Calcite is the most stable calcium carbonate polymorph at Earth’s surface.
Calcite and aragonite have the same chemical formula, CaCO₃, but different crystal structures. Calcite is stable across ordinary surface conditions, whereas aragonite is generally favored at higher pressures and can persist at the surface as a metastable mineral. Vaterite is another rarer polymorph.
Calcite is the principal mineral in many limestones and marbles. It also forms in caves, hydrothermal veins, shells, and marine sediments. Its crystals commonly show distinctive rhombohedral cleavage, and calcite can effervesce when dilute hydrochloric acid is applied.
The relationship between calcite and aragonite is a frequent source of confusion. Some organisms build shells from aragonite, while others use calcite; seawater chemistry can influence which form is biologically favored. Dolomite also contains calcium carbonate, but its formula includes magnesium and it is not a polymorph of calcite.