Aragonite

Aragonite

Aragonite is a carbonate mineral, a polymorph of the mineral calcite, both having the chemical composition CaCO₃. Its structure differs from calcite and leads to a different crystal shape, an orthorhombic system with acicular crystals. By repeated twinning pseudo-hexagonal forms result. It 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. A whole aragonite cave (the Ochtiná Aragonite Cave) is situated in Slovakia. In the USA, stalactitic aragonite is known from Carlsbad Caverns.

Aragonite crystal from Aragon, Spain.

Aragonite forms naturally in almost all mollusk shells, as well as in the ocean and in caves as inorganic precipitates called marine cements and speleothema, 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 revert 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.

See also

• list of minerals
• Nacre - otherwise known as "Mother-of-Pearl"
• Pearl
• Ocean acidification
• Ochtiná Aragonite Cave

External links

• Aragonite information
• The Ochtinska aragonite cave

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