Compositional and spectrochemical analyses of Cr-spinels in the Sittampundi Anorthosite Complex, Southern India: Implications for remote observation of spinels on the Moon

Abstract

The spectroscopic techniques such as Laser Raman, Fourier Transform Infrared (FTIR), and hyperspectral have been used widely to understand the mineral chemistry and crystal structure and identify the functional phases and organic molecules in geological materials on Earth and other planetary bodies. The present study used these spectroscopic techniques combined with X-ray Diffraction (XRD) and Electron Probe Micro Analysis (EPMA) to understand the spectral-compositional relationships of the Cr-spinel (Chromian spinel) present in chromitite bodies associated with Sittampundi Anorthosite Complex (SAC), southern India. The bands/lenses of Cr-spinel are found as layers (few centimeters to 6 m thick) intercalated with anorthosites and clinopyroxenites of the SAC. The cumulate Cr-spinels of the study area exhibit compositions ranging between Al-chromite and Cr-spinel. Fe-, Al-rich Cr-spinel is a characteristic of the SAC with Cr2O3 content ranging from ~32–37 wt% and Cr# (Cr/[Cr + Al]) in the 0.44 to 0.53 range. The XRD spectra of these Cr-spinels have shown characteristic peaks corresponding to its constituent phases, with the highest peak at 36.11°. The observed longward shift in the Raman A1g peak (~705–714 cm−1) is likely to be caused by the substitution of Al3+ in the spinel structure. The Raman A1g peak position near 705 cm−1 in the spectra is attributed to the coexistence of (Mg, Fe) in the tetrahedral site and (Al, Cr) in the octahedral site. The broader and stronger 2 µm band position in the hyperspectral data is at relatively shorter wavelengths than typical Cr-spinels due to enhanced Al content (Al2O3 ~ 25 wt%) in the SAC samples. The 2 µm band position is observed to have a longward shift with increasing Cr2O3 and Cr# abundances and a corresponding shortward shift with enhanced Al2O3 content in Cr-spinels. The linear relationship between the 2 µm band position and Cr/Al abundances indicates that this absorption band is significant in distinguishing Cr-spinels from Al-spinels. Based on spectral and compositional resemblance, Fe- and Al-rich Cr-spinels in SAC are considered as probable terrestrial (functional) analogues for similar lunar spinel compositions given that evidence-based correlation of intricate processes involved in their formation under the lunar and terrestrial conditions. The present study demonstrates the approach of applying spectrochemical characteristics of terrestrial analogue spinels for remote identification of lunar spinels and retrieving their compositional ranges from the spectral reflectance parameters.