Raman investigations of suspected airburst glasses from the Atacama Desert: Evidence for natural chemical vapour deposition of MoS2

Airbursts of extraterrestrial projectiles likely occurred frequently throughout the geological past but are rarely available in the geological record to study. Interestingly, natural glasses from Pica in northern Chile (Pica glass) possibly formed during a natural airburst event [1] and could be samples to study such an airburst. The origin of Pica glass, however, is controversial, with pyrometamorphic processes also proposed for their formation [2]. To shed light on the geological origin of Pica glass, we analysed its atomic structure and dynamics using Raman spectroscopy. Raman measurements were performed on thin sections of Pica glass samples, targeting both the glass matrix and crystalline inclusions. The Raman scattering collected from the matrix resembles that of an amorphous SiO₄ network containing significant amounts of broken Si-O-Si linkages, likely caused by alkali and/or alkali earth elements acting as network modifiers. Furthermore, distinct peak pairs at ~383/407 cm^-1 and ~385/406 cm^-1 were identified, corresponding to multilayer and bilayer molybdenum disulfide (MoS₂), respectively. These MoS₂ nanostructures are known from chemical vapour deposition experiments in which volatile MoO₃ reacts with S₂ gas [3]. The MoS₂ structures in Pica glasses are therefore interpreted to have formed by condensation of MoS₂ onto Pica glass melts – resembling deposition in a vapour plume. Preliminary EMP data show correlated, heterogeneous Mo and S concentrations in the glass matrix. Our results support the hypothesis that Pica glass formed by an atmospheric airburst event.

[1] Schultz et al. (2022) Geology, [2] Roperch et al. (2022) EPSL, [3] Pierucci et al. (2016) Sci.Rep.