Mineral formation and sorption mechanisms in marine ferromanganese-rich sediments

The phyllomanganate birnessite is the dominant Mn-bearing phase in oxic marine sediments and through coupled sorption and redox reactions exerts a strong control on the oceanic concentrations of micronutrient trace metals. However, during oxic diagenesis and under mild hydrothermal conditions, birnessite undergoes transformation to the tectomanganate todorokite. The mechanistic details of the transformation are important for the speciation and mobility of metals sequestered by birnessite, and are necessary in order to quantify the role of marine sediments in global trace element cycles.
This study provides new insight into the crystallization pathway and mechanism of todorokite formation from birnessite under conditions analogous to those found in marine diagenetic and hydrothermal settings. Using a combined approach employing X-ray diffraction, electron microscopy, infrared spectroscopy, X-ray absorption spectroscopy and wet chemical methods, I propose a new four-stage process for the transformation of birnessite to todorokite, beginning with todorokite nucleation, then crystal growth from solution to form todorokite primary particles, followed by their self-assembly and oriented growth via oriented attachment to form crystalline todorokite laths, culminating in traditional crystal ripening. Furthermore, the results of this study indicate that contrary to current understanding, the bioessential trace metal Ni impedes the transformation of birnessite to todorokite, and is eventually released into sediment porewaters. This mineralogical transformation may therefore provide a benthic flux of Ni and possibly other micronutrient trace metals to seawater.
Finally, I find that the uptake of Ni to the phyllomanganate birnessite under varying physiochemical conditions is accompanied by Ni stable isotope fractionation. During fractionation, the light Ni isotope is preferentially sorbed to birnessite, leaving the remaining solution heavy with respect to its Ni isotopic composition. These findings raise important questions about the mechanisms and processes responsible for the heavy δ60Ni isotopic compositions recently measured in marine ferromanganese-rich sediments.