Physics and Chemistry of Minerals

, Volume 46, Issue 7, pp 639–661 | Cite as

Structural iron in smectites with different charge locations

  • N. FinckEmail author
  • M. L. Schlegel
  • K. Dardenne
  • C. Adam
  • S. Kraft
  • A. Bauer
  • J.-L. Robert
Original Paper


The versatile structure of smectites can exhibit large variations in chemical compositions and cationic substitutions in different crystallographic sites, resulting in various locations of layer charge. Natural smectites can contain various amounts of structural iron, the chemical form of which can influence the reactivity of these minerals. The variety of Fe crystal chemistry in smectite was explored for eight natural smectites of distinct chemical compositions and charge locations, together with two synthetic ferric saponites used as reference compounds for tetrahedral Fe(III). All samples were identified as dioctahedral or trioctahedral smectite by X-ray diffraction and Fourier-transform infrared spectroscopy. The extent of [4]Al for [4]Si substitution was determined by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy. The Fe local chemical environment was probed by polarized X-ray absorption spectroscopy. Only Fe(III) could be detected in all samples, with no evidence of cluster formation. The O shell at 1.86 Å in synthetic saponites suggests Fe insertion in tetrahedral sites, and the absence of detected octahedral Fe implies quantitative substitution of [4]Fe3+ for [4]Si4+. In natural smectites, Fe(III) is bound to six O atoms at ~ 2.00 Å, suggesting insertion in octahedral sites. This inference is also supported by the detection of in-plane Mg/Al/Fe atoms at ~ 3.05 Å and out-of-plane Si/Al atoms at ~ 3.25 Å. In one Fe-rich nontronite, the detection of an O subshell at ~ 1.88 Å suggests a concomitant insertion of Fe(III) in tetrahedral sites. Low numbers of octahedral neighbors were detected in natural saponite and hectorite, presumably because of the presence of vacancies and/or Li(I) in adjacent octahedral sites balancing the local charge excess originating from the substitution of Fe(III) for Mg(II). The substitution of [4]Fe3+ for [4]Si4+ can be readily obtained under defined conditions in the laboratory, but seems more rare in natural samples, or present in amounts below the detection limit of spectroscopic methods used in this study.


Smectites Iron Polarized XAS FTIR 27Al and 29Si MAS-NMR 



We acknowledge the contribution of the late Dr. J.-L. Robert of IMPMC to this work. We thank E. Soballa (KIT-INE) for SEM-EDX analyses. We acknowledge the KIT Synchrotron Light Source and the Institute for Beam Physics and Technology (IBPT) for operation of the storage ring, the Karlsruhe Research Accelerator (KARA). We also thank the ESRF for provision of synchrotron radiation beam time and I. Kieffer for support at the BM30B (ESRF) beamline.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Institute for Nuclear Waste Disposal (INE)Karlsruhe Institute of Technology (KIT)KarlsruheGermany
  2. 2.DEN-Service d’Etudes Analytiques et de Réactivité des Surfaces (SEARS)CEA, Université Paris-SaclayGif-sur-YvetteFrance
  3. 3.IMPMC, UMR 7590, CNRS-Université Pierre et Marie CurieParis Cedex 05France

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