Magnesite (MgCO3) precipitation within the nanoconfined space of adsorbed H2O films (∼5 monolayers) was determined to have an apparent activation energy of only 36 ± 6 kJ mol-1, suggesting that Mg2+ under nanoconfinement adopts a hydration configuration that mimics that of aqueous Ca2+, at least energetically, if not also specifically in hydration structure.