Molecular simulations of hydrated phyllomanganates

AG Newton and KD Kwon, GEOCHIMICA ET COSMOCHIMICA ACTA, 235, 208-223 (2018).

DOI: 10.1016/j.gca.2018.05.021

Hydrated phyllomanganates are layered Mn-oxide minerals with interlayers that can possess variable water contents and react strongly with trace metals due to octahedral vacancies in the layer. The unique properties of phyllomanganates afford them a significant role in many environmental phenomena that affect soil and water composition mainly via cation exchange and adsorption of trace metals. Slight variations in the structural and chemical composition often result in a dramatic difference in the chemical reactivity of the minerals. Molecular simulations at the classical mechanical level of theory, which uses a simplified description of the interatomic potential energies, can provide an atomistic perspective of the relationship between the chemical composition and the bulk and interlayer structures. We introduce a set of interatomic potentials for hydrated phyllomanganates with variable vacancy and Mn3+ content and report the classical mechanical simulation results performed at standard temperature and pressure. The potentials we introduce provide not only a reasonable reproduction of the experimentally determined atomic structures of the chalcophanite group, but also new insights on similar phyllomanganate minerals with hexagonal symmetry and a range of vacancy contents. When a vacancy is protonated, Mn(3+ )is unstable in the hexagonal birnessite layer and occupies the interlayer as a cap on the associated vacancy. When a vacancy was charge-balanced by Mn3+ and Na+, considerable amounts of Mn3+ were incorporated into the hexagonal birnessite layer, but only at total Mn(III) contents greater than similar to 10% and with disordered layer stacking. The potentials also predicted a vacancy-free, triclinic Na-birnessite structure with Mn(III)-rich rows in the layer which were arranged parallel to the b-axis and separated by two rows of Mn(IV) octahedra. The dominant interlayer Na complex at a water content >= 0.7 H2O/MnO2 was an octahedrally-coordinated, split interlayer site with two birnessite O atoms in the axial positions and four interlayer H2O in the equatorial positions. Other interlayer Na complexes including some edge- and face-sharing complexes existed in trace amounts (<10%). These classical mechanical simulations represent a successful first test of the introduced interatomic potentials, which can be used to further explore the interlayer and surface complexes of phyllomanganate and birnessite-group minerals. (C) 2018 Elsevier Ltd. All rights reserved.

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