The Biogeochemistry of Layered Mn Oxides in Soils: A Crystallographic Perspective.

Birnessite-like phases occur in a wide variety of soil environments, and they can outcompete aluminosilicate clays in controlling the cycling of metals between solid and fluid phases because of their high redox and cation exchange capacities.  In the birnessite crystal structure, negatively charged sheets of MnO₆ octahedra are electrostatically balanced by hydrated interlayer cations.  The high valence state of octahedral Mn(III,IV) renders birnessite susceptible to reductive dissolution through biotic and abiotic agents.

We have explored the reductive dissolution of triclinic Na-birnessite [Na_0.58 (Mn⁴⁺_1.42 Mn³⁺_0.58) O₄ . 1.5 H₂O] by bacterial membranes, siderophores, and transition metal cations using a novel X-ray diffraction approach that allows for real-time observation of structural evolution during fluid interaction.  Birnessite powders in glass capillaries were irradiated with synchrotron X-rays in both closed cells (for anoxic experiments) and in open cells (for flow-through studies), and diffraction patterns were collected using fast image detectors with a temporal resolution of <1 minute.

Our results revealed that different agents of reductive dissolution generate distinct mechanisms of mineral breakdown.  Electron transfer from direct contact of the birnessite surface with the cell membrane of a dissimilatory metal-reducing bacterium, Shewanella oneidensis, induced a measurable contraction of the birnessite interlayer.  In contrast, the chelation and removal of Mn from the birnessite structure by bacterial siderophores did not result in a structural collapse of the mineral, despite the removal of 20 mol% Mn from the octahedral sheets.  Rather, the unit-cell parameters remained constant throughout the complete dissolution of birnessite.  When birnessite was abiotically reduced during the oxidation of aqueous Cr³⁺ to Cr⁶⁺, the triclinic structure transformed to one with hexagonal symmetry.  The specificity of birnessite’s response to different reduction/dissolution mechanisms suggests that crystallographic analysis of birnessite may serve as a useful biomarker to determine whether life forms have participated in mineral redox processes.