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Aragonite
Chemical Formula	CaCO3
Species	Carbonates
Crystal System	Orthorhombic
Mohs Scale	3-4
Specific Gravity	2.95
Color	White, red, yellow, orange, green , blue and brown
Streak	White
Luster	Vitreous, Resinous
Refractive Index	n = 1.529 - 1.530 n = 1.680 - 1.682 n = 1.685 - 1.686
Diaphaneity	Transparent, Translucent
Cleavage	Distinct/GoodOn distinct; On and very indistinct.
Fracture	Sub-Conchoidal
Crystal Habit：Pseudohexagonal, prismatic crystals, acicular, columnar, globular, reniform, pisolitic, coralloidal, stalactitic, internally banded
Geological Setting：As speleothemes in limestone caves; as pisolites, sinters and massive lamellar deposits at geysers and hot springs; as seafloor oolites; with siderite in iron deposits; with calcite and dolomite and other magnesium minerals in altered serpentinites, dunites and peridotites; and as a replacement mineral in various rock types and ore deposits, formed from low-temperature and pressure aqueous solutions.

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO₃ (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 ores at the Carinthian iron mines.

Occurrence

The type location 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 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 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 metastable and is thus commonly replaced by calcite in fossils. Aragonite older than the Carboniferous is essentially unknown.

Physical properties

Aragonite is thermodynamically 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.

Uses

In aquaria, aragonite is considered essential for the replication of reef conditions in aquariums.^{[citation needed]} It not only is the material that the sea life is evolved to use and live around, but also keeps the tank's pH close to its natural level.

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