Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses.
| Calcite | |
|---|---|
Clockwise from top left: scalenohedral, rhomboedral, stalactitic, and botryoidal calcite | |
| General | |
| Category | Carbonate mineral |
| Formula (repeating unit) | CaCO3 |
| Strunz classification | 5.AB.05 |
| Crystal system | Trigonal |
| Crystal class | Hexagonal scalenohedral (3m) H-M symbol: (3 2/m) |
| Space group | R3c |
| Unit cell | a = 4.9896(2) Å, c = 17.0610(11) Å; Z = 6 |
| Identification | |
| Color | Typically colorless or creamy white - may have shades of brownish colors |
| Crystal habit | Botryoidal, concretionary, druse, globular, granular, massive, rhombohedral, scalenohedral, stalactitic |
| Twinning | Common by four twin laws |
| Cleavage | Perfect on {1011} three directions with angle of 74° 55' |
| Fracture | Conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 3 (defining mineral) |
| Luster | Vitreous to pearly on cleavage surfaces |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.71 |
| Optical properties | Uniaxial (−); low relief |
| Refractive index | nω = 1.640–1.660 nε = 1.486 |
| Birefringence | δ = 0.154–0.174 |
| Fusibility | Infusible (decrepitates energetically) |
| Solubility | Soluble in dilute acids |
| Other characteristics | May fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent |
| References | |
Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable.
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