Comparative crystal chemistry of monovalent cation-exchanged birnessite

Abstract

Birnessite (ideally, Na0.54Mn2O4∙1.5H2O) is a layered manganese oxide mineral formed by edge-sharing of MnO6 octahedral sheets stacked in ∼7 Å spacing where variable amounts of water molecules and Na+ cations are occupied in a loosely disordered fashion. As such, the interlayer cations can be mixed with or substituted by other cations to contain diverse rare metals and chemical species to endow unique sensitivity on natural environments. To understand comparative crystal chemistry of birnessite as a function of interlayer cation substitution, a series of monovalent cation-exchanged forms, i.e., H-, Li-, K-, NH4-, Rb-, and Cs-exchanged birnessite, were produced from the original Na-birnessite under ambient conditions and analyzed using synchrotron X-ray powder diffraction, thermo-gravimetry, and X-ray photoluminescence spectroscopy. The (001) interplanar spacing generally expands as a function of ionic radius while it contracts as a function of the water content. Compared to the original Na-birnessite, MnO6 octahedral sheets of cation-exchanged forms become generally reduced and show higher MnIII to MnIV ratio, which affects the interlayer cation and water population. Based on the Rietveld structural refinements, the interlayer structural models are categorized into two groups depending on the distribution fashion of cations and water molecules, i.e., water molecules and cations in the Li-, K-, Rb-, Cs-birnessite models occupy separate interlayer sites whereas those are statistically mixed in the H-, Na-, NH4-birnessite models. The interlayer bonding between water molecules and oxygen atoms of the MnO6 layer appears to be a controlling factor for the interlayer spacing. The complex interplay among cation type, water content, Mn oxidation state, and related structural and chemical parameters are discussed to understand the comparative crystal chemistry of birnessite, which would serve as a basis to understanding its potential role in the geochemical cycle of cationic species in aqueous terrestrial environments.