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Molecular simulations of hydrated phyllomanganates
Newton, Aric G.,Kwon, Kideok D. Elsevier 2018 Geochimica et cosmochimica acta Vol.235 No.-
<P><B>Abstract</B></P> <P>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 Mn<SUP>3+</SUP> 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<SUP>3+</SUP> 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 Mn<SUP>3+</SUP> and Na<SUP>+</SUP>, considerable amounts of Mn<SUP>3+</SUP> were incorporated into the hexagonal birnessite layer, but only at total Mn(III) contents greater than ∼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 <I>b</I>-axis and separated by two rows of Mn(IV) octahedra. The dominant interlayer Na complex at a water content ≥0.7 H<SUB>2</SUB>O/MnO<SUB>2</SUB> was an octahedrally-coordinated, split interlayer site with two birnessite O atoms in the axial positions and four interlayer H<SUB>2</SUB>O 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.</P>
Manganese speciation in Mn-rich CaCO<sub>3</sub>: A density functional theory study
Son, Sangbo,Newton, Aric G.,Jo, Kyoung-nam,Lee, Jin-Yong,Kwon, Kideok D. Pergamon Press 2019 Geochimica et cosmochimica acta Vol.248 No.-
<P><B>Abstract</B></P> <P>The manganese content of aragonitic bivalve shells is a potential archival indicator of temporal Mn bioavailability in aquatic environments. The Mn speciation mechanism in biogenic aragonite minerals remains elusive because the analog is challenging to synthesize, and the metastable phase has yet to be fully resolved experimentally. In this study, we performed density functional theory (DFT) computations of hypothetical Mn-doped aragonite to examine its local coordination structure and thermodynamic and electronic properties. Our DFT calculations reproduced the experimental crystal structures and solubility product constants (<I>K</I> <SUB>sp</SUB>) of Mn-doped calcite. The magnetic moment of Mn was close to 5 <I>μ</I> <SUB>B</SUB> in both Mn-doped calcite and Mn-doped aragonite (Ca<SUB>1−</SUB> <I> <SUB>x</SUB> </I>Mn<I> <SUB>x</SUB> </I>CO<SUB>3</SUB>). The calculated <I>K</I> <SUB>sp</SUB> of Mn-doped aragonite was higher than that of Mn-doped calcite and increased with Mn content, indicative of the unfavorable coprecipitation of Mn with the aragonite phase. We found that the incorporation of a small mole fraction of Mn into aragonite created significant structural distortion around the Mn site, resulting in mixed coordination numbers of Mn (mainly five and seven). Valence-to-core X-ray emission spectroscopy (XES) measurements are useful in determining the coordination environment of Mn complexes. We calculated theoretical XES spectra, with a 1<I>s</I> core hole in Mn, for Mn-doped calcite and four versions of Mn-doped aragonite. The Boltzmann-averaged spectrum for different coordination numbers in Mn-doped aragonite was akin to an experimental XES spectrum of aragonitic bivalve shells. The energy position of the K<I>β</I> <SUB>2,5</SUB> band was calculated to be insensitive to Mn speciation in CaCO<SUB>3</SUB>; however, the band intensity was relatively sensitive to Mn speciation. The XES spectrum intensity decreased exponentially with increasing Mn–O distance. This quantitative XES relationship we report can reduce uncertainties in the spectral interpretation due to the absence of an Mn-doped aragonite reference spectrum.</P>