment, where calcite forms preferentially. It is relatively …   Lexicon of Cave and Karst Terminology

  • aragonite — aragonitas statusas T sritis chemija apibrėžtis Mineralas. formulė CaCO₃ atitikmenys: angl. aragonite rus. арагонит …   Chemijos terminų aiškinamasis žodynas

  • Aragonite sea — An aragonite sea contains aragonite and high magnesium calcite as the primary inorganic carbonate precipitates. In contrast, a calcite sea is one in which low magnesium calcite is the primary inorganic marine calcium carbonate precipitate.The… …   Wikipedia

  • aragonite — noun Etymology: German Aragonit, from Aragon, Spain Date: 1803 a mineral similar to calcite in consisting of calcium carbonate but differing from calcite in its orthorhombic crystallization, greater density, and less distinct cleavage •… …   New Collegiate Dictionary

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