Mid-infrared spectroscopy of lunar high-Ti basaltic glassy analogues

Haupt, CP; Stojic, AN; Morlok, A; Weber, I; Klemme, S; Hiesinger, H, Renggli, CJ

Abstract

Laboratory-based mid-infrared (MIR) spectroscopy of terrestrial and planetary analogue material combined with chemical and spectral insights from mission-derived data, provides critical tools for advancing our knowledge of planetary surfaces. 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 (0.5–18 wt%) and SiO2 between 35 and 52 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 non-existent 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 MIR datasets from terrestrial samples 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