Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; II, Results from chemical studies and EXAFS spectroscopy

Abstract

Other| October 01, 1997 Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; II, Results from chemical studies and EXAFS spectroscopy Ewen Silvester; Ewen Silvester University of Grenoble, Environmental Geochemistry Group, Grenoble, France Search for other works by this author on: GSW Google Scholar Alain Manceau; Alain Manceau CNRS, France Search for other works by this author on: GSW Google Scholar Victor A. Drits Victor A. Drits Russian Academy of Sciences, Russian Federation Search for other works by this author on: GSW Google Scholar Author and Article Information Ewen Silvester University of Grenoble, Environmental Geochemistry Group, Grenoble, France Alain Manceau CNRS, France Victor A. Drits Russian Academy of Sciences, Russian Federation Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X Copyright © 1997 by the Mineralogical Society of America American Mineralogist (1997) 82 (9-10): 962–978. https://doi.org/10.2138/am-1997-9-1013 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ewen Silvester, Alain Manceau, Victor A. Drits; Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; II, Results from chemical studies and EXAFS spectroscopy. American Mineralogist 1997;; 82 (9-10): 962–978. doi: https://doi.org/10.2138/am-1997-9-1013 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal (H+-exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural Na+ with solution H+ and the partial loss of Mn2+ to the aqueous phase. The desorption of Na+ in two temporally distinct steps indicates the existence of two types of binding sites for this cation. Mn2+ appears to originate from a partial disproportionation of Mn3+ in the NaBu layers, according to the sequenceMnlayer3++Mnlayer3+→Mnlayer4++Mnlayer2+→Mnlayer4++ Vacancy+Mnaq2+.EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing MnO6 octahedra, with no evidence for triple-corner (TC) sharing Mn. HBi is significantly different with strong evidence for TC-sharing Mn and therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-sharing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that the layer vacancies are restricted to every third row of Mn cations, with 50% of the Mn sites along these rows vacant. The density of vacancies in the entire layer is therefore one in six of layer Mn sites. Polarized EXAFS measurements on orientated films of NaBi and HBi confirm the absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at layer vacancy sites in HBi at pH 4 is dominantly Mn3+. The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples increases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer Mn3+ to interlayer TC-sharing positions and re-adsorption of Mn2+ from solution onto layer vacancy sites. The migration of Mn3+ cations into the interlayer releases the steric strain associated with the Jahn-Teller distortion of these octahedra. This model of the NaBu-to-HBi conversion explains the transformation from orthogonal to hexagonal layer symmetry, respectively, as reported by Drits et al. (1997).Analysis of the Zn EXAFS spectrum of Zn2+-exchanged birnessite shows that Zn2+ also occupies TC-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importance in adsorption and electron transfer processes occurring at the surface of this mineral. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.