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.