We detail the results of an experimental study on the kinetics of Fe isotope exchange between aqueous Fe(II)_aq and nanoparticulate mackinawite (FeS_m) at 25°C and 2°C over a one month period. The rate of isotopic exchange decreases synchronously with the growth of FeS_m nanoparticles. 100% isotopic exchange between bulk FeS_m and the solution is never reached and the extent of isotope exchange asymptotes to a maximum of ~75%. We demonstrate that particle growth driven by Ostwald ripening would produce much faster isotopic exchange than observed and would be limited by the extent of dissolution-recrystallisation. We show that Fe isotope exchange kinetics are consistent with i) FeS_m nanoparticles that have a core-shell structure, in which Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation-growth mechanism. We argue that because of the structure of FeS_m nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. FeS_m is a reactive component in diagenetic pyrite forming systems since FeS_m dissolves and reacts to form pyrite. Isotopic mobility and potential equilibration between FeS_m and Fe(II)_aq thus have direct implications for the ultimate Fe isotope signature recorded in sedimentary pyrite.