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(NAS Colloquium) Geology, Mineralogy, and Human Welfare (1999) / Chapter Skim
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Manganese Oxide Minerals: Crystal structures and Economic and Environmental Significance
Pages 3447-3454

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From page 3447...
... In recent years, however, investigations using transmission electron microscopy and powder x-ray and neutron diffraction methods have provided important new insights into the structures and properties of these materials. The crystal structures for todorokite and birnessite, two of the more common Mn oxide minerals in terrestrial deposits and ocean nodules, were determined by using powder x-ray diffraction data and the Rietveld refinement method.
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From page 3448...
... , suggesting that the cations are bound into the Mn oxides' atomic structures (27~. Additional insights into the nature of the heavy metals likely await a more detailed understanding of the atomic structures and crystal chemistries of Mn oxide minerals in ocean nodules.
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From page 3449...
... Unfortunately, the crystal structures, and consequently, the powder diffraction patterns are similar for many of the Mn oxide minerals. In many cases, it is necessary to supplement powder x-ray diffraction studies with other techniques, such as transmission electron microscopy (TEM)
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From page 3450...
... Chemical analyses of nsutite typically show minor amounts of Na, Ca, Mg, K, Zn, Ni, Fe, Al, and Si, and about 2-4 weight percent water (58)
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From page 3451...
... The crystal structure of todorokite recently was refined by using the Rietveld method and powder x-ray diffraction data, revealing for the first time the major water and cation positions in the tunnels (48~. Lower valence cations such as Mn(III)
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From page 3452...
... The crystal structures of synthetic Na-, K-, and Mg-birnessite-like phases, however, recently were determined by using TEM and powder x-ray diffraction (49~. The study confirmed that the basic structural unit is a sheet of MnO6 octahedra and revealed that the interlayer cations and water molecules occupied different positions in each of the three phases (Fig.
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From page 3453...
... Unfortunately, many Mn oxide minerals occur only as fine-grained, poorly crystalline aggregates and coatings, making crystal structure studies extremely challenging. In recent years, however, an arsenal of new techniques, such as TEM, Rietveld refinements using powder diffraction data, extended x-ray absorption fine-structure spectroscopy, and single-crystal studies using charge-coupled device detectors and synchrotron sources, have slowly started and are continuing to unravel many of the inner secrets of Mn oxide minerals.
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From page 3454...
... 3454 Colloquium Paper: Post 52. Turner, S
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Key Terms

  • crystal structure
  • crystal structures
  • chemical analyses
  • heavy metals
  • atomic structures

  • water molecules
  • poorly crystalline
  • cross sections
  • weight percent
  • triple chains

  • ocean nodules
  • typically occurs
  • ore deposits
  • diffraction pattern
  • diffraction data

  • geological settings
  • alteration product
  • tunnel cations
  • metal cations
  • double chains

  • square cross
  • minerals commonly
  • octahedral sites
  • transmission electron
  • share corners

  • oxide phases
  • using powder
  • minerals occur
  • oxidized zones
  • minor amounts


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