Phyllomanganate minerals exert strong control over the chemical speciation of natural and engineered systems as a result of their nanocrystalline structure and concomitant high surface reactivity. The high-capacity of Mn-oxides for adsorption of heavy metals and trace elements makes a comprehensive understanding of their structure and reactivity vital for effective environmental remediation and the future development of energy-related technologies. The reactivity of phyllomanganates correlates with the extent of octahedral Mn vacancies and the average Mn oxidation state. In addition, interlayer water influences cation exchange and enhances the structural stability of phyllomanganates. Yet, some aspects of the atomic structure of the phyllomanganates and the interlayer dynamics are not unambiguous. Atomistic simulations afford a unique perspective of the phyllomanganate mineral structure and its interlayer at a level of detail that can validate proposed structures and aid in the interpretation of spectroscopic data. Classical molecular dynamics (MD) which simplify the interatomic interactions by using a set of empirically-fitted potentials derived from structural and spectroscopic data are a computationally efficient means of exploring structural features and the interlayer dynamics of phyllomanganates that are inaccessible to experiment. In this presentation, we introduce MD simulations of Mn-oxides performed with our recently developed interatomic pair potentials at 298K and 1atm. The phyllomanganates chalcophanite (a hydrated Zn-manganese oxide with ~15% vacancies) and birnessite (a vacancy-free, hydrated manganese oxide with mixed MnIII/MnIV valence) are used in the MD simulations to examine i.) the effects of vacancies and Mn valence on the phyllomanganate structure, ii.) the role of interlayer water in structural stability, and iii.) cation surface complexes. These Mn-O interatomic potentials and MD simulations represent a valuable technique for the further explorations of the cationic surface complexes and the interlayer properties of biogenic Mn oxides.

Atomistic perspectives on cation complexes, octahedral vacancies, and the interlayer water of phyllomanganates through molecular dynamics simulations

253rd National Meeting (2017)

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