Time-resolved structural analysis of K- and Ba-exchange reactions with synthetic Na-birnessite using synchrotron X-ray diffraction

Christina L. Lopano, Peter J. Heaney, Jeffrey E. Post, Jonathan Hanson, Sridhar Komarneni

Research output: Contribution to journalArticle

55 Scopus citations

Abstract

Time-resolved Rietveld refinements using synchrotron X-ray diffraction (XRD) have documented real-time changes in unit-cell parameters in response to cation substitution in synthetic Na-birnessite. Potassium- and Ba-birnessite, like Na-birnessite, were found to have triclinic symmetry. Rietveld analyses of the XRD patterns for K- and Ba-exchanged birnessite revealed decreases in the a, c, and β unit-cell parameters, with a decrease of 1.7 and 0.5%, respectively, in unit-cell volume relative to Na-birnessite. Fourier electron difference syntheses revealed that the changes in the configuration of the interlayer species, and the charge, size, and hydration of the substituting cations, serve as the primary controls on changes in unit-cell parameters. Split electron density maxima with centers at (0 0 0.5) were present for Na, K, and Ba end-members; however, with increased substitution of K+ for Na+, the axis connecting the split-site maxima rotated from an orientation parallel to the b-axis to along the a-axis. Substitution of Ba2+ for Na+ did not result in rotation, but splitting of the interlayer site was more pronounced.

Original languageEnglish (US)
Pages (from-to)380-387
Number of pages8
JournalAmerican Mineralogist
Volume92
Issue number2-3
DOIs
StatePublished - Feb 1 2007

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

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