The kinetics of Fe(II)-carbonate precipitation
Keywords: Fe(II)-carbonate precipitation
Palaeoenvironmental reconstruction
Mineral-water interaction
Siderite
Kinetic isotope fractionation

This thesis investigates the processes, products, and rates of Fe(II)-carbonate precipitation using two experimental approaches. The first was to constrain the requisite critical supersaturation for homogeneous/spontaneous nucleation directly from solution, and to understand the mineralisation pathway of Fe(II)-carbonate once nucleated. The second set of experiments was designated to correlate the kinetic behaviour of siderite growth and solution chemistry, and to implement established rate laws to better understand the microscopic mechanisms that underpin crystal growth. This knowledge, in turn, allowed a further investigation into kinetic fractionation of carbon isotopes, enquiring how the isotopic composition reflects the kinetics of siderite growth and the chemistry of the parent solution. Nucleation experiments revealed that a precursor amorphous Fe(II)-carbonate (AFC) phase played a critical role in the homogeneous precipitation of Fe(II)-carbonate. Results indicate that, once the requisite critical supersaturation is surpassed, pervasive nucleation AFC rapidly decreases the solution saturation, and the nucleation rate behaves as an exponential function of solution saturation. The nucleated AFC subsequently crystallises into chukanovite (Fe2(OH)2CO3) or siderite (FeCO3) in days, depending on the availability and the ratio between Fe2+ and CO32- ions; a mineralisation pathway that is more intricate than previously appreciated. Growth experiments yielded a complete data set of the kinetic behaviour during siderite growth from near-nucleation (with respect to AFC) to near-equilibrium (with respect to siderite). These results delineate a chemical affinity-based kinetic model that predicts growth rate limited by the transport of dissolved species to the mineral surface at high solution saturation, but limited by the rate of surface reactions when the system approaches equilibrium. These observations are analogous to calcite precipitation studies, only that, siderite grows nearly 7 orders of magnitude slower than calcite at room temperature. The comparable growth mechanisms but contrasting growth rates between calcite and siderite can be attributed to the physiochemical difference between hydrated Ca2+ and Fe2+ ions. Due to the smaller ionic radius of Fe2+ (0.078 nm) than that of Ca2+ (0.1 nm), a higher activation energy is required to dehydrate the more densely charged Fe2+ ion on the crystal surface to form anhydrous FeCO3, thus siderite precipitate significantly slower than calcite and often involve the formation of hydrous/hydroxylated metastable phases. Isotopic analyses of both the solution and the precipitate indicate that siderite precipitated from supersaturated solution exhibit significant discrimination against 13C. Though information is still lacking, the observed isotopic compositions in precipitated siderite are satisfactorily explained with a kinetic fractionation model, which indicates the maximum kinetic isotope effect of C in abiotic siderite growth can be as much as 103 lnαsid-DIC = -12.13 ‰. On the basis of the kinetic constraint on siderite growth, experimental results presented in this thesis suggest that significant accumulations of abiotic siderite in the sediments require a considerable supersaturation to achieve an appreciable precipitation rate within the time frame of early diagenesis; inevitably, displays a negative δ13C signature. Geochemical constraints arising from this thesis prompt a re-evaluation on the palaeoenvironmental reconstruction based on ferruginous sediments, notably, late Archaean iron formation. Incorporating recent advances in high-resolution petrography, experimental geochemistry, and modelling studies of Fe-minerals, this study presents an updated understanding of the Fe cycle on early Earth, in which the deposition of ferruginous sediments was a direct response to enhanced Fe(II) influxes (e.g., hydrothermal venting), where Fe(II)-silicate nucleation was the dominant mechanism that attenuated hydrothermal Fe(II) fluxes to seawater, and Fe(II)-carbonate precipitation served as the ultimate sink of a comparatively small seawater Fe(II) reservoir.