Spectral variability of plagioclase-mafic mixtures (1): Effects of chemistry and modal abundance in reflectance spectra of rocks and mineral mixtures

Remote sensing from lunar highland and from Hermean surface showed reflectance spectra with no detectable absorption bands in the visible and near infrared, and recently M3 data from the lunar surface (e.g., Orientale Basin) have shown spectra with a weak absorption band centered at 1250 nm. Those terrains were generally interpreted as plagioclase rich regions. Plagioclase, however, has often been considered a spectrally transparent and almost featureless mineral. So it is difficult to recognize its presence and to quantify the abundance of this mineral even if it is one of the most important mineralogical component of the planetary surfaces. In this work we investigate the influence of plagioclase absorption band on the absorption of Fe, Mg minerals. We consider three plagioclases with different FeO wt.% contents which have been mixed with three mafic end-members (1) pyroxene-bearing and olivine-free, (2) olivine-poor and (3) olivine-rich, at two different grain size. Plagioclase’s influence has been expressed considering variation of different spectral parameters: band position, band intensity and band width. We show that plagioclase has spectroscopically different behavior when mixed with olivine-bearing or olivine-free end-members, but, in general, adding modal abundance of plagioclase produces higher albedo and reduced spectral contrast. Increasing FeO content in plagioclase and coarsening the grain size have similar effects on the mixture spectra. We also observed the spectral convergence of mixtures with different composition and grain size. With these results, we point out the importance of considering the plagioclase absorption on mixture’s mineralogical composition, when spectra from different terrain are considered. Moreover, accurate evaluation of the abundance of plagioclase in those mixture should be discussed in subsequent works.