Tunnel cation and water structures of todorokite: Insights into metal partitioning and isotopic fractionation
J Kim and KD Kwon, GEOCHIMICA ET COSMOCHIMICA ACTA, 333, 153-165 (2022).
Tectomanganate todorokite and phyllomanganate vernadite are the two major manganese minerals in marine ferroman-ganese sediments that contain large quantities of trace metals. Todorokite has a less clear metal partitioning mechanism than vernadite, mainly because the metal cation and water structures in the tunnel cavities remain poorly understood. Here, we report the composition-dependent tunnel-cation site disorder of todorokite obtained from molecular dynamics (MD) simulations. Our simulations of homocationic Ni2+-, Zn2+-, Mg2+-, Ca2+-, or Na+-todorokite revealed that tunnel cations form inner-sphere (IS) complexes with Mn octahedral surface at low hydration states, but at higher hydration states, outer-sphere (OS) complexes are the dominant cation species located at the center of the tunnel. However, K+ and Cs+ form only IS complexes regardless of the tunnel hydration state. Optimum water contents were determined based on the free energy changes associated with water intercalation into the tunnel cavities calculated as a function of water content for two heterocationic todorokite models: Mg0.5Na0.4Ca0.1Mn6O12.nH(2)O (Mg/Mn = 0.08, average Mn oxidation state = +3.73) and Mg1.1K0.1Ca0.1- Mn6O12.nH(2)O (Mg/Mn = 0.18, average Mn oxidation state = +3.58). These models emulate the chemical compositions of well-characterized terrestrial and marine todorokite samples, respectively. At each optimum water content, MD simulations revealed that OS complexes are the major Mg2+ species in the former model. However, IS complexes are the major Mg2+ species in the latter model, indicating that the tunnel cation speciation of diagenetic todorokite can be influenced by the sediment pore-fluid chemistry. The atomic tunnel cation and water structures of todorokite provide a molecular basis for under-standing of metal partitioning to Mn oxides and interpretation of metal stable isotopes recorded in ferromanganese nodules. (C) 2022 Elsevier Ltd. All rights reserved.
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