Spectral Characterization of Synthetic Silicate Glasses Analogous to Lunar Basalts

Volcanic glasses are widespread on the Moon and represent an important component of both pyroclastic deposits and the lunar regolith. These amorphous materials originate from explosive volcanic eruptions as well as from impact-related processes, and therefore preserve valuable information on the evolution of the lunar crust and mantle. Despite their relevance for the interpretation of remote sensing observations, amorphous silicate materials remain significantly underrepresented in current spectral libraries, especially when compared to crystalline phases. In this framework, laboratory investigations on well-characterized analogue materials are essential for improving the identification and interpretation of glass-rich planetary surfaces. In this work, we present a combined Visible-Near Infrared (VNIR, 400-2500 nm) and Mid-Infrared (MIR, 3000-25000 nm) spectral characterization of synthetic silicate glasses representative of lunar mafic to ultramafic compositions.

Four lunar analogue compositions derived from Apollo samples were synthesized in laboratory: a high-Ti mare basalt (MB), a low-Ti basalt (LTB), a KREEP basalt (KB), and an Apollo 15 green glass analogue (GG). The glasses were produced from high-purity oxide mixtures and subsequently separated into four grain size fractions (<38 μm, 38-63 μm, 63-106 μm, and 106-150 μm) to evaluate the influence of particle size on spectral behaviour.

Reflectance spectra were acquired using FTIR (Fourier Transform Infrared) spectroscopy across the VNIR and MIR spectral ranges. Additional XRPD (X-Ray Powder Diffractometry) analyses confirmed the predominantly amorphous nature of the synthesized materials, although the high-Ti MB sample displayed traces of crystallization, likely related to Fe-Ti oxide nucleation during quenching. Spectral analyses focused on the behaviour of albedo, spectral slopes, VNIR absorption bands, and MIR diagnostic features such as the Christiansen Feature (CF), Reststrahlen bands peak position (RB_peak), and Transparency Feature (TF).

In the VNIR domain, all samples display weak and broad absorption features centered near ~1100 nm and ~1900 nm, attributed to Fe²⁺ electronic transitions in octahedral and tetrahedral coordination. Compared to crystalline materials, these absorptions appear toned down and poorly resolved, reflecting the amorphous structure of the studied glasses. Spectral slopes and albedo are strongly affected by grain size, with finer fractions generally showing higher reflectance and steeper VIS slopes. However, significant compositional effects are also observed. In particular, the MB sample shows an anomalous spectral behaviour, characterized by increasing albedo toward coarser fractions and flattened VNIR slopes. The influence of composition was investigated through the SAT parameter (SiO₂+Al₂O₃+TiO₂), the SCFM index, and iron content. Overall, grain size exerts the dominant control on VNIR spectral variability, partially masking compositional trends.

In the MIR region, spectral behaviour is dominated by vibrational processes within the silicate framework and provides clearer compositional constraints. The CF position systematically shifts toward shorter wavelengths with increasing grain size and exhibits a strong correlation with parameters related to silicate network polymerization, particularly SAT and SiO₂ content. The finest grain size fractions (<38 μm) show the best agreement with trends previously reported in the literature for synthetic silicate glasses. In contrast, the RB_peak position displays a weaker and more scattered compositional dependence, although partial correlations with the SCFM parameter can be recognized. A weak TF is observed exclusively in the finest grain size fractions, confirming the strong grain size control on this spectral feature and the generally weak MIR spectral contrast of low-alkali silicate glasses.

In conclusion, the results demonstrate that MIR spectral parameters, particularly the Christiansen Feature, provide more robust compositional constraints than VNIR spectral properties, whereas VNIR data remain useful for identifying iron-rich glassy materials and evaluating grain size effects. The dataset presented here extends current spectral libraries toward primitive lunar mafic compositions and contributes to improving the interpretation of orbital and in situ lunar observations. This work contributes to improving spectral libraries of amorphous planetary materials and supports future investigations of glass-rich volcanic and impact deposits on the Moon and other rocky bodies.

Acknowledgements

We acknowledge the support of ASI under the ASI-UniPg agreement 2019-2-HH.0.