5:15 PM - 6:30 PM
[MIS14-P14] Synthesis and reversible redox properties of iron-saponite
Keywords:Smectite, Mars, Iron, Redox
Clay minerals widely distribute on the surface of Mars. In these clay minerals, (Fe,Mg)-smectites were commonly discovered and useful for understanding the history of Mars. The valence state of iron in smectite is one of the key properties to reveal the redox state at the smectite formation. It has been believed that the redox state can be revealed from the crystal structures of iron-rich smectite. In the smectite, ferrous (Fe2+) ions fully occupy the octahedral sites, while the ferric (Fe3+) can occupy only two third of the sites due to the charge balance. This difference should be identified from the X-ray diffraction (XRD) profile. From the XRD analysis, ferrous saponite can be an analogue of smectites discovered at Yellowknife Bay in Gale Crater [Chemtob et al., 2015]. However, the natural iron saponite on the Earth can also explain the XRD pattern and surprisingly the most iron was characterized by ferric (Fe3+) state [Treiman et al., 2014]. A central question is whether the XRD analysis can provide the oxidation state on formation of smectite.
A hypothesis was proposed for the crystal structure of ferric iron-rich saponite [Treiman et al., 2014]. The saponite was initially synthesized under reduced conditions and oxidized later. The oxidation may occur by the deprotonation from structural OH groups. If this is correct, the oxidation and reduction can occur reversibly without large deformation in the crystal structure. In this study, the reduction and oxidation of ferrian (ferric iron-rich) saponite were investigated to study the hypothesis.
Ferrian saponite was synthesized by the hydrothermal method similar to the method proposed for the synthesis of ferrous saponite [Baldermann et al., 2014]. The precipitates were dispersed in ultrapure water for one to five days. These samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES), humidity-controlled X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), and infrared spectroscopy.
The chemical composition of precipitates and their swelling behavior indicate that the precipitates are smectite. The crystal structure of synthesized samples is categorized to trioctahedral smectite which is characteristic to ferrous saponite, but the XANES and TEM-electron energy loss spectroscopy (EELS) analysis indicates that most iron is present as ferric iron. These results indicate that the smectite is ferrian saponite. The reduction treatment was conducted for the ferrian saponite for one hour. There is no difference in the XRD profile before and after the treatment, but about 40% of ferric ion was reduced to ferrous ion in the saponite. This reduction may be interpreted by the exchange of hydrogen in the crystal as proposed for the formation mechanism of natural ferrian saponite [Treiman et al., 2014].
Our results indicate that the ferrian saponite preserves the oxidation state on formation in the crystal structure, and the redox state of iron ions can be changed without deformation in the structure.
References
A. Baldermann, R. Dohrmann, S. Kaufhold, C. Nickel, I. Leftofsky-Papst, M. Dietzel, Clay Minerals, (2014) 49, 391-415.
S. M. Chemtob, R. D. Nickerson, R. V. Morris, D. G. Agresti, J. G. Gatalano, J. Geophys. Res. Planets, (2015) 120, 1119-1140.
A. H. Treiman et al., Am. Mineral., (2014) 99, 2234-2250.
A hypothesis was proposed for the crystal structure of ferric iron-rich saponite [Treiman et al., 2014]. The saponite was initially synthesized under reduced conditions and oxidized later. The oxidation may occur by the deprotonation from structural OH groups. If this is correct, the oxidation and reduction can occur reversibly without large deformation in the crystal structure. In this study, the reduction and oxidation of ferrian (ferric iron-rich) saponite were investigated to study the hypothesis.
Ferrian saponite was synthesized by the hydrothermal method similar to the method proposed for the synthesis of ferrous saponite [Baldermann et al., 2014]. The precipitates were dispersed in ultrapure water for one to five days. These samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES), humidity-controlled X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), and infrared spectroscopy.
The chemical composition of precipitates and their swelling behavior indicate that the precipitates are smectite. The crystal structure of synthesized samples is categorized to trioctahedral smectite which is characteristic to ferrous saponite, but the XANES and TEM-electron energy loss spectroscopy (EELS) analysis indicates that most iron is present as ferric iron. These results indicate that the smectite is ferrian saponite. The reduction treatment was conducted for the ferrian saponite for one hour. There is no difference in the XRD profile before and after the treatment, but about 40% of ferric ion was reduced to ferrous ion in the saponite. This reduction may be interpreted by the exchange of hydrogen in the crystal as proposed for the formation mechanism of natural ferrian saponite [Treiman et al., 2014].
Our results indicate that the ferrian saponite preserves the oxidation state on formation in the crystal structure, and the redox state of iron ions can be changed without deformation in the structure.
References
A. Baldermann, R. Dohrmann, S. Kaufhold, C. Nickel, I. Leftofsky-Papst, M. Dietzel, Clay Minerals, (2014) 49, 391-415.
S. M. Chemtob, R. D. Nickerson, R. V. Morris, D. G. Agresti, J. G. Gatalano, J. Geophys. Res. Planets, (2015) 120, 1119-1140.
A. H. Treiman et al., Am. Mineral., (2014) 99, 2234-2250.