Calcium carbonate

Calcium carbonate is the mineral skeleton of the macroscopic living world. It forms mollusc shells, the exoskeletons of phytoplanktonic coccoliths (covering ~30 % of fixed ocean carbon), foraminifera tests, scleractinian corals and fossilised bones. At large scale, these biological accumulations compact into sedimentary limestones — about 4 % of the Earth's crust by volume — from which we have cut, for 5,000 years, building stones, the marble of Greek sculptures (Penteli, Carrara), the chalk of the Alabaster Coast cliffs, and the rural drystone walls of Western Europe.

Industrially, CaCO₃ is consumed at about 200 Mt/yr. Its largest application is quicklime CaO production by calcination at 900 °C: CaCO₃ → CaO + CO₂. This reaction emits ~440 kg of CO₂ per tonne of lime produced — intrinsic carbonate-chemistry emissions, independent of the energy source. The cement industry (Portland process), which downstream converts CaO to calcium silicates, alone represents ~7-8 % of global CO₂ emissions, making it the second industrial emitter after steel. Decarbonisation pathways consider alternative cements (geopolymers, sulfo-aluminates), CO₂ capture-storage from kilning, or partial substitution by fly ash and slag.

In contemporary atmospheric-ocean chemistry, CaCO₃ plays a central role in the carbon balance: accelerated dissolution of calcareous shells by ocean acidification (surface pH dropped from 8.2 to 8.1 since 1850) threatens marine organisms fixing CaCO₃ and represents the main feedback loop between the carbon cycle and marine chemistry. Pteropods and coccoliths are already observed in calcification stress in polar zones.