Classical mechanical simulations of layer- and tunnel-structured manganese oxide minerals

AG Newton and KD Kwon, GEOCHIMICA ET COSMOCHIMICA ACTA, 291, 92-109 (2020).

DOI: 10.1016/j.gca.2020.04.034

Tecto- and phyllo-manganate minerals serve as important controls in the geochemical cycle of many elements in soils and sediments due to their high surface areas and chemical reactivity. These nanoscale Mn oxides are mainly composed of Mn octahedra in edge- and corner-sharing configurations that produce a range of tunnel and layer structures. Some fundamental aspects of their atomic structures, however, are difficult to characterize unambiguously because of dynamic structural transformations and a wide range of the structural and chemical compositions. Molecular dynamics (MD) simulations based on interatomic pair potentials provide a perspective on nanoscale Mn-oxides that can be inaccessible to experiment or density functional theory (DFT) calculations. We recently introduced a set of interatomic potentials for the atomistic simulation of Mn-oxides (MnFn) that reproduced many of the subtle phyllomanganate structural variations associated with pH, vacancy content, and Mn oxidation state. In this manuscript, we expand the roster of simulated Mn-oxide minerals to include hydrous and anhydrous tunnel structures and delta-MnO2 with octahedral vacancy contents up to 20%. The MnFn potentials reproduced the experimental tectomanganate lattice parameters and interatomic distances to within 5% and the relative energetics of tectomanganates as predicted by a recent DFT study. We also provide new insights into the Mg site occupancy in todorokite and the Zn coordination in vernadite through large-scale atomistic simulations with MnFn that are challenging to simulate with DFT. In the 3 x 3 tunnel structures of todorokite, the central tunnel site is the predominant site for fully-hydrated Mg2+center dot 6H(2)O with a minority of partially-hydrated Mg2+ cations binding to corner sites at decreased water/cation ratios. The occurrence of tetrahedral Zn complexes is dependent upon not only the Mn(IV) vacancy content but also the layer spacing and sheet registry. These atomistic simulation results demonstrate the ability of the MnFn potentials to reproduce the basic structures of nanophase Mn-oxides and the capability to perform large- scale modeling that is required to address the subtle structural variations associated with changes in environmental conditions. (C) 2020 Elsevier Ltd. All rights reserved.

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