Aragonite

Aragonite
Aragonite from Los Molinillos, Cuenca, Spain, sample width about 4 cm
General
Category	Carbonate minerals
Formula (repeating unit)	CaCO3
IMA symbol	Arg[1]
Crystal system	Orthorhombic
Unit cell	l a = 4.9598(5) Å, b = 7.9641(9) Å, and c = 5.7379(6) Å at 25 °C[2]
Identification
Color	Can come in a variety of colors, but commonly red or white
Crystal habit	Commonly dendritic or pseudo-hexagonal; can also be acicular, tabular, prismatic, coral-like
Twinning	Cyclic on {110}, forms pseudohexagonal aggregates. If polysynthetic, forms fine striations parallel to [110].
Cleavage	Good on [110], Poor on {110}.
Fracture	Subconchoidal
Tenacity	Very brittle
Mohs scale hardness	3.5–4
Luster	Vitreous, waxy, resinous
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	2.94
Optical properties	Biaxial (−)
Refractive index	nω = 1.550 nε = 1.650
Birefringence	δ = 0.155
2V angle	Measured 18–19°
Dispersion	Weak
Extinction	Parallel
Ultraviolet fluorescence	Faint white-blue to blue-violet
Solubility	Soluble in acids, and saltwater (but takes longer)
Common impurities	Commonly strontium, zirconium, lead
Other characteristics	Thermodynamically unstable, Morphs slowly back into calcite
References	[3][4]

Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (CaCO₃), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal.^[5] Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching helictitic forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.^[6]