Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments

The phyllomanganate birnessite is the main Mn-bearing phase in oxic marine sediments, and through coupled sorption and redox exerts a strong control on the oceanic concentration of micronutrient trace metals. However, under diagenesis and mild hydrothermal conditions, birnessite undergoes transformation to the tectomanganate todorokite. The mechanistic details of this 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. Here we transform a synthetic, poorly crystalline, hexagonal birnessite, analogous to marine birnessite, into todorokite under a mild reflux procedure, developed to mimic marine diagenesis and mild hydrothermal conditions. We characterize our birnessite and reflux products as a time series, employing X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), BET surface area analysis, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and extended X-ray absorption fine structure spectroscopy (EXAFS). We provide 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. Specifically we 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. We suggest that, contrary to current understanding, trace metals like Ni might retard the transformation of birnessite to todorokite and be released to marine sedimentary pore-waters during this diagenetic process, thus potentially providing a benthic flux of these micronutrients to seawater.