Integration of Raman, emission, and reflectance spectroscopy for earth and lunar mineralogy

Abstract

Spectroscopy plays a vital role in the identification and characterization of minerals on terrestrial and planetary surfaces. We review the three different spectroscopic techniques for characterizing minerals on the Earth and lunar surfaces separately. Seven sedimentary and metamorphic terrestrial rock samples were analyzed with three field-based spectrometers, i.e., Raman, Fourier transform infrared (FTIR), and visible to near infrared and shortwave infrared (Vis–NIR–SWIR) spectrometers. Similarly, a review of work done by previous researchers on lunar rock samples was also carried out for their Raman, Vis–NIR–SWIR, and thermal (mid-infrared) spectral responses. It has been found in both the cases that the spectral information such as Si-O-Si stretching (polymorphs) in Raman spectra, identification of impurities, Christiansen and Restrahlen band center variation in mid-infrared spectra, location of elemental substitution, the content of iron, and shifting of the band center of diagnostic absorption features at 1 and 2 μm in reflectance spectra are contributing to the characterization and identification of terrestrial and lunar minerals. We show that quartz can be better characterized by considering silica polymorphs from Raman spectra, emission features in the range of 8 to 14 μm in FTIR spectra, and reflectance absorption features from Vis–NIR–SWIR spectra. KREEP materials from Apollo 12 and 14 samples are also better characterized using integrated spectroscopic studies. Integrated spectral responses felicitate comprehensive characterization and better identification of minerals. We suggest that Raman spectroscopy and visible and NIR-thermal spectroscopy are the best techniques to explore the Earth’s and lunar mineralogy.