Interlayer water structure of phyllomanganates: Insights from MD simulations of chalcophanite-group oxide dehydration

S Han and KD Kwon, GEOCHIMICA ET COSMOCHIMICA ACTA, 318, 495-509 (2022).

DOI: 10.1016/j.gca.2021.11.036

Phyllomanganates transform and concentrate a variety of metals in marine and terrestrial environments through sorption and redox reactions. Water molecules present in the interlayer region of phyllomanganates are assumed to play an important role in the stability, cation exchange, and redox reactions of manganese oxide minerals. The molecular structure of interlayer water remains elusive due to insufficient analytical and computational capabilities for investigating these confined water molecules. This study explores the interlayer water structure of phyllomanganates based on classical molecular dynamics (MD) simulations performed for crystalline chalcophanite group oxides (Me2+ Mn3O7 center dot 3H(2)O, Me2+ = Zn2+, Ni2+, Mg2+, Mn2+, or Ca2+) as a function of the interlayer water content. As the oxides were dehydrated, MD simulations revealed collapse of the layer spacing in Zn- and Ni-chalcophanite and gradual decrease of the layer spacing in Mg-, Mn-, and Ca-chalcophanite. MD simulations also revealed that the cation-specific reduction in layer spacing with dehydration is controlled by the reorganization of interlayer water molecules, the extent of which strongly depends on the interlayer cations. Water molecules in chalcophanite group oxides occupied the midplane of the interlayer prior to dehydration, with the exception of Ca-chalcophanite which contained two split atomic planes of water molecules near the oxide surface. When Zn- and Ni-chalcophanite were monohydrated per formula unit, water molecules moved directly above or below the interlayer cations, leading to two split atomic planes of water molecules and a minor increase in the c-axis. In monohydrated Mg-, Mn-, and Ca-chalcophanite, water molecules occupied the midplane in the interlayer, with the c-axis reduced. Dehydration of Zn- and Ni- chalcophanite induced the dipole-moment vectors of water molecules to rotate simultaneously and become arranged perpendicular to the (001) basal plane, forming H-bonds between a water molecule and a single Mn octahedral sheet surface. However, in dehydrated Mg-, Mn-, and Ca- chalcophanite, water molecules were induced to rotate in the opposite direction; in this case, the dipole-moment vectors were nearly parallel to the basal plane, and H-bonds formed between a water molecule and two adjacent sheet surfaces. MD simulations with and without Mn4+-vacancy site disorder demonstrated that vacancy site disorder causes significant disorder in interlayer cation and water structures. The current study provides atomistic insight into the effects on interlayer water structure of chemical and structural disorders in birnessite-like phyllomanganates. (C) 2021 Elsevier Ltd. All rights reserved.

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