Aragonite

Aragonite
Aragonite from Salsignes Mine, Aude department, France Size: 30x30x20 cm
General
Category	Carbonate mineral
Formula (repeating unit)	CaCO3
Strunz classification	05.AB.15
Crystal system	Orthorhombic
Space group	Orthorhombic (2/m 2/m 2/m) - dipyramidal
Unit cell	a = 4.95 Å, b = 7.96 Å, c = 5.74 Å; Z = 4
Identification
Color	White, red, yellow, orange, green, purple, grey, blue and brown
Crystal habit	Pseudohexagonal, prismatic crystals, acicular, columnar, globular, reniform, pisolitic, coralloidal, stalactitic, internally banded
Twinning	Polysynthetic parallel to {100} cyclically on {110}
Cleavage	Distinct on {010}, imperfect {110} and {011}
Fracture	Subconchoidal
Tenacity	Brittle
Mohs scale hardness	3.5-4
Luster	Vitreous, resinous on fracture surfaces
Streak	White
Diaphaneity	Translucent to transparent
Specific gravity	2.95
Optical properties	Biaxial (-)
Refractive index	nα = 1.529 - 1.530 nβ = 1.680 - 1.682 nγ = 1.685 - 1.686
Birefringence	δ = 0.156
2V angle	18°
Solubility	Dilute acid
Other characteristics	Fluorescence: pale rose, yellow, white or bluish; phosphorescence: greenish or white (LW UV); yellowish (SW UV)
References

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). 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). 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. 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.

Gallery

• 
  Aragonite crystals from Cuenca, Castile-La Mancha, Spain
• 
  Cluster of twinned aragonite from Morocco
• 
  Remnant biogenic aragonite (thin, rainbow-colored shell) on the ammonite Baculites (Pierre Shale, Late Cretaceous, South Dakota).
• 
  Scanning electron microscope image of aragonite layers in the nacre of a blue mussel (Mytilus edulis).
• 
  Pink Aragonite Crystals
• 
  Fluorescence of Aragonite]]

See also

• Aragonite sea
• Ikaite, CaCO₃·6H₂O
• Monohydrocalcite, CaCO₃·H₂O
• Nacre, otherwise known as "Mother-of-Pearl"
• Oolitic aragonite sand

External links

• The Ochtinska aragonite cave
• Kosovo Caves Aragonite Formations