Structural iron in smectites with different charge locations
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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 Al for 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 Fe3+ for 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 Fe3+ for 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.
KeywordsSmectites 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.
- Bailey SW (1980) Structures of layer silicates. In: Brindley GW, Brown G (eds) Crystal structures of clay minerals and their X-Ray identification. Mineralogical society, London, pp 2–123Google Scholar
- Brindley GW (1980) Order-disorder in clay mineral structures. In: Brindley GW, Brown G (eds) Crystal structures of clay minerals and their X-ray identification. Mineralogical Society, London, pp 125–195Google Scholar
- Gates WP (2005) Infrared spectroscopy and the chemistry of dioctahedral smectites. In: Kloprogge T (ed) Vibrational spectroscopy of layer silicates and hydroxides, CMS workshop lecture series, vol 13. The Clay Mineral Society, Chantilly, pp 125–168Google Scholar
- Komarneni S, Fyfe CA, Kennedy GJ, Strobl H (1986) Characterization of synthetic and naturally-occuring clays by Al-27 and Si-29 magic angle spinning NMR spectroscopy. J Am Ceram Soc 69(3):C45–C47Google Scholar
- Manceau A (1990) Distribution of cations among the octahedra of phyllosilicates—insight from EXAFS. Can Mineral 28:321–328Google Scholar
- Meunier A (2005) Clays. Springer-Verlag, Berlin Heidelberg, p 472Google Scholar
- Moore DM, Reynolds RC Jr (1997) X-ray diffraction and the identification and analysis of clay minerals. Oxford University Press, Oxford, p 400Google Scholar
- Semenova TF, Rozhdestvenskaya IV, Frankkamenetsky VA (1977) Refinement of the crystal structure of tetraferriphlogopite. Kristallografiya 22(6):1196–1201Google Scholar
- Woessner DE (1989) Characterization of clay minerals by Al-27 nuclear magnetic resonance spectroscopy. Am Miner 74(1–2):203–215Google Scholar