Model VNIR optical constants of silicates mixtures analogues to igneous rocks

Details Model VNIR optical constants of silicates mixtures analogues to igneous rocks Model VNIR optical constants of silicates mixtures analogues to igneous rocks

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p43c1696c&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P43C-1696

Physics

[3934] Mineral Physics / Optical, Infrared, And Raman Spectroscopy, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties

Scientific paper

Remote-sensing studies have revealed that most of the inner planets surfaces are composed of silicate bearing rocks with variable relative mineral abundances and compositions that could be indicative of genetically related rocks. Quantifying and modeling those phases in mineral mixtures is an important task to characterize the surface compositions and to understand the evolution of the crust. One approach to reach this aim is to invert the bidirectional reflectance equation(Hapke,1993), to derive optical constants of powdered materials. This approach has been applied with positive results to obtain the optical constants of mafic silicates, e.g., olivine and pyroxene, and plagioclase taking into account the effects of composition and grain-size variations. Mixtures of those mineral phases were modeled successfully starting from the optical constants of each end-member, but only few works modeled spectra of mixtures where some of the end-members could not be individually characterized. Here we present preliminary results on the determination of the optical constants of complex silicates mixtures obtained applying the Hapke's radiative transfer model to visible-near infrared reflectance spectra. The bidirectional reflectance spectra are acquired at the SLAB (Spectroscopy Laboratory, Iasf-INAF, Roma) in the wavelength range 0.35- 2.50 μm. We prepared mixtures starting from three different plagioclases with variable amount of FeO and two different mafic end-members, mixtures of pyroxenes and pyroxenes + olivine, respectively. For each plagioclase composition, mixtures varying from 20 to 90 wt.% plagioclase and multimineral mafic grains were prepared, in two grain size classes (63-125 μm and 125-250 μm), for a total of 64 samples. Moreover, to optimize the calculation of the optical constant, we have acquired spectra of the end-members in other two different grain size classes (50-75 μm and 100-125 μm). We have retrieved the single scattering albedo (w) for each wavelength under the assumption of isotropic single scattering applying Hapke's equation of bidirectional reflectance to the laboratory spectra of all plagioclase and mafic end-members. Then, keeping fixed the grain-size distribution and the real part of the refractive index (n+ik), we inverted the formulation of w given in Hapke (1993) to compute the imaginary part (k) for the entire wavelength range. The resulting k values have been used to model spectra of intimate mixtures of plagioclase and mafic assemblages. This laboratory activity is financially supported by an ASI grant.

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