STRUCTURAL STUDY OF BIOTIC AND ABIOTIC NANOCRYSTALLINE MANGANESE OXIDES USING ATOMIC PAIR DISTRIBUTION FUNCTION TECHNIQUE

ZHU, Mengqiang¹, FARROW, Christopher², POST, Jeffrey E.³, LIVI, Kenneth⁴, BILLINGE, Simon², GINDER-VOGEL, Matthew⁵ and SPARKS, Donald⁶, (1)Department of Geochemistry, Earth Science Division, Lawrence Berkeley National Laboratory, 307 McCone Hall, University of California-Berkeley, Berkeley, CA 94720, (2)Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, (3)Dept. of Mineral Sciences, Smithsonian Institution, P.O. Box 37012, Washington, DC 20013-7012, (4)Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218, (5)Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, 152 Townsend Hall, Newark, DE 19707, (6)Department of Plant and Soil Sciences, University of Delaware, 152 Townsend Hall, 531 South College Avenue, Newark, DE 19713, mzhu@lbl.gov

Atomic pair distribution function (PDF) analysis is a powerful technique to determine crystal structures of nanoparticulate, poorly crystalline and amorphous materials. The PDF technique recently has been applied in the structural characterization of several disordered environmental minerals, such as ferrihydrite and imogolite. In this study, we used PDF analysis to investigate the structures of nanophase manganese (Mn) oxides. Mn-oxides, including both layered and tunnel structures, are strong oxidants and extraordinary metal sorbents in nature. Naturally-occurring Mn-oxides often have poor crystallinity as determined via XRD, and neither XRD nor EXAFS techniques are able to fully interpret, or even differentiate, their structures. For example, amorphous todorokite (3×3 tunnel structure) and birnessite (layered structure) cannot be differentiated by XRD and EXAFS if they are so disordered that 7 or 10 Å d-spacing is not observed. δ-MnO₂, polymeric MnO₂ and biogenic Mn-oxides are most often used as analogues to investigate the environmental behaviors of natural poorly crystalline Mn-oxides. Our PDF results indicate that δ-MnO₂, polymeric MnO₂ and a biogenic Mn-oxide sample are layered structures with nearly hexagonal symmetry. The particle size and shape are refined during the structural modeling for the PDF simulation. The best model accurately allowing simulation of the PDFs is a monoclinic structure with the C2/m space group using a disk-like shape factor with a high degree of stacking disorder. These results help in understanding the reactivity and mineralogy of several naturally-occurring Mn-oxide minerals.

Session No. 18

T102. Structure, Properties, and Geochemistry of Nanoparticles, Nanoclusters, and Nanocomposites in Biogeochemical Systems I: In Honor of Benjamin Gilbert, Recipient of the 2010 MSA Award

Sunday, 31 October 2010: 8:00 AM-12:00 PM

Mile High Ballroom 2AB/3AB (Colorado Convention Center)

Geological Society of America Abstracts with Programs. Vol. 42, No. 5, p.62

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