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Synthesis and characterization of Fe(III)-Fe(II)-Mg-Al smectite solid solutions and implications for planetary science

Fox, Valerie K. and Kupper, Robert J. and Ehlmann, Bethany L. and Catalano, Jeffrey G. and Razzell-Hollis, Joseph and Abbey, William J. and Schild, Dirk J. and Nickerson, Ryan D. and Peters, Jonas C. and Katz, Sydney M. and White, Andrew A. (2021) Synthesis and characterization of Fe(III)-Fe(II)-Mg-Al smectite solid solutions and implications for planetary science. American Mineralogist, 106 (6). pp. 964-982. ISSN 0003-004X. doi:10.2138/am-2020-7419ccbyncnd.

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This study demonstrates the synergies and limits of multiple measurement types for the detection of smectite chemistry and oxidation state on planetary surfaces to infer past geochemical conditions. Smectite clay minerals are common products of water-rock interactions throughout the solar system, and their detection and characterization provides important clues about geochemical conditions and past environments if sufficient information about their composition can be discerned. Here, we synthesize and report on the spectroscopic properties of a suite of smectite samples that span the intermediate compositional range between Fe(II), Fe(III), Mg, and Al end-member species using bulk chemical analyses, X-ray diffraction, Vis/IR reflectance spectroscopy, UV and green-laser Raman spectroscopy, and Mössbauer spectroscopy. Our data show that smectite composition and the oxidation state of octahedral Fe can be reliably identified in the near infrared on the basis of combination and fundamental metal-OH stretching modes between 2.1–2.9 μm, which vary systematically with chemistry. Smectites dominated by Mg or Fe(III) have spectrally distinct fundamental and combination stretches, whereas Al-rich and Fe(II)-rich smectites have similar fundamental minima near 2.76 μm, but have distinct combination M-OH features between 2.24 and 2.36 μm. We show that with expanded spectral libraries that include intermediate composition smectites and both Fe(III) and Fe(II) oxidation states, more refined characterization of smectites from MIR data is now possible, as the position of the 450 cm⁻¹ absorption shifts systematically with octahedral Fe content, although detailed analysis is best accomplished in concert with other characterization methods. Our data also provide the first Raman spectral libraries of smectite clays as a function of chemistry, and we demonstrate that Raman spectroscopy at multiple excitation wavelengths can qualitatively distinguish smectite clays of different structures and can enhance interpretation by other types of analyses. Our sample set demonstrates how X-ray diffraction can distinguish between dioctahedral and trioctahedral smectites using either the (02,11) or (06,33) peaks, but auxiliary information about chemistry and oxidation state aids in specific identifications. Finally, the temperature-dependent isomer shift and quadrupole splitting in Mössbauer data are insensitive to changes in Fe content but reliability differentiates Fe within the smectite mineral structure.

Item Type:Article
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URLURL TypeDescription
Ehlmann, Bethany L.0000-0002-2745-3240
Catalano, Jeffrey G.0000-0001-9311-977X
Peters, Jonas C.0000-0002-6610-4414
Additional Information:© 2021 by the Mineralogical Society of America. This is an open-access article distributed under the terms of the Creative Commons Attribution CC-BY-NC-ND 4.0 License, which permits users to copy and redistribute the work, provided this is not done for commercial purposes and further does not permit distribution of the work if it is changed or edited in any way, and provided that the user gives appropriate credit, provides a link to the license, and that the licensor is not represented as endorsing use of the work.Open access: Article available to all readers online. This article is CC BY-NC-ND. Manuscript received December 31, 2019; Manuscript accepted November 25, 2020. Manuscript handled by Sylvain Grangeon. Thanks to George Rossman for the use of his ATR and Raman instruments, as well as sage advice. Thanks also to Rohit Bhartia for use of the DUV Raman system. We also thank our reviewers, Benoit Dubacq and Jebril Hadi, for extremely thoughtful reviews that greatly improved the quality of our manuscript. This work was funded by NASA Solar Systems Workings (NNX15AH53G), Mars Science Laboratory Participating Scientist, and Mars 2020 SHERLOC Co-Investigator grants, all to B.L.E.
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Issue or Number:6
Record Number:CaltechAUTHORS:20210608-102819867
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Official Citation:Valerie K. Fox, Robert J. Kupper, Bethany L. Ehlmann, Jeffrey G. Catalano, Joseph Razzell-Hollis, William J. Abbey, Dirk J. Schild, Ryan D. Nickerson, Jonas C. Peters, Sydney M. Katz, Andrew A. White; Synthesis and characterization of Fe(III)-Fe(II)-Mg-Al smectite solid solutions and implications for planetary science. American Mineralogist 2021;; 106 (6): 964–982. doi:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109440
Deposited By: Tony Diaz
Deposited On:09 Jun 2021 18:33
Last Modified:09 Jun 2021 18:33

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