Startseite Oxidation-reduction mechanism of iron in dioctahedral smectites: II. Crystal chemistry of reduced Garfield nontronite
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Oxidation-reduction mechanism of iron in dioctahedral smectites: II. Crystal chemistry of reduced Garfield nontronite

  • A. Manceau EMAIL logo , V.A. Drits , B. Lanson , D. Chateigner , J. Wu , D. Huo , W.P. Gates und J.W. Stucki
Veröffentlicht/Copyright: 25. März 2015
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American Mineralogist
Aus der Zeitschrift American Mineralogist Band 85 Heft 1

Abstract

The crystallochemical structure of reduced Garfield nontronite was studied by X-ray absorption pre-edge and infrared (IR) spectroscopy, powder X-ray diffraction, polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy, and texture goniometry. Untreated and highly reduced (>99% of total Fe as Fe2+) nontronite samples were analyzed to determine the coordination number and the crystallographic site occupation of Fe2+, changes in in-plane and out-of-plane layer structure and mid-range order between Fe centers, and to monitor the changes in structural and adsorbed OH/H2O groups in the structure of reduced nontronite. Contrary to earlier models predicting the formation of fivefold coordinated Fe in the structure of nontronites upon reduction, these new results revealed that Fe maintains sixfold coordination after complete reduction. In-plane PEXAFS evidence indicates that some of the Fe atoms occupy trans-sites in the reduced state, forming small trioctahedral domains within the structure of reduced nontronite. Migration of Fe from cisto trans sites during the reduction process was corroborated by simulations of X-ray diffraction patterns which revealed that about 28% of Fe2+ cations exist in trans sites of the reduced nontronite, rather than fully cis occupied, as in oxidized nontronite. Out-of-plane P-EXAFS results indicated that the reduction of Fe suppressed basal oxygen corrugation typical of dioctahedral smectites, and resulted in a flat basal surface which is characteristic of trioctahedral layer silicates. IR spectra of reduced nontronite revealed that the dioctahedral nature of the nontronite was lost and a band near 3623 cm-1 formed, which is thought to be associated with trioctahedral [Fe2+]3OH stretching vibrations. On the basis of these results, a structural model for the reduction mechanism of Fe3+ to Fe2+ in Garfield nontronite is proposed that satisfies all structural data currently available. The migration of reduced Fe ions from cis-octahedra to adjacent trans-octahedra is accompanied by a dehydroxylation reaction due to the protonation of OH groups initially coordinated to Fe. This structural modification results in the formation of trioctahedral Fe2+ clusters separated by clusters of vacancies in which the oxygen ligands residing at the boundary between trioctahedral and vacancy domains are greatly coordination undersaturated. The charge of these O atoms is compensated by the incorporation of protons, and by the displacement of Fe2+ atoms from their ideal octahedral position toward the edges of trioctahedral clusters, thus accounting for the incoherency of the Fe-Fe1 and Fe-Fe2 distances. From these results, the ideal structural formula of reduced Garfield nontronite is Na1.30[Si7.22Al0.78] [Fe2+3.65Al0.32Mg0.04]O17.93(OH)5 in which the increased layer charge due to reduction of Fe3+ to Fe2+ is satisfied by the incorporation of protons and interlayer Na.

Received: 1998-8-3
Accepted: 1999-8-15
Published Online: 2015-3-25
Published in Print: 2000-1-1

© 2015 by Walter de Gruyter Berlin/Boston

Artikel in diesem Heft

  1. Eclogite-facies relics and inferred ultrahigh-pressure metamorphism in the North Dabie Complex, central-eastern China
  2. Application of INVEQ to the geothermobarometry of metamorphic rocks near a kyanite-sillimanite isograd, Mica Creek, British Columbia
  3. Microstructural characterization of metamorphic magnetite crystals with implications for oxygen isotope distribution
  4. Coexisting monazite and allanite in peraluminous granitoids of the TribečMountains, Western Carpathians
  5. Effect of alkalis, phosphorus, and water on the surface tension of haplogranite melt
  6. Direct observation of immiscibility in pyrope-almandine-grossular garnet
  7. On the cluster analysis of grains and crystals in rocks
  8. Experimental approach to constrain second critical end points in fluid/silicate systems: Near-solidus fluids and melts in the system albite-H2O
  9. Estimation and testing of standard molar thermodynamic properties of tourmaline end-members using data of natural samples
  10. An investigation of matrix effects in the analysis of fluorine in humite-group minerals by EMPA, SIMS, and SREF
  11. New data on the crystal-chemistry of fluoborite by means of SREF, SIMS, and EMP analysis
  12. Effect of aluminum on Ti-coordination in silicate glasses: A XANES study
  13. XAFS study of Cu model compounds and Cu2+ sorption products on amorphous SiO2, γ-Al2O3, and anatase
  14. Oxidation-reduction mechanism of iron in dioctahedral smectites: I. Crystal chemistry of oxidized reference nontronites
  15. Oxidation-reduction mechanism of iron in dioctahedral smectites: II. Crystal chemistry of reduced Garfield nontronite
  16. 15N NMR study of nitrate ion structure and dynamics in hydrotalcite-like compounds
  17. 29Si and 27Al MAS-NMR spectroscopy of b-eucryptite (LiAlSiO4): The enthalpy of Si,Al ordering
  18. The nuclear and magnetic structure of “white rust”—Fe(OH0.86D0.14)2
  19. In-situ study of the R3̅ to R3̅c phase transition in the ilmenite-hematite solid solution using time-of-flight neutron powder diffraction
  20. In situ X-ray diffraction investigation of lawsonite and zoisite at high pressures and temperatures
  21. A shell model for the simulation of rhombohedral carbonate minerals and their point defects
  22. Optical properties of natural and cation-exchanged heulandite group zeolites
  23. The crystal structure of gearksutite, CaAlF4(OH)· H2O
  24. A new anhydrous amphibole from the Eifel region, Germany: Description and crystal structure of obertiite, NaNa2(Mg3Fe3+Ti4+)Si8O22O2
  25. Kinoshitalite, Ba(Mg)3(Al2Si2)O10(OH,F)2, a brittle mica from a manganese deposit in Oman: Paragenesis and crystal chemistry
  26. The crystal structure of parisite-(Ce), Ce2CaF2(CO3)3
  27. High-pressure synthesis of Na2Mg6Si6O18(OH)2—a new hydrous silicate phase isostructural with aenigmatite
https://doi.org/10.2138/am-2000-0115

Artikel in diesem Heft

  1. Eclogite-facies relics and inferred ultrahigh-pressure metamorphism in the North Dabie Complex, central-eastern China
  2. Application of INVEQ to the geothermobarometry of metamorphic rocks near a kyanite-sillimanite isograd, Mica Creek, British Columbia
  3. Microstructural characterization of metamorphic magnetite crystals with implications for oxygen isotope distribution
  4. Coexisting monazite and allanite in peraluminous granitoids of the TribečMountains, Western Carpathians
  5. Effect of alkalis, phosphorus, and water on the surface tension of haplogranite melt
  6. Direct observation of immiscibility in pyrope-almandine-grossular garnet
  7. On the cluster analysis of grains and crystals in rocks
  8. Experimental approach to constrain second critical end points in fluid/silicate systems: Near-solidus fluids and melts in the system albite-H2O
  9. Estimation and testing of standard molar thermodynamic properties of tourmaline end-members using data of natural samples
  10. An investigation of matrix effects in the analysis of fluorine in humite-group minerals by EMPA, SIMS, and SREF
  11. New data on the crystal-chemistry of fluoborite by means of SREF, SIMS, and EMP analysis
  12. Effect of aluminum on Ti-coordination in silicate glasses: A XANES study
  13. XAFS study of Cu model compounds and Cu2+ sorption products on amorphous SiO2, γ-Al2O3, and anatase
  14. Oxidation-reduction mechanism of iron in dioctahedral smectites: I. Crystal chemistry of oxidized reference nontronites
  15. Oxidation-reduction mechanism of iron in dioctahedral smectites: II. Crystal chemistry of reduced Garfield nontronite
  16. 15N NMR study of nitrate ion structure and dynamics in hydrotalcite-like compounds
  17. 29Si and 27Al MAS-NMR spectroscopy of b-eucryptite (LiAlSiO4): The enthalpy of Si,Al ordering
  18. The nuclear and magnetic structure of “white rust”—Fe(OH0.86D0.14)2
  19. In-situ study of the R3̅ to R3̅c phase transition in the ilmenite-hematite solid solution using time-of-flight neutron powder diffraction
  20. In situ X-ray diffraction investigation of lawsonite and zoisite at high pressures and temperatures
  21. A shell model for the simulation of rhombohedral carbonate minerals and their point defects
  22. Optical properties of natural and cation-exchanged heulandite group zeolites
  23. The crystal structure of gearksutite, CaAlF4(OH)· H2O
  24. A new anhydrous amphibole from the Eifel region, Germany: Description and crystal structure of obertiite, NaNa2(Mg3Fe3+Ti4+)Si8O22O2
  25. Kinoshitalite, Ba(Mg)3(Al2Si2)O10(OH,F)2, a brittle mica from a manganese deposit in Oman: Paragenesis and crystal chemistry
  26. The crystal structure of parisite-(Ce), Ce2CaF2(CO3)3
  27. High-pressure synthesis of Na2Mg6Si6O18(OH)2—a new hydrous silicate phase isostructural with aenigmatite
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