[Uwe]

It is sometimes stated that CaCO3 precipitation in seawater can occur either following

(1) Ca2+ + Co32- -> CaCO3


or

(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

[Borek]

Think about it this way: first equation refers to the overall process consisting of a dissociation followed by the precipitation.