The inorganic cycle begins with the production of carbonic acid (HCO) from rainwater and gaseous carbon dioxide. Due to this process, normal rain has a pH of around 5.6. Carbonic acid is a weak acid, but over long timescales, it can dissolve silicate rocks (as well as carbonate rocks). Most of the Earth's crust (and mantle) is composed of silicates. These substances break down into dissolved ions as a result. For example, calcium silicate (CaSiO), or wollastonite, reacts with carbon dioxide and water to yield a calcium ion, Ca, a bicarbonate ion, HCO, and dissolved silica. This reaction structure is representative of general silicate weathering of calcium silicate minerals. The chemical pathway is as follows:
River runoff carries these products to the ocean, where marine calcifying organisms use Ca and HCO to build their shells and skeletons, a process called carbonate precipitation:
Two molecules of CO are required for silicate rock weathering; marine calcification releases one molecule back to the atmosphere. The calcium carbonate (CaCO) contained in shells and skeletons sinks after the marine organism dies and is deposited on the ocean floor.
The final stage of the process involves the movement of the seafloor. At subduction zones, the carbonate sediments are buried and forced back into the mantle. Some carbonate may be carried deep into the mantle where high pressure and temperature conditions allow it to combine metamorphically with SiO to form CaSiO and CO, which is released from the interior into the atmosphere via volcanism, thermal vents in the ocean, or soda springs, which are natural springs that contain carbon dioxide gas or soda water:
This final step returns the second CO molecule to the atmosphere and closes the inorganic carbon budget. 99.6% of all carbon on Earth (equating to roughly 10 billion tons of carbon) is sequestered in the longterm rock reservoir. And essentially all carbon has spent time in the form of carbonate. By contrast, only 0.002% of carbon exists in the biosphere.