It is sometimes stated that CaCO3 precipitation in seawater can occur either following
(1) Ca2+ + Co32- -> CaCO3
(2) Ca2+ + HCO32- -> CaCO3 + H+
In an abiotic system, in which CaCO3 precipitation only follows thermodynamics, why would CaCO3 precipitation follow equation 2?
It is sometime stated that this would occur as DIC at a the pH range of seawater is mainly composed of HCO32-. But the CaCO3 lattice is composed of Ca2+ and carbonate, not bicarbonate ions, so ultimately the precipitation reaction will again be equation 1