¹W. M. Lawrence,¹B. L. Ehlmann
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2025JE009377]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
Published by arrangement with John Wiley & Sons

CM (Mighei‐type) carbonaceous chondrites host abundant OH/H2O‐bearing phyllosilicatesformed from water‐rock reactions in primitive planetesimals. Their infrared (IR) spectral features resemblethose of C‐type asteroids, making laboratory analyses of CMs essential for interpreting asteroid observations.However, CM chondrites are often breccias composed of lithologies with variable degrees of aqueousalteration, complicating their interpretation. Here we use in situ analytical techniques to characterize spectral‐compositional relationships for phyllosilicates in 8 CM lithologies across two meteorite samples. Micro‐Fourier Transform Infrared (μ‐FTIR) spectra collected from phyllosilicate‐rich matrix regions show that bandpositions of the 3‐μm feature and Si‐O stretch Reststrahlen band (RB) systematically vary with alteration.Additional data from spatially correlated electron microprobe and μ‐FTIR measurements tie spectral variationsto specific cation substitutions in serpentines: the 3‐μm feature shifts from 2.78 to 2.70 μm with increased Mg/Fe in octahedral sites, and the Si‐O stretch RB shifts from 10.8 to 9.8 μm with increased Si/Fe3+ in tetrahedralsites. Co‐variation of these features across the studied CM lithologies defines two successive alteration stages:(1) the Si‐O stretch RB and 3‐μm feature shift to longer and shorter wavelengths, respectively, as Mg‐ andcronstedtite‐rich phyllosilicates form from incipient chondrule alteration; (2) Si‐O stretch RB shifts to shorterwavelengths as Mg‐serpentines replace cronstedtite and Mg‐rich chondrules. These patterns align with inferredchanges in composition and redox state for altering fluids on the CM parent body. Similar features in thespectra of C‐type asteroids may reveal information about conditions of aqueous alteration and constrain modelsof their evolution.

^1,2C. P. Haupt,³A. N. Stojic,³A. Morlok,³I. Weber,¹S. Klemme,³H. Hiesinger,^1,4C. J. Renggli
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2024JE008895]
¹Institut für Mineralogie, Universität Münster, Münster, Germany
²ISTO, UMR 7327, University of Orléans, CNRS, BRGM, OSUC, Orléans, France
³Institut für Planetologie, Universität Münster, Münster, Germany
⁴Max-Planck-Institute for Solar System Research, Göttingen, Germany
Published by arrangement with John Wiley & Sons

Laboratory-based mid-infrared (MIR) spectroscopy of terrestrial and planetary analogue materials, combined with chemical and spectral insights from mission-derived data, provides critical tools for advancing our knowledge of planetary surfaces. The returned lunar samples provide information on the chemical variability of the lunar surface. Lunar basalts are notably enriched in TiO2 when compared to their terrestrial equivalents, and are ideal candidates to study the influence of composition on MIR spectral features. We characterized 25 synthetic lunar glasses with variable TiO2 (0.6–18.7 wt%) and SiO2 (35.6–52.1 wt%) in the thermal infrared range using micro-Fourier Transform Infrared Spectrometry (μ-FTIR). Our data reveal a strong linear relationship between the intensity of a spectral shoulder at 14.25 μm (702 cm−1) and the TiO2 content of the analyzed glasses. We suggest that the relationship in our samples reflects an increased distortion of the silicate network with increasing TiO2 concentrations. We observe that TiO2 acts as a network former in specific concentration intervals, thereby affecting the intensity of the observed spectral features in the MIR. This linear relationship is virtually nonexistent in samples that are developing stages of short-range order in the glasses and those samples that show only moderate to low amounts of TiO2. Comparison with data sets from Earth and Mercury analogue materials confirms that the Christiansen Feature (CF) consistently correlates with the SiO2 content, underscoring its robustness as a proxy for glass polymerization across planetary compositions. Finally, we emphasize that incipient crystal nucleation in glassy surfaces affects spectral features in the MIR range.