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Carbonate–silicate cycle


The carbonate–silicate geochemical cycle describes the transformation of silicate rocks to carbonate rocks by weathering and sedimentation at Earth's surface and the transformation of carbonate rocks back into silicates by metamorphism and magmatism. It plays a large part in the carbon cycle, since the equilibrium point of the carbonate-silicate cycle dictates the pace of carbon release from the lithosphere.

The carbonate-silicate cycle involves several chemical reactions that occur in different environments. In the atmosphere, gaseous carbon dioxide (CO2) dissolves in rainwater, forming natural carbonic acid (H2CO3). This weak acid weathers silicate rocks on continents, slowly dissolving the rock and releasing aqueous minerals through the chemical reaction CaSiO3(s) (wollastonite) + 2CO2(g) + H2O(l) → Ca2+
(aq)
+ 2HCO
3
(aq) (bicarbonate) + SiO2(aq) (dissolved silica). These dissolved minerals are eventually carried by water to the ocean, where they are used by living organisms such as foraminifera, radiolarians, coccolithopores, and diatoms to create shells of CaCO3 (calcite) or SiO2 (opal) through the reactions Ca2+(aq) + 2HCO3
(aq)
→ CaCO3(s) + CO2(g) + H2O(l) (for calcite precipitation) and SiO2(aq) → SiO2(s) (for opal precipitation). When these organisms die, many shells are remineralized but some shells fall all the way to the sea floor and are buried. The cycle is completed when the sea floor is subducted and carbonate minerals recombine with silicate minerals under temperatures above 300 °C to reform calcium silicates and release gaseous CO2 through volcanism (CaCO3(s) + SiO2(s) → CaSiO3(s) + CO2(g)).


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