Reflectance of silicate glasses in the mid-infrared region (MIR): Implications for planetary research

Volcanic phenomaena shaped the surface of all terrestrial planets in the solar system, and silicate glasses represent a major component in pyroclastic deposits and lavas. Spectral features of silicate glasses therefore influence spectral characteristics of large portions of planetary surfaces. In this study, experimental petrology techniques have been used to produce 19 silicate glass samples having natural chemical composition corresponding to four of the most common magmatic series on planet Earth. Reflectance of such products was investigated in the mid-infrared region (MIR) to observe the evolution of their spectral characteristics with changing degree of evolution (expressed as silica content) and alkaline content. We have observed how chemical features have a clear influence in shifting the spectral features (to lower wave-lengths with increasing silica, such as for previously studied volcanic rocks) and on the spectral shape, which is substantially different between mafic and highly silicic products. This allowed us to propose a model to retrieve chemical information (SiO2 and SiO2 + Al2O3 + TiO2 content) from the wavelength at which spectral features (CF and RBpeak) occur. Moreover, by comparing our results with previous MIR studies we have observed that our model can be applied, to a certain extent, to interpret chemical fingerpint volcanic rocks in general. Here, it is also shown how granulometry influences spectral shape, but does not affect spectral shift. This study will be useful to interpret planetary information and assess how amorphous silicate phases influence spectral characteristics of volcanic areas on planetary surfaces.