Vaterite

Vaterite
Vaterite from San Vito quarry, San Vito, Monte Somma, Somma-Vesuvius Complex, Italy
General
Category	Carbonate minerals
Formula (repeating unit)	CaCO3
Strunz classification	5.AB.20
Crystal system	Hexagonal
Crystal class	Dihexagonal dipyramidal (6mmm) H-M symbol: (6/m 2/m 2/m)
Space group	P63/mmc {P63/m 2/m 2/c}
Unit cell	a = 4.13, c = 8.49 [Å]; Z = 6
Identification
Color	Colorless
Crystal habit	Fine fibrous crystals, typically less than 0.1 mm, in spherulitic aggregates.
Fracture	Irregular to uneven, splintery
Tenacity	Brittle
Mohs scale hardness	3
Luster	Sub-vitreous, waxy
Diaphaneity	Transparent to semi-transparent
Specific gravity	2.54
Optical properties	Uniaxial (+)
Refractive index	nω = 1.550 nε = 1.650
Birefringence	δ = 0.100
References

Vaterite (CaCO₃) is a mineral, a polymorph of calcium carbonate. It was named after the German mineralogist Heinrich Vater. It is also known as mu-calcium carbonate (μ-CaCO₃) and has a JCPDS number of 13-192. Vaterite, like aragonite, is a metastable phase of calcium carbonate at ambient conditions at the surface of the earth. As it is less stable than either calcite or aragonite, vaterite has a higher solubility than either of these phases. Therefore, once vaterite is exposed to water, it converts to calcite (at low temperature) or aragonite (at high temperature: ~60 °C). However, vaterite does occur naturally in mineral springs, organic tissue, gallstones, and urinary calculi. In those circumstances, some impurities (metal ions or organic matter) may stabilize the vaterite and prevent its transformation into calcite or aragonite. Vaterite is usually colorless, its shape is spherical, and its diameter is small, ranging from 0.05 to 5 μm.

Vaterite can be produced as the first mineral deposits repairing natural or experimentally-induced shell damage in some aragonite-shelled mollusks (e.g. gastropods). Subsequent shell deposition occurs as aragonite.

Vaterite belongs to the hexagonal crystal system, whereas calcite is trigonal and aragonite is orthorhombic.