Effect of bulk chemistry in the spectral variability of igneous rocks in VIS-NIR region: Implications to remote compositional mapping

In the present study, a range of igneous rocks with weight percentage of silica ranging from 45% to 70% were used to generate reflectance spectra in the VIS-NIR region. The laboratory generated reflectance spectra of these rocks were used to study the effect of chemical composition and mineralogy on the spectral properties. The characteristic spectral features were evaluated based on the mineralogical and chemical characteristics of the rocks. The main spectral features in the VIS–NIR region are the 0.7 μm absorption band due to the inter valance charge transfer between Fe2+ and Fe3+ termed as Band F, the 1 μm broad absorption band from Fe2+ at the octahedral sites in pyroxene termed as Band I, the 1.9 μm and 2.3 μm narrow absorption bands due to H2O or OH functional group in hydrated minerals. The 2 μm absorption feature (Band II; Cloutis and Gaffey, 1991) is observed as a weak feature in all the mafic rocks. The analysis of Band I with the bulk chemistry and mineralogy, we observed a positive correlation to the bulk Ca abundance. Rocks with high bulk calcic content exemplify Band I as a prominent spectral feature towards longer wavelength. Consequently, basalt, gabbro and anorthositic rocks show Band I as a strong feature. However, rocks with low bulk Calcic content show Band I as weak absorption feature observed towards shorter wavelength. Thus, igneous rocks of alkaline affinity have subdued Band I feature that appears towards shorter wavelength. The analysis of Band F with the bulk chemistry and mineralogy showed a positive correlation to the bulk Fe abundance. The results of the present study have implications towards remote compositional mapping and lithological discrimination for Planetary Studies.