Near-infrared Spectra of Lunar Ferrous Mineral Mixtures

Iron-bearing minerals are a major component of materials on the lunar surface, and many of them can be distinguished based on the diagnostic absorption features in visible and near-infrared reflectance spectra. The relationship between the 1 mu m absorption (Band I) center and the band area ratio (BAR), defined as the ratio of 2-1 mu m absorption areas, provides a sensitive way to estimate the relative abundance of olivine (Ol) and pyroxene. In the plot of the BAR value versus the Band I center, the Ol-orthopyroxene (Opx) mixing line (derived from terrestrial materials) is strictly applicable only to Ol-Opx mixtures. Based on published database of laboratory spectra and compositional data for lunar rocks and mineral separates, this study investigated the spectral characteristics of a variety of common Fe2+-bearing lunar minerals and rocks, such as clinopyroxene (Cpx), ilmenite-rich basalt, pyroxene-bearing anorthosite, and glass-rich impact melt. The lunar Ol-Cpx-Opx mixing line for rocks and minerals is determined, which contains less curvature than the Ol-Opx mixing line, consistent with both the higher Fe2+ content of lunar mafic silicates and the presence of appreciable Cpx. This study suggests that some of the pyroxene-bearing lunar materials that are rich in ilmenite, glass, or plagioclase can also be distinguished by this method. These results indicate that the plot of the BAR value versus the Band I center is a useful tool for spectral analysis of lunar composition and mineralogy, especially for those having pyroxene-dominated spectra. Plain Language Summary Iron-bearing minerals are a major component of materials on the lunar surface, many of which can be distinguished by their different diagnostic absorption features in reflectance spectra. The relationship between two spectral parameters, which are the wavelength position of the 1 mu m absorption (Band I) center and the band area ratio (BAR, defined as the ratio of 2-1 mu m absorption areas), provides a sensitive method for estimating the relative abundance of two common ironbearing minerals (olivine and orthopyroxene). However, there is a systematic offset in the identification of lunar materials. Based on published laboratory spectral database, this study investigated the spectral characteristics of various common iron-bearing lunar minerals and rocks and determined the distribution trend of lunar olivine and pyroxene (orthopyroxene and clinopyroxene) mixtures in the plot of the BAR value versus the Band I center. This study also suggests that some of the pyroxene-bearing lunar materials rich in ilmenite, glass, or plagioclase can also be distinguished by this method. This result indicates that this method is useful for spectral analysis of composition and mineralogy on the lunar surface.