Clays and Clay Minerals

, Volume 39, Issue 3, pp 254–263 | Cite as

High-Resolution Transmission Electron Microscopy Study of Mn-Oxyhydroxide Transformations and Accompanying Phases in a Lateritic Profile of Moanda, Gabon

  • M. Amouric
  • S. Parc
  • D. Nahon


Unheated natural mixtures of manganite and secondary pyrolusite, from the same lateritic manganiferous sequence, were studied in different orientations by high-resolution transmission electron microscopy (HRTEM), electron diffraction, and energy-dispersive X-ray analysis (EDX) to determine the fine structure of these phases, their possible crystallographic relations, and the genetic processes that led to the formation of the pyrolusite. Typical palisadic texture was observed for both minerals. Characteristic cracks parallel t. (010) of the pyrolusite structure and in particular <210> microfissures in manganite were noted as signs of structural accommodation accompanying the transformation phenomenon between these two minerals. A previously unreported manganese oxide of the spinel-type (γ-Mn2O3 or Mn3O4) was also identified in the original mixture. This oxide gave pure microdomains as intergrowths with pyrolusite adjacent to manganite. This is the first report of a natural occurrence of γ-Mn2O3. The manganite-pyrolusite transformation process and an unsuspected γ-Mn2O3 (Mn3O4)-pyrolusite transition were directly illustrated in detail for the first time. Interfaces between the concerned phases were not sharp or smooth, but exhibited strong strain contrasts and interferential periodicities. Lattice images and microdiffraction patterns proved that both transformations were oriented, suggestive of topotactic relations. In addition, the principal minerals in the matrix (illite, kaolinite, and goethite) were examined for a better understanding of their role in Mn-oxyhydroxides transformations.

Key Words

Energy-dispersive X-ray analysis Hausmannite High-resolution transmission electron microscopy Laterite Manganese Manganite Pyrolusite Topotactic transformation 


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Copyright information

© The Clay Minerals Society 1991

Authors and Affiliations

  • M. Amouric
    • 1
  • S. Parc
    • 2
  • D. Nahon
    • 2
  1. 1.Centre de Recherche sur les Mécanismes de la Croissance CristallineC.N.R.S.-Campus Luminy, case 913Marseille Cedex 9France
  2. 2.Laboratoire de Géosciences de l’EnvironnementURA C.N.R.S. 132Marseille Cedex 13France

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