Aragonite

Aragonite
Aragonite from Salsigne mine, Salsigne, Aude, France - Size: 30x30x20 cm
General
Category	Carbonate mineral
Formula (repeating unit)	CaCO3
Strunz classification	5.AB.15
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pmcn
Unit cell	a = 4.95, b = 7.96 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 most 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 crystal system with acicular crystal. 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.

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 US, 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.