calcite

{{Infobox mineral | name = Calcite | category = Carbonate mineral | boxwidth = | boxbgcolor = #d4d1cd | image = Calcite Variation.png | imagesize = | caption = Clockwise from top left: scalenohedral, rhomboedral, stalactitic, and botryoidal calcite | formula = CaCO3 | IMAsymbol = Cal | molweight = | strunz = 5.AB.05 | system = Trigonal | class = Hexagonal scalenohedral (m) H-M symbol: ( 2/m) | symmetry = Rc | unit cell = a = 4.9896(2) Å, c = 17.0610(11) Å; Z = 6 | color = Typically colorless or creamy white - may have shades of brownish colors | habit = Botryoidal, concretionary, druse, globular, granular, massive, rhombohedral, scalenohedral, stalactitic | twinning = Common by four twin laws | cleavage = Perfect on {101} three directions with angle of 74° 55' | fracture = Conchoidal | tenacity = Brittle | mohs = 3 (defining mineral) | luster = Vitreous to pearly on cleavage surfaces | refractive = nω = 1.640–1.660 nε = 1.486 | opticalprop = Uniaxial (−); low relief | birefringence = δ = 0.154–0.174 | pleochroism = | streak = White | gravity = 2.71 | density = | melt = | fusibility = Infusible (decrepitates energetically) | diagnostic = | solubility = Soluble in dilute acids | diaphaneity = Transparent to translucent | other = May fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent | references = }}

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.