Aragonite

Aragonite
Aragonite.
General
Category	Carbonate mineral
Formula (repeating unit)	CaCO3
Crystal system	Orthorhombic with pseudo hexagonal aggregates
Identification
Color	White, red, yellow, orange, green , blue and brown
Twinning	Polysynthetic parallel to [100] cyclically on {110}
Cleavage	Distinct on [010]
Fracture	Subconchoidal
Mohs scale hardness	3.5-4
Luster	dull or vitreous
Streak	White
Diaphaneity	Translucent to transparent
Specific gravity	2.947
Optical properties	Biaxial (-)
Refractive index	nα = 1.529 - 1.530 nβ = 1.680 - 1.682 nγ = 1.685 - 1.686
Birefringence	δ = 0.156
References

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.

Occurrence

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, aragonite in the form of stalactites and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves. 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.

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 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.

Gallery

• 
  Aragonite crystals (~ 4 cm in size)
• 
  Crystal structure of aragonite. O is red, Ca blue and C black. Carbonate groups are black triangles.
• 
  Remnant biogenic aragonite (thin, rainbow-colored shell) on the ammonite Baculites (Pierre Shale, Late Cretaceous, South Dakota).

See also

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

References

• Webmineral data
• Mineral Data Publishing

External links

• The Ochtinska aragonite cave

• Kosovo Caves Aragonite Formations