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

1,2Valerie K. Fox et al. (>10)
American Mineralogist 106, 964–982 Link to Article [DOI: https://doi.org/10.2138/am-2020-7419CCBYNCND]
1California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, U.S.A
2University of Minnesota, John T. Tate Hall, 116 Church Street
SE, Minneapolis, MN 55455-0149, U.S.A.
Copyright: The Mineralogical Society of America

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.

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