The role of very fine particle sizes in the reflectance spectroscopy of plagioclase-bearing mixtures: New understanding for the interpretation of the finest sizes of the lunar regolith

Abstract

The lunar surface consists of a regolith layer that covers the underlying bedrocks, and is generally char- acterized by particulates < 1 cm. Lunar soil is the fine fraction of the regolith, and is generally between 60 and 80 μm. Sizes < 10 μm, accounting for ca. 5–20% of the soil, were recognized and petrologically classified. The coarsest sizes of the regolith are chemically and mineralogically similar, while the finest fractions are more feldspathic, probably due to easier fracturing of plagioclase than mafic minerals. Due to the more feldspathic nature of the very fine lunar soils, in this paper, we quantitatively inves- tigate the influence of very fine ( < 10 μm) plagioclase on the absorption bands of mafic minerals using the Modified Gaussian Model. We considered two plagioclases with different iron content and two mafic end-members (1) 56% orthopyroxene and 44% clinopyroxene, and (2) 30% orthopyroxene and 70% olivine. We also compared our results with the deconvolution of the same mixtures at coarser sizes. Our results mainly show that: (1) fine sizes act principally on reflectance and on spectral contrast (with the former increasing and the latter decreasing); (2) very fine plagioclase has a blue slope in the Near Infrared and very shallow 1250 nm band depth, close to zero; (3) consequently, the plagioclase band is always shallower than mafic bands; (4) in mixtures with olivine, the composite band center always shows the typical olivine value, differently from coarser mixtures; and (5) mafic materials have a blue slope in the Short Wavelength Infrared Region, a more V-shaped 1 μm pyroxene absorption and the 1 μm mafic band centers are shifted by ca. 40 nm vs. coarse sizes, reflecting a different weight within the crystal field absorption of the mafic component in very fine size. We also evidenced that a coarse plagioclase could be overestimated, while a very fine one could be underestimated if compared with the 63–125 μm size.